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[debian/debian-policy.git] / policy.sgml

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  <book>
    <titlepag>
      <title>Debian Policy Manual</title>
      <author><qref id="authors">The Debian Policy Mailing List</qref></author>
      <version>version &version;, &date;</version>

      <abstract>
        This manual describes the policy requirements for the Debian
        GNU/Linux distribution.  This includes the structure and
        contents of the Debian archive and several design issues of
        the operating system, as well as technical requirements that
        each package must satisfy to be included in the distribution.
      </abstract>

      <copyright>
        <copyrightsummary>
          Copyright &copy; 1996,1997,1998 Ian Jackson
          and Christian Schwarz.
        </copyrightsummary>
        <p>
          This manual is free software; you may redistribute it and/or
          modify it under the terms of the GNU General Public License
          as published by the Free Software Foundation; either version
          2, or (at your option) any later version.
        </p>

        <p>
          This is distributed in the hope that it will be useful, but
          <em>without any warranty</em>; without even the implied
          warranty of merchantability or fitness for a particular
          purpose.  See the GNU General Public License for more
          details.
        </p>

        <p>
          A copy of the GNU General Public License is available as
          <file>/usr/share/common-licenses/GPL</file> in the Debian GNU/Linux
          distribution or on the World Wide Web at
          <url id="http://www.gnu.org/copyleft/gpl.html"
               name="the GNU General Public Licence">. You can also
          obtain it by writing to the Free Software Foundation, Inc.,
          51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA.
        </p>
      </copyright>
    </titlepag>

    <toc detail="sect1">

    <chapt id="scope">
      <heading>About this manual</heading>
      <sect>
        <heading>Scope</heading>
        <p>
          This manual describes the policy requirements for the Debian
          GNU/Linux distribution. This includes the structure and
          contents of the Debian archive and several design issues of the
          operating system, as well as technical requirements that
          each package must satisfy to be included in the
          distribution.
        </p>

        <p>
          This manual also describes Debian policy as it relates to
          creating Debian packages. It is not a tutorial on how to build
          packages, nor is it exhaustive where it comes to describing
          the behavior of the packaging system. Instead, this manual
          attempts to define the interface to the package management
          system that the developers have to be conversant with.<footnote>
              Informally, the criteria used for inclusion is that the
              material meet one of the following requirements:
              <taglist compact="compact">
                <tag>Standard interfaces</tag>
                <item>
                    The material presented represents an interface to
                    the packaging system that is mandated for use, and
                    is used by, a significant number of packages, and
                    therefore should not be changed without peer
                    review. Package maintainers can then rely on this
                    interfaces not changing, and the package
                    management software authors need to ensure
                    compatibility with these interface
                    definitions. (Control file and changelog file
                    formats are examples.)
                </item>
                <tag>Chosen Convention</tag>
                <item>
                    If there are a number of technically viable choices
                    that can be made, but one needs to select one of
                    these options for inter-operability. The version
                    number format is one example.
                </item>
              </taglist>
              Please note that these are not mutually exclusive;
              selected conventions often become parts of standard
              interfaces.
          </footnote>
        </p>

        <p>
          The footnotes present in this manual are
          merely informative, and are not part of Debian policy itself.
        </p>

        <p>
          The appendices to this manual are not necessarily normative,
          either. Please see <ref id="pkg-scope"> for more information.
        </p>

        <p>
          In the normative part of this manual,
          the words <em>must</em>, <em>should</em> and
          <em>may</em>, and the adjectives <em>required</em>,
          <em>recommended</em> and <em>optional</em>, are used to
          distinguish the significance of the various guidelines in
          this policy document. Packages that do not conform to the
          guidelines denoted by <em>must</em> (or <em>required</em>)
          will generally not be considered acceptable for the Debian
          distribution. Non-conformance with guidelines denoted by
          <em>should</em> (or <em>recommended</em>) will generally be
          considered a bug, but will not necessarily render a package
          unsuitable for distribution. Guidelines denoted by
          <em>may</em> (or <em>optional</em>) are truly optional and
          adherence is left to the maintainer's discretion.
        </p>

        <p>
          These classifications are roughly equivalent to the bug
          severities <em>serious</em> (for <em>must</em> or
          <em>required</em> directive violations), <em>minor</em>,
          <em>normal</em> or <em>important</em>
          (for <em>should</em> or <em>recommended</em> directive
          violations) and <em>wishlist</em> (for <em>optional</em>
          items).
          <footnote>
                Compare RFC 2119.  Note, however, that these words are
                used in a different way in this document.
          </footnote>
        </p>

        <p>
          Much of the information presented in this manual will be
          useful even when building a package which is to be
          distributed in some other way or is intended for local use
          only.
        </p>
      </sect>

      <sect>
        <heading>New versions of this document</heading>

        <p>
          This manual is distributed via the Debian package
          <package><url name="debian-policy"
              id="http://packages.debian.org/debian-policy"></package> 
          (<httpsite>packages.debian.org</httpsite> 
          <httppath>/debian-policy</httppath>).
        </p>

        <p>
          The current version of this document is also available from
          the Debian web mirrors at
          <tt><url name="/doc/debian-policy/"
                id="http://www.debian.org/doc/debian-policy/"></tt>.
          (
          <httpsite>www.debian.org</httpsite>
          <httppath>/doc/debian-policy/</httppath>)
          Also available from the same directory are several other
          formats: <file>policy.html.tar.gz</file>
          (<httppath>/doc/debian-policy/policy.html.tar.gz</httppath>),
          <file>policy.pdf.gz</file> 
          (<httppath>/doc/debian-policy/policy.pdf.gz</httppath>)
          and <file>policy.ps.gz</file>
          (<httppath>/doc/debian-policy/policy.ps.gz</httppath>).
        </p>

        <p>
          The <package>debian-policy</package> package also includes the file
          <file>upgrading-checklist.txt.gz</file> which indicates policy
          changes between versions of this document.
        </p>
      </sect>

      <sect id="authors">
        <heading>Authors and Maintainers</heading>

        <p>
          Originally called "Debian GNU/Linux Policy Manual", this
          manual was initially written in 1996 by Ian Jackson.
          It was revised on November 27th, 1996 by David A. Morris.
          Christian Schwarz added new sections on March 15th, 1997,
          and reworked/restructured it in April-July 1997.
          Christoph Lameter contributed the "Web Standard".
          Julian Gilbey largely restructured it in 2001.
        </p>

        <p>
          Since September 1998, the responsibility for the contents of
          this document lies on the <url name="debian-policy mailing list"
          id="mailto:debian-policy@lists.debian.org">. Proposals
          are discussed there and inserted into policy after a certain
          consensus is established.
          <!-- insert shameless policy-process plug here eventually -->
          The actual editing is done by a group of maintainers that have
          no editorial powers. These are the current maintainers:

          <enumlist>
            <item>Julian Gilbey</item>
            <item>Branden Robinson</item>
            <item>Josip Rodin</item>
            <item>Manoj Srivastava</item>
          </enumlist>
        </p>

        <p>
          While the authors of this document have tried hard to avoid
          typos and other errors, these do still occur. If you discover
          an error in this manual or if you want to give any
          comments, suggestions, or criticisms please send an email to
          the Debian Policy List,
          <email>debian-policy@lists.debian.org</email>, or submit a
          bug report against the <tt>debian-policy</tt> package.
        </p>

        <p>
          Please do not try to reach the individual authors or maintainers
          of the Policy Manual regarding changes to the Policy.
        </p>
      </sect>

      <sect id="related">
        <heading>Related documents</heading>

        <p>
          There are several other documents other than this Policy Manual
          that are necessary to fully understand some Debian policies and
          procedures.
        </p>

        <p>
          The external "sub-policy" documents are referred to in:
          <list compact="compact">
            <item><ref id="fhs"></item>
            <item><ref id="virtual_pkg"></item>
            <item><ref id="menus"></item>
            <item><ref id="mime"></item>
            <item><ref id="perl"></item>
            <item><ref id="maintscriptprompt"></item>
            <item><ref id="emacs"></item>
          </list>
        </p>

        <p>
          In addition to those, which carry the weight of policy, there
          is the Debian Developer's Reference. This document describes
          procedures and resources for Debian developers, but it is
          <em>not</em> normative; rather, it includes things that don't
          belong in the Policy, such as best practices for developers.
        </p>

        <p>
          The Developer's Reference is available in the
          <package>developers-reference</package> package.
          It's also available from the Debian web mirrors at
          <tt><url name="/doc/developers-reference/"
                id="http://www.debian.org/doc/developers-reference/"></tt>.
        </p>
      </sect>

    </chapt>


    <chapt id="archive">
      <heading>The Debian Archive</heading>

      <p>
        The Debian GNU/Linux system is maintained and distributed as a
        collection of <em>packages</em>. Since there are so many of
        them (currently well over 15000), they are split into
        <em>sections</em> and given <em>priorities</em> to simplify
        the handling of them.
      </p>

      <p>
        The effort of the Debian project is to build a free operating
        system, but not every package we want to make accessible is
        <em>free</em> in our sense (see the Debian Free Software
        Guidelines, below), or may be imported/exported without
        restrictions. Thus, the archive is split into the distribution
        areas or categories based on their licenses and other restrictions.
      </p>

      <p>
        The aims of this are:

        <list compact="compact">
          <item>to allow us to make as much software available as we can</item>
          <item>to allow us to encourage everyone to write free software,
                and</item>
          <item>to allow us to make it easy for people to produce
                CD-ROMs of our system without violating any licenses,
                import/export restrictions, or any other laws.</item>
        </list>
      </p>

      <p>
        The <em>main</em> category  forms the
        <em>Debian GNU/Linux distribution</em>.
      </p>

      <p>
        Packages in the other distribution areas (<tt>contrib</tt>,
        <tt>non-free</tt>) are not considered to be part of the Debian
        distribution, although we support their use and provide
        infrastructure for them (such as our bug-tracking system and
        mailing lists). This Debian Policy Manual applies to these
        packages as well.
      </p>

      <sect id="dfsg">
        <heading>The Debian Free Software Guidelines</heading>
        <p>
          The Debian Free Software Guidelines (DFSG) form our
          definition of "free software".  These are:
            <taglist>
              <tag>Free Redistribution
              </tag>
              <item>
                  The license of a Debian component may not restrict any
                  party from selling or giving away the software as a
                  component of an aggregate software distribution
                  containing programs from several different
                  sources. The license may not require a royalty or
                  other fee for such sale.
              </item>
              <tag>Source Code
              </tag>
              <item>
                  The program must include source code, and must allow
                  distribution in source code as well as compiled form.
              </item>
              <tag>Derived Works
              </tag>
              <item>
                  The license must allow modifications and derived
                  works, and must allow them to be distributed under the
                  same terms as the license of the original software.
              </item>
              <tag>Integrity of The Author's Source Code
              </tag>
              <item>
                  The license may restrict source-code from being
                  distributed in modified form <em>only</em> if the
                  license allows the distribution of "patch files"
                  with the source code for the purpose of modifying the
                  program at build time. The license must explicitly
                  permit distribution of software built from modified
                  source code. The license may require derived works to
                  carry a different name or version number from the
                  original software.  (This is a compromise. The Debian
                  Project encourages all authors to not restrict any
                  files, source or binary, from being modified.)
              </item>
              <tag>No Discrimination Against Persons or Groups
              </tag>
              <item>
                  The license must not discriminate against any person
                  or group of persons.
              </item>
              <tag>No Discrimination Against Fields of Endeavor
              </tag>
              <item>
                  The license must not restrict anyone from making use
                  of the program in a specific field of endeavor. For
                  example, it may not restrict the program from being
                  used in a business, or from being used for genetic
                  research.
              </item>
              <tag>Distribution of License
              </tag>
              <item>
                  The rights attached to the program must apply to all
                  to whom the program is redistributed without the need
                  for execution of an additional license by those
                  parties.
              </item>
              <tag>License Must Not Be Specific to Debian
              </tag>
              <item>
                  The rights attached to the program must not depend on
                  the program's being part of a Debian system. If the
                  program is extracted from Debian and used or
                  distributed without Debian but otherwise within the
                  terms of the program's license, all parties to whom
                  the program is redistributed must have the same
                  rights as those that are granted in conjunction with
                  the Debian system.
              </item>
              <tag>License Must Not Contaminate Other Software
              </tag>
              <item>
                  The license must not place restrictions on other
                  software that is distributed along with the licensed
                  software. For example, the license must not insist
                  that all other programs distributed on the same medium
                  must be free software.
              </item>
              <tag>Example Licenses
              </tag>
              <item>
                  The "GPL," "BSD," and "Artistic" licenses are examples of
                  licenses that we consider <em>free</em>.
              </item>
            </taglist>
        </p>
      </sect>

      <sect id="sections">
        <heading>Categories</heading>

        <sect1 id="main">
          <heading>The main category</heading>

          <p>
            Every package in <em>main</em> must comply with the DFSG
            (Debian Free Software Guidelines).
          </p>

          <p>
            In addition, the packages in <em>main</em>
            <list compact="compact">
              <item>
                  must not require a package outside of <em>main</em>
                  for compilation or execution (thus, the package must
                  not declare a "Depends", "Recommends", or
                  "Build-Depends" relationship on a non-<em>main</em>
                  package),
              </item>
              <item>
                  must not be so buggy that we refuse to support them,
                  and
              </item>
              <item>
                  must meet all policy requirements presented in this
                  manual.
              </item>
            </list>
          </p>

        </sect1>

        <sect1 id="contrib">
          <heading>The contrib category</heading>

          <p>
            Every package in <em>contrib</em> must comply with the DFSG.
          </p>

          <p>
            In addition, the packages in <em>contrib</em>
            <list compact="compact">
              <item>
                  must not be so buggy that we refuse to support them,
                  and
              </item>
              <item>
                  must meet all policy requirements presented in this
                  manual.
              </item>
            </list>
          </p>


          <p>
            Examples of packages which would be included in
            <em>contrib</em> are:
            <list compact="compact">
              <item>
                  free packages which require <em>contrib</em>,
                  <em>non-free</em> packages or packages which are not
                  in our archive at all for compilation or execution,
                  and
              </item>
              <item>
                  wrapper packages or other sorts of free accessories for
                  non-free programs.
              </item>
            </list>
          </p>
        </sect1>

        <sect1 id="non-free">
          <heading>The non-free category</heading>

          <p>
            Packages must be placed in <em>non-free</em> if they are
            not compliant with the DFSG or are encumbered by patents
            or other legal issues that make their distribution
            problematic.
          </p>

          <p>
            In addition, the packages in <em>non-free</em>
            <list compact="compact">
              <item>
                  must not be so buggy that we refuse to support them,
                  and
              </item>
              <item>
                  must meet all policy requirements presented in this
                  manual that it is possible for them to meet.
                  <footnote>
                      It is possible that there are policy
                      requirements which the package is unable to
                      meet, for example, if the source is
                      unavailable.  These situations will need to be
                      handled on a case-by-case basis.
                  </footnote>
              </item>
            </list>
          </p>
        </sect1>

      </sect>

      <sect id="pkgcopyright">
        <heading>Copyright considerations</heading>

        <p>
          Every package must be accompanied by a verbatim copy of
          its copyright and distribution license in the file
          <file>/usr/share/doc/<var>package</var>/copyright</file>
          (see <ref id="copyrightfile"> for further details).
        </p>

        <p>
          We reserve the right to restrict files from being included
          anywhere in our archives if
          <list compact="compact">
            <item>
                  their use or distribution would break a law,
            </item>
            <item>
                  there is an ethical conflict in their distribution or
                  use,
            </item>
            <item>
                  we would have to sign a license for them, or
            </item>
            <item>
                  their distribution would conflict with other project
                  policies.
            </item>
          </list>
        </p>

        <p>
          Programs whose authors encourage the user to make
          donations are fine for the main distribution, provided
          that the authors do not claim that not donating is
          immoral, unethical, illegal or something similar; in such
          a case they must go in <em>non-free</em>.
        </p>

        <p>
          Packages whose copyright permission notices (or patent
          problems) do not even allow redistribution of binaries
          only, and where no special permission has been obtained,
          must not be placed on the Debian FTP site and its mirrors
          at all.
        </p>

        <p>
          Note that under international copyright law (this applies
          in the United States, too), <em>no</em> distribution or
          modification of a work is allowed without an explicit
          notice saying so.  Therefore a program without a copyright
          notice <em>is</em> copyrighted and you may not do anything
          to it without risking being sued! Likewise if a program
          has a copyright notice but no statement saying what is
          permitted then nothing is permitted.
        </p>

        <p>
          Many authors are unaware of the problems that restrictive
          copyrights (or lack of copyright notices) can cause for
          the users of their supposedly-free software.  It is often
          worthwhile contacting such authors diplomatically to ask
          them to modify their license terms. However, this can be a
          politically difficult thing to do and you should ask for
          advice on the <tt>debian-legal</tt> mailing list first, as
          explained below.
        </p>

        <p>
          When in doubt about a copyright, send mail to
          <email>debian-legal@lists.debian.org</email>.  Be prepared
          to provide us with the copyright statement.  Software
          covered by the GPL, public domain software and BSD-like
          copyrights are safe; be wary of the phrases "commercial
          use prohibited" and "distribution restricted".
        </p>
      </sect>

      <sect id="subsections">
        <heading>Sections</heading>

        <p>
          The packages in the categories <em>main</em>,
          <em>contrib</em> and <em>non-free</em> are grouped further
          into <em>sections</em> to simplify handling.
        </p>

        <p>
          The category and section for each package should be
          specified in the package's <tt>Section</tt> control record
          (see <ref id="f-Section">).  However, the maintainer of the
          Debian archive may override this selection to ensure the
          consistency of the Debian distribution.  The
          <tt>Section</tt> field should be of the form:
          <list compact="compact">
            <item>
                  <em>section</em> if the package is in the
                  <em>main</em> category,
            </item>
            <item>
                  <em>segment/section</em> if the package is in
                  the <em>contrib</em> or <em>non-free</em>
                  distribution areas.
            </item>
          </list>
        </p>

        <p>
          The Debian archive maintainers provide the authoritative
          list of sections.  At present, they are:
          <em>admin</em>, <em>base</em>, <em>comm</em>,
          <em>contrib</em>, <em>devel</em>, <em>doc</em>,
          <em>editors</em>, <em>electronics</em>, <em>embedded</em>,
          <em>games</em>, <em>gnome</em>, <em>graphics</em>,
          <em>hamradio</em>, <em>interpreters</em>, <em>kde</em>,
          <em>libs</em>, <em>libdevel</em>, <em>mail</em>,
          <em>math</em>, <em>misc</em>, <em>net</em>, <em>news</em>,
          <em>non-free</em>, <em>oldlibs</em>,
          <em>otherosfs</em>, <em>perl</em>, <em>python</em>,
          <em>science</em>, <em>shells</em>,
          <em>sound</em>, <em>tex</em>, <em>text</em>,
          <em>utils</em>, <em>web</em>, <em>x11</em>.
        </p>
      </sect>

      <sect id="priorities">
        <heading>Priorities</heading>

        <p>
          Each package should have a <em>priority</em> value, which is
          included in the package's <em>control record</em>
          (see <ref id="f-Priority">).
          This information is used by the Debian package management tools to
          separate high-priority packages from less-important packages.
        </p>

        <p>
          The following <em>priority levels</em> are recognized by the
          Debian package management tools.
          <taglist>
            <tag><tt>required</tt></tag>
            <item>
                Packages which are necessary for the proper
                functioning of the system (usually, this means that
                dpkg functionality depends on these packages).
                Removing a <tt>required</tt> package may cause your
                system to become totally broken and you may not even
                be able to use <prgn>dpkg</prgn> to put things back,
                so only do so if you know what you are doing.  Systems
                with only the <tt>required</tt> packages are probably
                unusable, but they do have enough functionality to
                allow the sysadmin to boot and install more software.
            </item>
            <tag><tt>important</tt></tag>
            <item>
                Important programs, including those which one would
                expect to find on any Unix-like system.  If the
                expectation is that an experienced Unix person who
                found it missing would say "What on earth is going on,
                where is <prgn>foo</prgn>?", it must be an
                <tt>important</tt> package.<footnote>
                    This is an important criterion because we are
                    trying to produce, amongst other things, a free
                    Unix.
                </footnote>
                Other packages without which the system will not run
                well or be usable must also have priority
                <tt>important</tt>.  This does
                <em>not</em> include Emacs, the X Window System, TeX
                or any other large applications.  The
                <tt>important</tt> packages are just a bare minimum of
                commonly-expected and necessary tools.
            </item>
            <tag><tt>standard</tt></tag>
            <item>
                These packages provide a reasonably small but not too
                limited character-mode system.  This is what will be
                installed by default if the user doesn't select anything
                else.  It doesn't include many large applications.
            </item>
            <tag><tt>optional</tt></tag>
            <item>
                (In a sense everything that isn't required is
                optional, but that's not what is meant here.) This is
                all the software that you might reasonably want to
                install if you didn't know what it was and don't have
                specialized requirements. This is a much larger system
                and includes the X Window System, a full TeX
                distribution, and many applications. Note that
                optional packages should not conflict with each other.
            </item>
            <tag><tt>extra</tt></tag>
            <item>
                This contains all packages that conflict with others
                with required, important, standard or optional
                priorities, or are only likely to be useful if you
                already know what they are or have specialized
                requirements.
            </item>
          </taglist>
        </p>

        <p>
          Packages must not depend on packages with lower priority
          values (excluding build-time dependencies).  In order to
          ensure this, the priorities of one or more packages may need
          to be adjusted.
        </p>
      </sect>

    </chapt>


    <chapt id="binary">
      <heading>Binary packages</heading>

      <p>
        The Debian GNU/Linux distribution is based on the Debian
        package management system, called <prgn>dpkg</prgn>. Thus,
        all packages in the Debian distribution must be provided
        in the <tt>.deb</tt> file format.
      </p>

      <sect>
        <heading>The package name</heading>

        <p>
          Every package must have a name that's unique within the Debian
          archive.
        </p>

        <p>
          The package name is included in the control field
          <tt>Package</tt>, the format of which is described
          in <ref id="f-Package">.
          The package name is also included as a part of the file name
          of the <tt>.deb</tt> file.
        </p>
      </sect>

      <sect id="versions">
        <heading>The version of a package</heading>

        <p>
          Every package has a version number recorded in its
          <tt>Version</tt> control file field, described in
          <ref id="f-Version">.
        </p>

        <p>
          The package management system imposes an ordering on version
          numbers, so that it can tell whether packages are being up- or
          downgraded and so that package system front end applications
          can tell whether a package it finds available is newer than
          the one installed on the system.  The version number format
          has the most significant parts (as far as comparison is
          concerned) at the beginning.
        </p>

        <p>
          If an upstream package has problematic version numbers they
          should be converted to a sane form for use in the
          <tt>Version</tt> field.
        </p>

        <sect1>
          <heading>Version numbers based on dates</heading>

          <p>
            In general, Debian packages should use the same version
            numbers as the upstream sources.
          </p>

          <p>
            However, in some cases where the upstream version number is
            based on a date (e.g., a development "snapshot" release) the
            package management system cannot handle these version
            numbers without epochs. For example, dpkg will consider
            "96May01" to be greater than "96Dec24".
          </p>

          <p>
            To prevent having to use epochs for every new upstream
            version, the date based portion of the version number
            should be changed to the following format in such cases:
            "19960501", "19961224". It is up to the maintainer whether
            they want to bother the upstream maintainer to change
            the version numbers upstream, too.
          </p>

          <p>
            Note that other version formats based on dates which are
            parsed correctly by the package management system should
            <em>not</em> be changed.
          </p>

          <p>
            Native Debian packages (i.e., packages which have been
            written especially for Debian) whose version numbers include
            dates should always use the "YYYYMMDD" format.
          </p>
        </sect1>

      </sect>

      <sect>
        <heading>The maintainer of a package</heading>

        <p>
          Every package must have a Debian maintainer (the
          maintainer may be one person or a group of people
          reachable from a common email address, such as a mailing
          list).  The maintainer is responsible for ensuring that
          the package is placed in the appropriate distributions.
        </p>

        <p>
          The maintainer must be specified in the
          <tt>Maintainer</tt> control field with their correct name
          and a working email address.  If one person maintains
          several packages, they should try to avoid having
          different forms of their name and email address in
          the <tt>Maintainer</tt> fields of those packages.
        </p>

        <p>
          The format of the <tt>Maintainer</tt> control field is
          described in <ref id="f-Maintainer">.
        </p>

        <p>
          If the maintainer of a package quits from the Debian
          project, "Debian QA Group"
          <email>packages@qa.debian.org</email> takes over the
          maintainer-ship of the package until someone else
          volunteers for that task. These packages are called
          <em>orphaned packages</em>.<footnote>
                The detailed procedure for doing this gracefully can
                be found in the Debian Developer's Reference,
                see <ref id="related">.
          </footnote>
        </p>
      </sect>

      <sect id="descriptions">
        <heading>The description of a package</heading>

        <p>
          Every Debian package must have an extended description
          stored in the appropriate field of the control record.
          The technical information about the format of the
          <tt>Description</tt> field is in <ref id="f-Description">.
        </p>

        <p>
          The description should describe the package (the program) to a
          user (system administrator) who has never met it before so that
          they have enough information to decide whether they want to
          install it. This description should not just be copied verbatim
          from the program's documentation.
        </p>

        <p>
          Put important information first, both in the synopsis and
          extended description.  Sometimes only the first part of the
          synopsis or of the description will be displayed.  You can
          assume that there will usually be a way to see the whole
          extended description.
        </p>

        <p>
          The description should also give information about the
          significant dependencies and conflicts between this package
          and others, so that the user knows why these dependencies and
          conflicts have been declared.
        </p>

        <p>
          Instructions for configuring or using the package should
          not be included (that is what installation scripts,
          manual pages, info files, etc., are for).  Copyright
          statements and other administrivia should not be included
          either (that is what the copyright file is for).
        </p>

        <sect1 id="synopsis"><heading>The single line synopsis</heading>

          <p>
            The single line synopsis should be kept brief - certainly
            under 80 characters.
          </p>

          <p>
            Do not include the package name in the synopsis line.  The
            display software knows how to display this already, and you
            do not need to state it.  Remember that in many situations
            the user may only see the synopsis line - make it as
            informative as you can.
          </p>

        </sect1>

        <sect1 id="extendeddesc"><heading>The extended description</heading>

          <p>
            Do not try to continue the single line synopsis into the
            extended description.  This will not work correctly when
            the full description is displayed, and makes no sense
            where only the summary (the single line synopsis) is
            available.
          </p>

          <p>
            The extended description should describe what the package
            does and how it relates to the rest of the system (in terms
            of, for example, which subsystem it is which part of).
          </p>

          <p>
            The description field needs to make sense to anyone, even
            people who have no idea about any of the things the
            package deals with.<footnote>
                The blurb that comes with a program in its
                announcements and/or <prgn>README</prgn> files is
                rarely suitable for use in a description.  It is
                usually aimed at people who are already in the
                community where the package is used.
            </footnote>
          </p>

        </sect1>

      </sect>

      <sect>
        <heading>Dependencies</heading>

        <p>
          Every package must specify the dependency information
          about other packages that are required for the first to
          work correctly.
        </p>

        <p>
          For example, a dependency entry must be provided for any
          shared libraries required by a dynamically-linked executable
          binary in a package.
        </p>

        <p>
          Packages are not required to declare any dependencies they
          have on other packages which are marked <tt>Essential</tt>
          (see below), and should not do so unless they depend on a
          particular version of that package.<footnote>
            <p>
              Essential is defined as the minimal set of functionality
              that must be available and usable on the system even
              when packages are in an unconfigured (but unpacked)
              state.  This is needed to avoid unresolvable dependency
              loops on upgrade.  If packages add unnecessary
              dependencies on packages in this set, the chances that
              there <strong>will</strong> be an unresolvable
              dependency loop caused by forcing these Essential
              packages to be configured first before they need to be
              is greatly increased.  It also increases the chances
              that frontends will be unable to
              <strong>calculate</strong> an upgrade path, even if one
              exists.
            </p>
            <p>
              Also, it's pretty unlikely that functionality from
              Essential shall ever be removed (which is one reason why
              care must be taken before adding to the Essential
              packages set), but <em>packages</em> have been removed
              from the Essential set when the functionality moved to a
              different package. So depending on these packages
              <em>just in case</em> they stop being essential does way
              more harm than good.
            </p>
          </footnote>
        </p>

        <p>
          Sometimes, a package requires another package to be installed
          <em>and</em> configured before it can be installed. In this
          case, you must specify a <tt>Pre-Depends</tt> entry for
          the package.
        </p>

        <p>
          You should not specify a <tt>Pre-Depends</tt> entry for a
          package before this has been discussed on the
          <tt>debian-devel</tt> mailing list and a consensus about
          doing that has been reached.
        </p>

        <p>
          The format of the package interrelationship control fields is
          described in <ref id="relationships">.
        </p>
      </sect>

      <sect id="virtual_pkg">
        <heading>Virtual packages</heading>

        <p>
          Sometimes, there are several packages which offer
          more-or-less the same functionality. In this case, it's
          useful to define a <em>virtual package</em> whose name
          describes that common functionality.  (The virtual
          packages only exist logically, not physically; that's why
          they are called <em>virtual</em>.) The packages with this
          particular function will then <em>provide</em> the virtual
          package. Thus, any other package requiring that function
          can simply depend on the virtual package without having to
          specify all possible packages individually.
        </p>

        <p>
          All packages should use virtual package names where
          appropriate, and arrange to create new ones if necessary.
          They should not use virtual package names (except privately,
          amongst a cooperating group of packages) unless they have
          been agreed upon and appear in the list of virtual package
          names. (See also <ref id="virtual">)
        </p>

        <p>
          The latest version of the authoritative list of virtual
          package names can be found in the <tt>debian-policy</tt> package.
          It is also available from the Debian web mirrors at
          <tt><url name="/doc/packaging-manuals/virtual-package-names-list.txt"
                id="http://www.debian.org/doc/packaging-manuals/virtual-package-names-list.txt"></tt>.
        </p>

        <p>
          The procedure for updating the list is described in the preface
          to the list.
        </p>

      </sect>

      <sect>
        <heading>Base system</heading>

        <p>
          The <tt>base system</tt> is a minimum subset of the Debian
          GNU/Linux system that is installed before everything else
          on a new system. Thus, only very few packages are allowed
          to go into the <tt>base</tt> section to keep the required
          disk usage very small.
        </p>

        <p>
          Most of these packages will have the priority value
          <tt>required</tt> or at least <tt>important</tt>, and many
          of them will be tagged <tt>essential</tt> (see below).
        </p>
      </sect>

      <sect>
        <heading>Essential packages</heading>

        <p>
          Some packages are tagged <tt>essential</tt> for a system
          using the <tt>Essential</tt> control file field.
          The format of the <tt>Essential</tt> control field is
          described in <ref id="f-Essential">.
        </p>

        <p>
          Since these packages cannot be easily removed (one has to
          specify an extra <em>force option</em> to
          <prgn>dpkg</prgn> to do so), this flag must not be used
          unless absolutely necessary.  A shared library package
          must not be tagged <tt>essential</tt>; dependencies will
          prevent its premature removal, and we need to be able to
          remove it when it has been superseded.
        </p>

        <p>
          Since dpkg will not prevent upgrading of other packages
          while an <tt>essential</tt> package is in an unconfigured
            state, all <tt>essential</tt> packages must supply all of
            their core functionality even when unconfigured. If the
            package cannot satisfy this requirement it must not be
            tagged as essential, and any packages depending on this
            package must instead have explicit dependency fields as
            appropriate.
        </p>

        <p>
          You must not tag any packages <tt>essential</tt> before
          this has been discussed on the <tt>debian-devel</tt>
          mailing list and a consensus about doing that has been
          reached.
        </p>
      </sect>

      <sect id="maintscripts">
        <heading>Maintainer Scripts</heading>

        <p>
          The package installation scripts should avoid producing
          output which is unnecessary for the user to see and
          should rely on <prgn>dpkg</prgn> to stave off boredom on
          the part of a user installing many packages.  This means,
          amongst other things, using the <tt>--quiet</tt> option on
          <prgn>install-info</prgn>.
        </p>

        <p>
          Errors which occur during the execution of an installation
          script must be checked and the installation must not
          continue after an error.
        </p>

        <p>
          Note that in general <ref id="scripts"> applies to package
          maintainer scripts, too.
        </p>

        <p>
          You should not use <prgn>dpkg-divert</prgn> on a file
          belonging to another package without consulting the
          maintainer of that package first.
        </p>

        <p>
          All packages which supply an instance of a common command
          name (or, in general, filename) should generally use
          <prgn>update-alternatives</prgn>, so that they may be
          installed together.  If <prgn>update-alternatives</prgn>
          is not used, then each package must use
          <tt>Conflicts</tt> to ensure that other packages are
          de-installed.  (In this case, it may be appropriate to
          specify a conflict against earlier versions of something
          that previously did not use
          <prgn>update-alternatives</prgn>; this is an exception to
          the usual rule that versioned conflicts should be
          avoided.)
        </p>

        <sect1 id="maintscriptprompt">
          <heading>Prompting in maintainer scripts</heading>
          <p>
            Package maintainer scripts may prompt the user if
            necessary. Prompting should be done by communicating
            through a program, such as <prgn>debconf</prgn>, which
            conforms to the Debian Configuration management
            specification, version 2 or higher.  Prompting the user by
            other means, such as by hand<footnote>
                From the Jargon file: by hand 2. By extension,
                writing code which does something in an explicit or
                low-level way for which a presupplied library
                (<em>debconf, in this instance</em>) routine ought
                to have been available.
            </footnote>, is now deprecated.
          </p>

          <p>
            The Debian Configuration management specification is included
            in the <file>debconf_specification</file> files in the
            <package>debian-policy</package> package.
            It is also available from the Debian web mirrors at
            <tt><url name="/doc/packaging-manuals/debconf_specification.html"
                id="http://www.debian.org/doc/packaging-manuals/debconf_specification.html"></tt>.
          </p>

          <p>
            Packages which use the Debian Configuration management
            specification may contain an additional
            <prgn>config</prgn> script and a <tt>templates</tt>
            file in their control archive<footnote>
                The control.tar.gz inside the .deb.
                See <manref name="deb" section="5">.
            </footnote>.
            The <prgn>config</prgn> script might be run before the
            <prgn>preinst</prgn> script, and before the package is unpacked
            or any of its dependencies or pre-dependencies are satisfied.
            Therefore it must work using only the tools present in
            <em>essential</em> packages.<footnote>
                  <package>Debconf</package> or another tool that
                  implements the Debian Configuration management
                  specification will also be installed, and any
                  versioned dependencies on it will be satisfied
                  before preconfiguration begins.
            </footnote>
          </p>

          <p>
            Packages which use the Debian Configuration management
            specification must allow for translation of their messages
            by using a gettext-based system such as the one provided by
            the <package>po-debconf</package> package.
          </p>

          <p>
            Packages should try to minimize the amount of prompting
            they need to do, and they should ensure that the user
            will only ever be asked each question once.  This means
            that packages should try to use appropriate shared
            configuration files (such as <file>/etc/papersize</file> and
            <file>/etc/news/server</file>), and shared
            <package>debconf</package> variables rather than each
            prompting for their own list of required pieces of
            information.
          </p>

          <p>
            It also means that an upgrade should not ask the same
            questions again, unless the user has used
            <tt>dpkg --purge</tt> to remove the package's configuration.
            The answers to configuration questions should be stored in an
            appropriate place in <file>/etc</file> so that the user can
            modify them, and how this has been done should be
            documented.
          </p>

          <p>
            If a package has a vitally important piece of
            information to pass to the user (such as "don't run me
            as I am, you must edit the following configuration files
            first or you risk your system emitting badly-formatted
            messages"), it should display this in the
            <prgn>config</prgn> or <prgn>postinst</prgn> script and
            prompt the user to hit return to acknowledge the
            message.  Copyright messages do not count as vitally
            important (they belong in
            <file>/usr/share/doc/<var>package</var>/copyright</file>);
            neither do instructions on how to use a program (these
            should be in on-line documentation, where all the users
            can see them).
          </p>

          <p>
            Any necessary prompting should almost always be confined
            to the <prgn>config</prgn> or <prgn>postinst</prgn>
            script. If it is done in the <prgn>postinst</prgn>, it
            should be protected with a conditional so that
            unnecessary prompting doesn't happen if a package's
            installation fails and the <prgn>postinst</prgn> is
            called with <tt>abort-upgrade</tt>,
            <tt>abort-remove</tt> or <tt>abort-deconfigure</tt>.
          </p>
        </sect1>

      </sect>

    </chapt>


    <chapt id="source">
      <heading>Source packages</heading>

      <sect id="standardsversion">
        <heading>Standards conformance</heading>

        <p>
          Source packages should specify the most recent version number
          of this policy document with which your package complied
          when it was last updated.
        </p>

        <p>
          This information may be used to file bug reports
          automatically if your package becomes too much out of date.
        </p>

        <p>
          The version is specified in the <tt>Standards-Version</tt>
          control field.
          The format of the <tt>Standards-Version</tt> field is
          described in <ref id="f-Standards-Version">.
        </p>

        <p>
          You should regularly, and especially if your package has
          become out of date, check for the newest Policy Manual
          available and update your package, if necessary. When your
          package complies with the new standards you should update the
          <tt>Standards-Version</tt> source package field and
          release it.<footnote>
                See the file <file>upgrading-checklist</file> for
                information about policy which has changed between
                different versions of this document.
          </footnote>
        </p>

      </sect>

      <sect id="pkg-relations">
        <heading>Package relationships</heading>

        <p>
          Source packages should specify which binary packages they
          require to be installed or not to be installed in order to
          build correctly.  For example, if building a package
          requires a certain compiler, then the compiler should be
          specified as a build-time dependency.
        </p>

        <p>
          It is not necessary to explicitly specify build-time
          relationships on a minimal set of packages that are always
          needed to compile, link and put in a Debian package a
          standard "Hello World!" program written in C or C++.  The
          required packages are called <em>build-essential</em>, and
          an informational list can be found in
          <file>/usr/share/doc/build-essential/list</file> (which is
          contained in the <tt>build-essential</tt>
          package).<footnote>
            Rationale:
                <list compact="compact">
                  <item>
                      This allows maintaining the list separately
                      from the policy documents (the list does not
                      need the kind of control that the policy
                      documents do).
                  </item>
                  <item>
                      Having a separate package allows one to install
                      the build-essential packages on a machine, as
                      well as allowing other packages such as tasks to
                      require installation of the build-essential
                      packages using the depends relation.
                  </item>
                  <item>
                      The separate package allows bug reports against
                      the list to be categorized separately from
                      the policy management process in the BTS.
                  </item>
                </list>
          </footnote>
        </p>

        <p>
          When specifying the set of build-time dependencies, one
          should list only those packages explicitly required by the
          build.  It is not necessary to list packages which are
          required merely because some other package in the list of
          build-time dependencies depends on them.<footnote>
                The reason for this is that dependencies change, and
                you should list all those packages, and <em>only</em>
                those packages that <em>you</em> need directly.  What
                others need is their business.  For example, if you
                only link against <file>libimlib</file>, you will need to
                build-depend on <package>libimlib2-dev</package> but
                not against any <tt>libjpeg*</tt> packages, even
                though <tt>libimlib2-dev</tt> currently depends on
                them: installation of <package>libimlib2-dev</package>
                will automatically ensure that all of its run-time
                dependencies are satisfied.
          </footnote>
        </p>

        <p>
          If build-time dependencies are specified, it must be
          possible to build the package and produce working binaries
          on a system with only essential and build-essential
          packages installed and also those required to satisfy the
          build-time relationships (including any implied
          relationships).  In particular, this means that version
          clauses should be used rigorously in build-time
          relationships so that one cannot produce bad or
          inconsistently configured packages when the relationships
          are properly satisfied.
        </p>

        <p>
          <ref id="relationships"> explains the technical details.
        </p>
      </sect>

      <sect>
        <heading>Changes to the upstream sources</heading>

        <p>
          If changes to the source code are made that are not
          specific to the needs of the Debian system, they should be
          sent to the upstream authors in whatever form they prefer
          so as to be included in the upstream version of the
          package.
        </p>

        <p>
          If you need to configure the package differently for
          Debian or for Linux, and the upstream source doesn't
          provide a way to do so, you should add such configuration
          facilities (for example, a new <prgn>autoconf</prgn> test
          or <tt>#define</tt>) and send the patch to the upstream
          authors, with the default set to the way they originally
          had it.  You can then easily override the default in your
          <file>debian/rules</file> or wherever is appropriate.
        </p>

        <p>
          You should make sure that the <prgn>configure</prgn> utility
          detects the correct architecture specification string
          (refer to <ref id="arch-spec"> for details).
        </p>

        <p>
          If you need to edit a <prgn>Makefile</prgn> where GNU-style
          <prgn>configure</prgn> scripts are used, you should edit the
          <file>.in</file> files rather than editing the
          <prgn>Makefile</prgn> directly.  This allows the user to
          reconfigure the package if necessary.  You should
          <em>not</em> configure the package and edit the generated
          <prgn>Makefile</prgn>!  This makes it impossible for someone
          else to later reconfigure the package without losing the
          changes you made.
        </p>

      </sect>

      <sect id="dpkgchangelog">
        <heading>Debian changelog: <file>debian/changelog</file></heading>

        <p>
          Changes in the Debian version of the package should be
          briefly explained in the Debian changelog file
          <file>debian/changelog</file>.<footnote>
            <p>
              Mistakes in changelogs are usually best rectified by
              making a new changelog entry rather than "rewriting
              history" by editing old changelog entries.
            </p>
          </footnote>
          This includes modifications
          made in the Debian package compared to the upstream one
          as well as other changes and updates to the package.
          <footnote>
              Although there is nothing stopping an author who is also
              the Debian maintainer from using this changelog for all
              their changes, it will have to be renamed if the Debian
              and upstream maintainers become different people. In such
              a case, however, it might be better to maintain the package
              as a non-native package.
          </footnote>
        </p>

        <p>
          
        </p>

        <p>
          The format of the <file>debian/changelog</file> allows the
          package building tools to discover which version of the package
          is being built and find out other release-specific information.
        </p>

        <p>
          That format is a series of entries like this:

<example compact="compact">
<var>package</var> (<var>version</var>) <var>distribution(s)</var>; urgency=<var>urgency</var>
            <var>
              [optional blank line(s), stripped]
            </var>
  * <var>change details</var>
    <var>more change details</var>
            <var>
              [blank line(s), included in output of dpkg-parsechangelog]
            </var>
  * <var>even more change details</var>
            <var>
              [optional blank line(s), stripped]
            </var>
 -- <var>maintainer name</var> &lt;<var>email address</var>&gt;<var>[two spaces]</var>  <var>date</var>
</example>
        </p>

        <p>
          <var>package</var> and <var>version</var> are the source
          package name and version number.
        </p>

        <p>
          <var>distribution(s)</var> lists the distributions where
          this version should be installed when it is uploaded - it
          is copied to the <tt>Distribution</tt> field in the
          <file>.changes</file> file.  See <ref id="f-Distribution">.
        </p>

        <p>
          <var>urgency</var> is the value for the <tt>Urgency</tt>
          field in the <file>.changes</file> file for the upload
          (see <ref id="f-Urgency">). It is not possible to specify
          an urgency containing commas; commas are used to separate
          <tt><var>keyword</var>=<var>value</var></tt> settings in the
          <prgn>dpkg</prgn> changelog format (though there is
          currently only one useful <var>keyword</var>,
          <tt>urgency</tt>).
        </p>

        <p>
          The change details may in fact be any series of lines
          starting with at least two spaces, but conventionally each
          change starts with an asterisk and a separating space and
          continuation lines are indented so as to bring them in
          line with the start of the text above.  Blank lines may be
          used here to separate groups of changes, if desired.
        </p>

        <p>
          If this upload resolves bugs recorded in the Bug Tracking
          System (BTS), they may be automatically closed on the
          inclusion of this package into the Debian archive by
          including the string: <tt>closes: Bug#<var>nnnnn</var></tt>
          in the change details.<footnote>
              To be precise, the string should match the following
              Perl regular expression:
              <example>
/closes:\s*(?:bug)?\#?\s?\d+(?:,\s*(?:bug)?\#?\s?\d+)*/i
              </example>
              Then all of the bug numbers listed will be closed by the
              archive maintenance script (<prgn>katie</prgn>), or in
              the case of an NMU, marked as fixed.
          </footnote>
          This information is conveyed via the <tt>Closes</tt> field
          in the <tt>.changes</tt> file (see <ref id="f-Closes">).
        </p>

        <p>
          The maintainer name and email address used in the changelog
          should be the details of the person uploading <em>this</em>
          version.  They are <em>not</em> necessarily those of the
          usual package maintainer.  The information here will be
          copied to the <tt>Changed-By</tt> field in the
          <tt>.changes</tt> file (see <ref id="f-Changed-By">),
          and then later used to send an acknowledgement when the
          upload has been installed.
        </p>

        <p>
          The <var>date</var> should be in RFC822 format<footnote>
              This is generated by <tt>date -R</tt>.
          </footnote>; it should include the time zone specified
          numerically, with the time zone name or abbreviation
          optionally present as a comment in parentheses.
        </p>

        <p>
          The first "title" line with the package name should start
          at the left hand margin; the "trailer" line with the
          maintainer and date details should be preceded by exactly
          one space.  The maintainer details and the date must be
          separated by exactly two spaces.
        </p>

        <p>
          For more information on placement of the changelog files
          within binary packages, please see <ref id="changelogs">.
        </p>

        <sect1><heading>Alternative changelog formats</heading>

          <p>
            In non-experimental packages you must use a format for
            <file>debian/changelog</file> which is supported by the most
            recent released version of <prgn>dpkg</prgn>.
          </p>

          <p>
            It is possible to use a format different from the standard
            one by providing a changelog parser for the format you wish
            to use. The parser must have an API compatible with that
            expected by <prgn>dpkg-genchanges</prgn> and
            <prgn>dpkg-gencontrol</prgn>, and it must not interact with
            the user at all.
            <footnote>
              If there is general interest in the new format, you should
              contact the <package>dpkg</package> maintainer to have the
              parser script for it included in the <prgn>dpkg</prgn>
              package.  (You will need to agree that the parser and its
              man page may be distributed under the GNU GPL, just as the rest
              of <prgn>dpkg</prgn> is.)
            </footnote>
          </p>
        </sect1>
      </sect>
      <sect id="dpkgcopyright">
        <heading>Copyright: <file>debian/copyright</file></heading>
        <p>
          Every package must be accompanied by a verbatim copy of
          its copyright and distribution license in the file
          <file>/usr/share/doc/<var>package</var>/copyright</file>
          (see <ref id="copyrightfile"> for further details). Also see
          <ref id="pkgcopyright"> for further considerations relayed
          to copyrights for packages.
        </p>
      </sect>
      <sect>
        <heading>Error trapping in makefiles</heading>

        <p>
          When <prgn>make</prgn> invokes a command in a makefile
          (including your package's upstream makefiles and
          <file>debian/rules</file>), it does so using <prgn>sh</prgn>.  This
          means that <prgn>sh</prgn>'s usual bad error handling
          properties apply: if you include a miniature script as one
          of the commands in your makefile you'll find that if you
          don't do anything about it then errors are not detected
          and <prgn>make</prgn> will blithely continue after
          problems.
        </p>

        <p>
          Every time you put more than one shell command (this
          includes using a loop) in a makefile command you
          must make sure that errors are trapped.  For
          simple compound commands, such as changing directory and
          then running a program, using <tt>&amp;&amp;</tt> rather
          than semicolon as a command separator is sufficient.  For
          more complex commands including most loops and
          conditionals you should include a separate <tt>set -e</tt>
          command at the start of every makefile command that's
          actually one of these miniature shell scripts.
        </p>
      </sect>

      <sect id="timestamps">
        <heading>Time Stamps</heading>
        <p>
          Maintainers should preserve the modification times of the
          upstream source files in a package, as far as is reasonably
          possible.<footnote>
              The rationale is that there is some information conveyed
              by knowing the age of the file, for example, you could
              recognize that some documentation is very old by looking
              at the modification time, so it would be nice if the
              modification time of the upstream source would be
              preserved.
          </footnote>
        </p>
      </sect>

      <sect id="restrictions">
        <heading>Restrictions on objects in source packages</heading>

        <p>
          The source package may not contain any hard links<footnote>
            <p>
              This is not currently detected when building source
              packages, but only when extracting
              them.
            </p>
            <p>
              Hard links may be permitted at some point in the
              future, but would require a fair amount of
              work.
            </p>
          </footnote>, device special files, sockets or setuid or
          setgid files.<footnote>
              Setgid directories are allowed.
          </footnote>
        </p>
      </sect>

      <sect id="debianrules">
        <heading>Main building script: <file>debian/rules</file></heading>

        <p>
          This file must be an executable makefile, and contains the
          package-specific recipes for compiling the package and
          building binary package(s) from the source.
        </p>

        <p>
          It must start with the line <tt>#!/usr/bin/make -f</tt>,
          so that it can be invoked by saying its name rather than
          invoking <prgn>make</prgn> explicitly.
        </p>

        <p>
          Since an interactive <file>debian/rules</file> script makes it
          impossible to auto-compile that package and also makes it
          hard for other people to reproduce the same binary
          package, all <em>required targets</em> MUST be
          non-interactive. At a minimum, required targets are the
          ones called by <prgn>dpkg-buildpackage</prgn>, namely,
          <em>clean</em>, <em>binary</em>, <em>binary-arch</em>,
          <em>binary-indep</em>, and <em>build</em>. It also follows
          that any target that these targets depend on must also be
          non-interactive.
        </p>

        <p>
          The targets are as follows (required unless stated otherwise):
          <taglist>
            <tag><tt>build</tt></tag>
            <item>
              <p>
                The <tt>build</tt> target should perform all the
                configuration and compilation of the package.
                If a package has an interactive pre-build
                configuration routine, the Debianized source package
                must either be built after this has taken place (so
                that the binary package can be built without rerunning
                the configuration) or the configuration routine
                modified to become non-interactive.  (The latter is
                preferable if there are architecture-specific features
                detected by the configuration routine.)
              </p>

              <p>
                For some packages, notably ones where the same
                source tree is compiled in different ways to produce
                two binary packages, the <tt>build</tt> target
                does not make much sense.  For these packages it is
                good enough to provide two (or more) targets
                (<tt>build-a</tt> and <tt>build-b</tt> or whatever)
                for each of the ways of building the package, and a
                <tt>build</tt> target that does nothing.  The
                <tt>binary</tt> target will have to build the
                package in each of the possible ways and make the
                binary package out of each.
              </p>

              <p>
                The <tt>build</tt> target must not do anything
                that might require root privilege.
              </p>

              <p>
                The <tt>build</tt> target may need to run the
                <tt>clean</tt> target first - see below.
              </p>

              <p>
                When a package has a configuration and build routine
                which takes a long time, or when the makefiles are
                poorly designed, or when <tt>build</tt> needs to
                run <tt>clean</tt> first, it is a good idea to
                <tt>touch build</tt> when the build process is
                complete.  This will ensure that if <tt>debian/rules
                build</tt> is run again it will not rebuild the whole
                program.<footnote>
                    Another common way to do this is for <tt>build</tt>
                    to depend on <prgn>build-stamp</prgn> and to do
                    nothing else, and for the <prgn>build-stamp</prgn>
                    target to do the building and to <tt>touch
                    build-stamp</tt> on completion.  This is
                    especially useful if the build routine creates a
                    file or directory called <tt>build</tt>; in such a
                    case, <tt>build</tt> will need to be listed as
                    a phony target (i.e., as a dependency of the
                    <tt>.PHONY</tt> target).  See the documentation of
                    <prgn>make</prgn> for more information on phony
                    targets.
                </footnote>
              </p>
            </item>

            <tag><tt>build-arch</tt> (optional),
                 <tt>build-indep</tt> (optional)
            </tag>
            <item>
              <p>
                A package may also provide both of the targets
                <tt>build-arch</tt> and <tt>build-indep</tt>.
                The <tt>build-arch</tt> target, if provided, should
                perform all the configuration and compilation required
                for producing all architecture-dependant binary packages
                (those packages for which the body of the
                <tt>Architecture</tt> field in <tt>debian/control</tt>
                is not <tt>all</tt>).
                Similarly, the <tt>build-indep</tt> target, if
                provided, should perform all the configuration and
                compilation required for producing all
                architecture-independent binary packages
                (those packages for which the body of the
                <tt>Architecture</tt> field in <tt>debian/control</tt>
                is <tt>all</tt>).
                The <tt>build</tt> target should depend on those of the
                targets <tt>build-arch</tt> and <tt>build-indep</tt> that
                are provided in the rules file.
              </p>

              <p>
                If one or both of the targets <tt>build-arch</tt> and
                <tt>build-indep</tt> are not provided, then invoking
                <file>debian/rules</file> with one of the not-provided
                targets as arguments should produce a exit status code
                of 2.  Usually this is provided automatically by make
                if the target is missing.
              </p>

              <p>
                The <tt>build-arch</tt> and <tt>build-indep</tt> targets
                must not do anything that might require root privilege.
              </p>
            </item>

            <tag><tt>binary</tt>, <tt>binary-arch</tt>,
              <tt>binary-indep</tt>
            </tag>
            <item>
              <p>
                The <tt>binary</tt> target must be all that is
                necessary for the user to build the binary package(s)
                produced from this source package.  It is
                split into two parts: <prgn>binary-arch</prgn> builds
                the binary packages which are specific to a particular
                architecture, and <tt>binary-indep</tt> builds
                those which are not.
              </p>
              <p>
                <tt>binary</tt> may be (and commonly is) a target with
                no commands which simply depends on
                <tt>binary-arch</tt> and <tt>binary-indep</tt>.
              </p>
              <p>
                Both <tt>binary-*</tt> targets should depend on the
                <tt>build</tt> target, or on the appropriate
                <tt>build-arch</tt> or <tt>build-indep</tt> target, if
                provided, so that the package is built if it has not
                been already.  It should then create the relevant
                binary package(s), using <prgn>dpkg-gencontrol</prgn> to
                make their control files and <prgn>dpkg-deb</prgn> to
                build them and place them in the parent of the top
                level directory.
              </p>

              <p>
                Both the <tt>binary-arch</tt> and
                <tt>binary-indep</tt> targets <em>must</em> exist.
                If one of them has nothing to do (which will always be
                the case if the source generates only a single binary
                package, whether architecture-dependent or not), it
                must still exist and must always succeed.
              </p>

              <p>
                The <tt>binary</tt> targets must be invoked as
                root.<footnote>
                    The <prgn>fakeroot</prgn> package often allows one
                    to build a package correctly even without being
                    root.
                </footnote>
              </p>
            </item>

            <tag><tt>clean</tt></tag>
            <item>
              <p>
                This must undo any effects that the <tt>build</tt>
                and <tt>binary</tt> targets may have had, except
                that it should leave alone any output files created in
                the parent directory by a run of a <tt>binary</tt>
                target.
              </p>

              <p>
                If a <tt>build</tt> file is touched at the end of
                the <tt>build</tt> target, as suggested above, it
                should be removed as the first action that
                <tt>clean</tt> performs, so that running
                <tt>build</tt> again after an interrupted
                <tt>clean</tt> doesn't think that everything is
                already done.
              </p>

              <p>
                The <tt>clean</tt> target may need to be
                invoked as root if <tt>binary</tt> has been
                invoked since the last <tt>clean</tt>, or if
                <tt>build</tt> has been invoked as root (since
                <tt>build</tt> may create directories, for
                example).
              </p>
            </item>

            <tag><tt>get-orig-source</tt> (optional)</tag>
            <item>
              <p>
                This target fetches the most recent version of the
                original source package from a canonical archive site
                (via FTP or WWW, for example), does any necessary
                rearrangement to turn it into the original source
                tar file format described below, and leaves it in the
                current directory.
              </p>

              <p>
                This target may be invoked in any directory, and
                should take care to clean up any temporary files it
                may have left.
              </p>

              <p>
                This target is optional, but providing it if
                possible is a good idea.
              </p>
            </item>
          </taglist>

        <p>
          The <tt>build</tt>, <tt>binary</tt> and
          <tt>clean</tt> targets must be invoked with the current
          directory being the package's top-level directory.
        </p>


        <p>
          Additional targets may exist in <file>debian/rules</file>,
          either as published or undocumented interfaces or for the
          package's internal use.
        </p>

        <p>
          The architectures we build on and build for are determined
          by <prgn>make</prgn> variables using the utility
          <qref id="pkg-dpkg-architecture"><prgn>dpkg-architecture</prgn></qref>.
          You can determine the
          Debian architecture and the GNU style architecture
          specification string for the build machine (the machine type
          we are building on) as well as for the host machine (the
          machine type we are building for).  Here is a list of
          supported <prgn>make</prgn> variables:
          <list compact="compact">
            <item>
                <tt>DEB_*_ARCH</tt> (the Debian architecture)
            </item>
            <item>
                <tt>DEB_*_GNU_TYPE</tt> (the GNU style architecture
                specification string)
            </item>
            <item>
                <tt>DEB_*_GNU_CPU</tt> (the CPU part of
                <tt>DEB_*_GNU_TYPE</tt>)
            </item>
            <item>
                <tt>DEB_*_GNU_SYSTEM</tt> (the System part of
                <tt>DEB_*_GNU_TYPE</tt>)
          </list>
          where <tt>*</tt> is either <tt>BUILD</tt> for specification of
          the build machine or <tt>HOST</tt> for specification of the
          host machine.
        </p>

        <p>
          Backward compatibility can be provided in the rules file
          by setting the needed variables to suitable default
          values; please refer to the documentation of
          <prgn>dpkg-architecture</prgn> for details.
        </p>

        <p>
          It is important to understand that the <tt>DEB_*_ARCH</tt>
          string only determines which Debian architecture we are
          building on or for. It should not be used to get the CPU
          or system information; the GNU style variables should be
          used for that.
        </p>
      </sect>

<!-- FIXME: section pkg-srcsubstvars is the same as substvars -->
      <sect id="substvars">
        <heading>Variable substitutions: <file>debian/substvars</file></heading>

        <p>
          When <prgn>dpkg-gencontrol</prgn>,
          <prgn>dpkg-genchanges</prgn> and <prgn>dpkg-source</prgn>
          generate control files they perform variable substitutions
          on their output just before writing it.  Variable
          substitutions have the form <tt>${<var>variable</var>}</tt>.
          The optional file <file>debian/substvars</file> contains
          variable substitutions to be used; variables can also be set
          directly from <file>debian/rules</file> using the <tt>-V</tt>
          option to the source packaging commands, and certain
          predefined variables are also available.
        </p>

        <p>
          The <file>debian/substvars</file> file is usually generated and
          modified dynamically by <file>debian/rules</file> targets, in
          which case it must be removed by the <tt>clean</tt> target.
        </p>

        <p>
          See <manref name="dpkg-source" section="1"> for full
          details about source variable substitutions, including the
          format of <file>debian/substvars</file>.</p>
      </sect>

      <sect id="debianwatch">
        <heading>Optional upstream source location: <file>debian/watch</file></heading>

        <p>
          This is an optional, recommended control file for the
          <tt>uscan</tt> utility which defines how to automatically
          scan ftp or http sites for newly available updates of the
          package. This is used by <url id="
          http://dehs.alioth.debian.org/"> and other Debian QA tools
          to help with quality control and maintenance of the
          distribution as a whole.
        </p>

      </sect>

      <sect id="debianfiles">
        <heading>Generated files list: <file>debian/files</file></heading>

        <p>
          This file is not a permanent part of the source tree; it
          is used while building packages to record which files are
          being generated.  <prgn>dpkg-genchanges</prgn> uses it
          when it generates a <file>.changes</file> file.
        </p>

        <p>
          It should not exist in a shipped source package, and so it
          (and any backup files or temporary files such as
          <file>files.new</file><footnote>
              <file>files.new</file> is used as a temporary file by
              <prgn>dpkg-gencontrol</prgn> and
              <prgn>dpkg-distaddfile</prgn> - they write a new
              version of <tt>files</tt> here before renaming it,
              to avoid leaving a corrupted copy if an error
              occurs.
          </footnote>) should be removed by the
          <tt>clean</tt> target.  It may also be wise to
          ensure a fresh start by emptying or removing it at the
          start of the <tt>binary</tt> target.
        </p>

        <p>
          When <prgn>dpkg-gencontrol</prgn> is run for a binary
          package, it adds an entry to <file>debian/files</file> for the
          <file>.deb</file> file that will be created when <tt>dpkg-deb
          --build</tt> is run for that binary package.  So for most
          packages all that needs to be done with this file is to
          delete it in the <tt>clean</tt> target.
        </p>

        <p>
          If a package upload includes files besides the source
          package and any binary packages whose control files were
          made with <prgn>dpkg-gencontrol</prgn> then they should be
          placed in the parent of the package's top-level directory
          and <prgn>dpkg-distaddfile</prgn> should be called to add
          the file to the list in <file>debian/files</file>.</p>
      </sect>

      <sect id="embeddedfiles">
        <heading>Convenience copies of code</heading>

        <p>
          Some software packages include in their distribution convenience
          copies of code from other software packages, generally so that
          users compiling from source don't have to download multiple
          packages.  Debian packages should not make use of these
          convenience copies unless the included package is explicitly
          intended to be used in this way.<footnote>
            For example, parts of the GNU build system work like this.
          </footnote>
          If the included code is already in the Debian archive in the
          form of a library, the Debian packaging should ensure that
          binary packages reference the libraries already in Debian and
          the convenience copy is not used.  If the included code is not
          already in Debian, it should be packaged separately as a
          prerequisite if possible.
          <footnote>
            Having multiple copies of the same code in Debian is
            inefficient, often creates either static linking or shared
            library conflicts, and, most importantly, increases the
            difficulty of handling security vulnerabilities in the
            duplicated code.
          </footnote>
        </p>
      </sect>

    </chapt>


    <chapt id="controlfields">
      <heading>Control files and their fields</heading>

      <p>
        The package management system manipulates data represented in
        a common format, known as <em>control data</em>, stored in
        <em>control files</em>.
        Control files are used for source packages, binary packages and
        the <file>.changes</file> files which control the installation
        of uploaded files<footnote>
            <prgn>dpkg</prgn>'s internal databases are in a similar
            format.
        </footnote>.
      </p>

      <sect id="controlsyntax">
        <heading>Syntax of control files</heading>

        <p>
          A control file consists of one or more paragraphs of
          fields<footnote>
                The paragraphs are also sometimes referred to as stanzas.
          </footnote>.
          The paragraphs are separated by blank lines.  Some control
          files allow only one paragraph; others allow several, in
          which case each paragraph usually refers to a different
          package.  (For example, in source packages, the first
          paragraph refers to the source package, and later paragraphs
          refer to binary packages generated from the source.)
        </p>

        <p>
          Each paragraph consists of a series of data fields; each
          field consists of the field name, followed by a colon and
          then the data/value associated with that field.  It ends at
          the end of the (logical) line.  Horizontal whitespace
          (spaces and tabs) may occur immediately before or after the
          value and is ignored there; it is conventional to put a
          single space after the colon.  For example, a field might
          be:
          <example compact="compact">
Package: libc6
          </example>
          the field name is <tt>Package</tt> and the field value
          <tt>libc6</tt>.
        </p>

        <p>
          Many fields' values may span several lines; in this case
          each continuation line must start with a space or a tab.
          Any trailing spaces or tabs at the end of individual
          lines of a field value are ignored. 
        </p>

        <p>
          In fields where it is specified that lines may not wrap,
          only a single line of data is allowed and whitespace is not
          significant in a field body. Whitespace must not appear
          inside names (of packages, architectures, files or anything
          else) or version numbers, or between the characters of
          multi-character version relationships.
        </p>

        <p>
          Field names are not case-sensitive, but it is usual to
          capitalize the field names using mixed case as shown below.
        </p>

        <p>
          Blank lines, or lines consisting only of spaces and tabs,
          are not allowed within field values or between fields - that
          would mean a new paragraph.
        </p>

      </sect>

      <sect id="sourcecontrolfiles">
        <heading>Source package control files -- <file>debian/control</file></heading>

        <p>
          The <file>debian/control</file> file contains the most vital
          (and version-independent) information about the source package
          and about the binary packages it creates.
        </p>

        <p>
          The first paragraph of the control file contains information about
          the source package in general. The subsequent sets each describe a
          binary package that the source tree builds.
        </p>

        <p>
          The fields in the general paragraph (the first one, for the source
          package) are:

          <list compact="compact">
            <item><qref id="f-Source"><tt>Source</tt></qref> (mandatory)</item>
            <item><qref id="f-Maintainer"><tt>Maintainer</tt></qref> (mandatory)</item>
            <item><qref id="f-Uploaders"><tt>Uploaders</tt></qref></item>
            <item><qref id="f-Section"><tt>Section</tt></qref> (recommended)</item>
            <item><qref id="f-Priority"><tt>Priority</tt></qref> (recommended)</item>
            <item><qref id="sourcebinarydeps"><tt>Build-Depends</tt> et al</qref></item>
            <item><qref id="f-Standards-Version"><tt>Standards-Version</tt></qref> (recommended)</item>
          </list>
        </p>

        <p>
          The fields in the binary package paragraphs are:

          <list compact="compact">
            <item><qref id="f-Package"><tt>Package</tt></qref> (mandatory)</item>
            <item><qref id="f-Architecture"><tt>Architecture</tt></qref> (mandatory)</item>
            <item><qref id="f-Section"><tt>Section</tt></qref> (recommended)</item>
            <item><qref id="f-Priority"><tt>Priority</tt></qref> (recommended)</item>
            <item><qref id="f-Essential"><tt>Essential</tt></qref></item>
            <item><qref id="binarydeps"><tt>Depends</tt> et al</qref></item>
            <item><qref id="f-Description"><tt>Description</tt></qref> (mandatory)</item>
          </list>
        </p>

        <p>
          The syntax and semantics of the fields are described below.
        </p>

<!-- stuff -->

        <p>
          These fields are used by <prgn>dpkg-gencontrol</prgn> to
          generate control files for binary packages (see below), by
          <prgn>dpkg-genchanges</prgn> to generate the
          <tt>.changes</tt> file to accompany the upload, and by
          <prgn>dpkg-source</prgn> when it creates the
          <file>.dsc</file> source control file as part of a source
          archive. Many fields are permitted to span multiple lines in
          <file>debian/control</file> but not in any other control
          file. These tools are responsible for removing the line
          breaks from such fields when using fields from
          <file>debian/control</file> to generate other control files.
        </p>

        <p>
          The fields here may contain variable references - their
          values will be substituted by <prgn>dpkg-gencontrol</prgn>,
          <prgn>dpkg-genchanges</prgn> or <prgn>dpkg-source</prgn>
          when they generate output control files.
          See <ref id="substvars"> for details.
        </p>

      </sect>

      <sect id="binarycontrolfiles">
        <heading>Binary package control files -- <file>DEBIAN/control</file></heading>

        <p>
          The <file>DEBIAN/control</file> file contains the most vital
          (and version-dependent) information about a binary package.
        </p>

        <p>
          The fields in this file are:

          <list compact="compact">
            <item><qref id="f-Package"><tt>Package</tt></qref> (mandatory)</item>
            <item><qref id="f-Source"><tt>Source</tt></qref></item>
            <item><qref id="f-Version"><tt>Version</tt></qref> (mandatory)</item>
            <item><qref id="f-Section"><tt>Section</tt></qref> (recommended)</item>
            <item><qref id="f-Priority"><tt>Priority</tt></qref> (recommended)</item>
            <item><qref id="f-Architecture"><tt>Architecture</tt></qref> (mandatory)</item>
            <item><qref id="f-Essential"><tt>Essential</tt></qref></item>
            <item><qref id="binarydeps"><tt>Depends</tt> et al</qref></item>
            <item><qref id="f-Installed-Size"><tt>Installed-Size</tt></qref></item>
            <item><qref id="f-Maintainer"><tt>Maintainer</tt></qref> (mandatory)</item>
            <item><qref id="f-Description"><tt>Description</tt></qref> (mandatory)</item>
          </list>
        </p>
      </sect>

      <sect id="debiansourcecontrolfiles">
        <heading>Debian source control files -- <tt>.dsc</tt></heading>

        <p>
          This file contains a series of fields, identified and
          separated just like the fields in the control file of
          a binary package.  The fields are listed below; their
          syntax is described above, in <ref id="pkg-controlfields">.

        <list compact="compact">
          <item><qref id="f-Format"><tt>Format</tt></qref> (mandatory)</item>
          <item><qref id="f-Source"><tt>Source</tt></qref> (mandatory)</item>
          <item><qref id="f-Version"><tt>Version</tt></qref> (mandatory)</item>
          <item><qref id="f-Maintainer"><tt>Maintainer</tt></qref> (mandatory)</item>
          <item><qref id="f-Uploaders"><tt>Uploaders</tt></qref></item>
          <item><qref id="f-Binary"><tt>Binary</tt></qref></item>
          <item><qref id="f-Architecture"><tt>Architecture</tt></qref></item>
          <item><qref id="sourcebinarydeps"><tt>Build-Depends</tt> et al</qref></item>
          <item><qref id="f-Standards-Version"><tt>Standards-Version</tt></qref> (recommended)</item>
          <item><qref id="f-Files"><tt>Files</tt></qref> (mandatory)</item>
        </list>
        </p>

        <p>
          The source package control file is generated by
          <prgn>dpkg-source</prgn> when it builds the source
          archive, from other files in the source package,
          described above.  When unpacking, it is checked against
          the files and directories in the other parts of the
          source package.
        </p>

      </sect>

      <sect id="debianchangesfiles">
        <heading>Debian changes files -- <file>.changes</file></heading>

        <p>
          The .changes files are used by the Debian archive maintenance
          software to process updates to packages. They contain one
          paragraph which contains information from the
          <tt>debian/control</tt> file and other data about the
          source package gathered via <tt>debian/changelog</tt>
          and <tt>debian/rules</tt>.
        </p>

        <p>
          The fields in this file are:

          <list compact="compact">
            <item><qref id="f-Format"><tt>Format</tt></qref> (mandatory)</item>
            <item><qref id="f-Date"><tt>Date</tt></qref> (mandatory)</item>
            <item><qref id="f-Source"><tt>Source</tt></qref> (mandatory)</item>
            <item><qref id="f-Binary"><tt>Binary</tt></qref> (mandatory)</item>
            <item><qref id="f-Architecture"><tt>Architecture</tt></qref> (mandatory)</item>
            <item><qref id="f-Version"><tt>Version</tt></qref> (mandatory)</item>
            <item><qref id="f-Distribution"><tt>Distribution</tt></qref> (mandatory)</item>
            <item><qref id="f-Urgency"><tt>Urgency</tt></qref> (recommended)</item>
            <item><qref id="f-Maintainer"><tt>Maintainer</tt></qref> (mandatory)</item>
            <item><qref id="f-Changed-By"><tt>Changed-By</tt></qref></item>
            <item><qref id="f-Description"><tt>Description</tt></qref> (mandatory)</item>
            <item><qref id="f-Closes"><tt>Closes</tt></qref></item>
            <item><qref id="f-Changes"><tt>Changes</tt></qref> (mandatory)</item>
            <item><qref id="f-Files"><tt>Files</tt></qref> (mandatory)</item>
          </list>
        </p>
      </sect>

      <sect id="controlfieldslist">
        <heading>List of fields</heading>

        <sect1 id="f-Source">
          <heading><tt>Source</tt></heading>

          <p>
            This field identifies the source package name.
          </p>

          <p>
            In <file>debian/control</file> or a <file>.dsc</file> file,
            this field must contain only the name of the source package.
          </p>

          <p>
            In a binary package control file or a <file>.changes</file>
            file, the source package name may be followed by a version
            number in parentheses<footnote>
                It is customary to leave a space after the package name
                if a version number is specified.
            </footnote>.
            This version number may be omitted (and is, by
            <prgn>dpkg-gencontrol</prgn>) if it has the same value as
            the <tt>Version</tt> field of the binary package in
            question.  The field itself may be omitted from a binary
            package control file when the source package has the same
            name and version as the binary package.
          </p>
        </sect1>

        <sect1 id="f-Maintainer">
          <heading><tt>Maintainer</tt></heading>

          <p>
            The package maintainer's name and email address.  The name
            should come first, then the email address inside angle
            brackets <tt>&lt;&gt</tt> (in RFC822 format).
          </p>

          <p>
            If the maintainer's name contains a full stop then the
            whole field will not work directly as an email address due
            to a misfeature in the syntax specified in RFC822; a
            program using this field as an address must check for this
            and correct the problem if necessary (for example by
            putting the name in round brackets and moving it to the
            end, and bringing the email address forward).
          </p>
        </sect1>

        <sect1 id="f-Uploaders">
          <heading><tt>Uploaders</tt></heading>

          <p>
            List of the names and email addresses of co-maintainers of
            the package, if any. If the package has other maintainers
            beside the one named in the 
            <qref id="f-Maintainer">Maintainer field</qref>, their
            names and email addresses should be listed here. The
            format is the same as that of the Maintainer tag, and
            multiple entries should be comma separated. Currently,
            this field is restricted to a single line of data.  This
            is an optional field.
          </p>
          <p>
            Any parser that interprets the Uploaders field in
            <file>debian/control</file> must permit it to span multiple
            lines.  Line breaks in an Uploaders field that spans multiple
            lines are not significant and the semantics of the field are
            the same as if the line breaks had not been present.
          </p>
        </sect1>

        <sect1 id="f-Changed-By">
          <heading><tt>Changed-By</tt></heading>

          <p>
            The name and email address of the person who changed the
            said package. Usually the name of the maintainer.
            All the rules for the Maintainer field apply here, too.
          </p>
        </sect1>

        <sect1 id="f-Section">
          <heading><tt>Section</tt></heading>

          <p>
            This field specifies an application area into which the package
            has been classified. See <ref id="subsections">.
          </p>

          <p>
            When it appears in the <file>debian/control</file> file,
            it gives the value for the subfield of the same name in
            the <tt>Files</tt> field of the <file>.changes</file> file.
            It also gives the default for the same field in the binary
            packages.
          </p>
        </sect1>

        <sect1 id="f-Priority">
          <heading><tt>Priority</tt></heading>

          <p>
            This field represents how important that it is that the user
            have the package installed. See <ref id="priorities">.
          </p>

          <p>
            When it appears in the <file>debian/control</file> file,
            it gives the value for the subfield of the same name in
            the <tt>Files</tt> field of the <file>.changes</file> file.
            It also gives the default for the same field in the binary
            packages.
          </p>
        </sect1>

        <sect1 id="f-Package">
          <heading><tt>Package</tt></heading>

          <p>
            The name of the binary package.
          </p>

          <p>
            Package names must consist only of lower case letters
            (<tt>a-z</tt>), digits (<tt>0-9</tt>), plus (<tt>+</tt>)
            and minus (<tt>-</tt>) signs, and periods (<tt>.</tt>).
            They must be at least two characters long and must start
            with an alphanumeric character.
          </p>
        </sect1>

        <sect1 id="f-Architecture">
          <heading><tt>Architecture</tt></heading>

          <p>
            Depending on context and the control file used, the
            <tt>Architecture</tt> field can include the following sets of
            values:
            <list>
                <item>A unique single word identifying a Debian machine
                      architecture, see <ref id="arch-spec">.
                <item><tt>all</tt>, which indicates an
                      architecture-independent package.
                <item><tt>any</tt>, which indicates a package available
                      for building on any architecture.
                <item><tt>source</tt>, which indicates a source package.
            </list>
          </p>

          <p>
            In the main <file>debian/control</file> file in the source
            package, or in the source package control file
            <file>.dsc</file>, one may specify a list of architectures
            separated by spaces, or the special values <tt>any</tt> or
            <tt>all</tt>.
          </p>

          <p>
            Specifying <tt>any</tt> indicates that the source package
            isn't dependent on any particular architecture and should
            compile fine on any one. The produced binary package(s)
            will be specific to whatever the current build architecture
            is.<footnote>
                This is the most often used setting, and is recommended
                for new packages that aren't <tt>Architecture: all</tt>.
            </footnote>
          </p>

          <p>
            Specifying a list of architectures indicates that the source
            will build an architecture-dependent package, and will only
            work correctly on the listed architectures.<footnote>
                This is a setting used for a minority of cases where the
                program is not portable. Generally, it should not be used
                for new packages.
            </footnote>
          </p>

          <p>
            In a <file>.changes</file> file, the <tt>Architecture</tt>
            field lists the architecture(s) of the package(s)
            currently being uploaded.  This will be a list; if the
            source for the package is also being uploaded, the special
            entry <tt>source</tt> is also present.
          </p>

          <p>
            See <ref id="debianrules"> for information how to get the
            architecture for the build process.
          </p>
        </sect1>

        <sect1 id="f-Essential">
          <heading><tt>Essential</tt></heading>

          <p>
            This is a boolean field which may occur only in the
            control file of a binary package or in a per-package fields
            paragraph of a main source control data file.
          </p>

          <p>
            If set to <tt>yes</tt> then the package management system
            will refuse to remove the package (upgrading and replacing
            it is still possible). The other possible value is <tt>no</tt>,
            which is the same as not having the field at all.
          </p>
        </sect1>

        <sect1>
          <heading>Package interrelationship fields:
            <tt>Depends</tt>, <tt>Pre-Depends</tt>,
            <tt>Recommends</tt>, <tt>Suggests</tt>,
            <tt>Breaks</tt>, <tt>Conflicts</tt>,
            <tt>Provides</tt>, <tt>Replaces</tt>, <tt>Enhances</tt>
          </heading>

          <p>
            These fields describe the package's relationships with
            other packages.  Their syntax and semantics are described
            in <ref id="relationships">.</p>
        </sect1>

        <sect1 id="f-Standards-Version">
          <heading><tt>Standards-Version</tt></heading>

          <p>
            The most recent version of the standards (the policy
            manual and associated texts) with which the package
            complies.
          </p>

          <p>
            The version number has four components: major and minor
            version number and major and minor patch level.  When the
            standards change in a way that requires every package to
            change the major number will be changed.  Significant
            changes that will require work in many packages will be
            signaled by a change to the minor number.  The major patch
            level will be changed for any change to the meaning of the
            standards, however small; the minor patch level will be
            changed when only cosmetic, typographical or other edits
            are made which neither change the meaning of the document
            nor affect the contents of packages.
          </p>

          <p>
            Thus only the first three components of the policy version
            are significant in the <em>Standards-Version</em> control
            field, and so either these three components or the all
            four components may be specified.<footnote>
                In the past, people specified the full version number
                in the Standards-Version field, for example "2.3.0.0".
                Since minor patch-level changes don't introduce new
                policy, it was thought it would be better to relax
                policy and only require the first 3 components to be
                specified, in this example "2.3.0".  All four
                components may still be used if someone wishes to do so.
            </footnote>
          </p>

        </sect1>

        <sect1 id="f-Version">
          <heading><tt>Version</tt></heading>

          <p>
            The version number of a package. The format is:
            [<var>epoch</var><tt>:</tt>]<var>upstream_version</var>[<tt>-</tt><var>debian_revision</var>]
          </p>

          <p>
            The three components here are:
            <taglist>
              <tag><var>epoch</var></tag>
              <item>
                <p>
                  This is a single (generally small) unsigned integer.  It
                  may be omitted, in which case zero is assumed.  If it is
                  omitted then the <var>upstream_version</var> may not
                  contain any colons.
                </p>

                <p>
                  It is provided to allow mistakes in the version numbers
                  of older versions of a package, and also a package's
                  previous version numbering schemes, to be left behind.
                </p>
              </item>

              <tag><var>upstream_version</var></tag>
              <item>
                <p>
                  This is the main part of the version number.  It is
                  usually the version number of the original ("upstream")
                  package from which the <file>.deb</file> file has been made,
                  if this is applicable.  Usually this will be in the same
                  format as that specified by the upstream author(s);
                  however, it may need to be reformatted to fit into the
                  package management system's format and comparison
                  scheme.
                </p>

                <p>
                  The comparison behavior of the package management system
                  with respect to the <var>upstream_version</var> is
                  described below.  The <var>upstream_version</var>
                  portion of the version number is mandatory.
                </p>

                <p>
                  The <var>upstream_version</var> may contain only
                  alphanumerics<footnote>
                        Alphanumerics are <tt>A-Za-z0-9</tt> only.
                  </footnote>
                  and the characters <tt>.</tt> <tt>+</tt> <tt>-</tt>
                  <tt>:</tt> <tt>~</tt> (full stop, plus, hyphen, colon,
                  tilde) and should start with a digit.  If there is no
                  <var>debian_revision</var> then hyphens are not allowed;
                  if there is no <var>epoch</var> then colons are not
                  allowed.
                </p>
              </item>

              <tag><var>debian_revision</var></tag>
              <item>
                <p>
                  This part of the version number specifies the version of
                  the Debian package based on the upstream version.  It
                  may contain only alphanumerics and the characters
                  <tt>+</tt> <tt>.</tt> <tt>~</tt> (plus, full stop,
                  tilde) and is compared in the same way as the
                  <var>upstream_version</var> is.
                </p>

                <p>
                  It is optional; if it isn't present then the
                  <var>upstream_version</var> may not contain a hyphen.
                  This format represents the case where a piece of
                  software was written specifically to be turned into a
                  Debian package, and so there is only one "debianisation"
                  of it and therefore no revision indication is required.
                </p>

                <p>
                  It is conventional to restart the
                  <var>debian_revision</var> at <tt>1</tt> each time the
                  <var>upstream_version</var> is increased.
                </p>

                <p>
                  The package management system will break the version
                  number apart at the last hyphen in the string (if there
                  is one) to determine the <var>upstream_version</var> and
                  <var>debian_revision</var>.  The absence of a
                  <var>debian_revision</var> compares earlier than the
                  presence of one (but note that the
                  <var>debian_revision</var> is the least significant part
                  of the version number).
                </p>
              </item>
            </taglist>
          </p>

          <p>
            The <var>upstream_version</var> and <var>debian_revision</var>
            parts are compared by the package management system using the
            same algorithm:
          </p>

          <p>
            The strings are compared from left to right.
          </p>

          <p>
            First the initial part of each string consisting entirely of
            non-digit characters is determined.  These two parts (one of
            which may be empty) are compared lexically.  If a difference
            is found it is returned.  The lexical comparison is a
            comparison of ASCII values modified so that all the letters
            sort earlier than all the non-letters and so that a tilde
            sorts before anything, even the end of a part.  For example,
            the following parts are in sorted order from earliest to
            latest: <tt>~~</tt>, <tt>~~a</tt>, <tt>~</tt>, the empty part,
            <tt>a</tt>.<footnote>
              One common use of <tt>~</tt> is for upstream pre-releases.
              For example, <tt>1.0~beta1~svn1245</tt> sorts earlier than
              <tt>1.0~beta1</tt>, which sorts earlier than <tt>1.0</tt>.
            </footnote>
          </p>

          <p>
            Then the initial part of the remainder of each string which
            consists entirely of digit characters is determined.  The
            numerical values of these two parts are compared, and any
            difference found is returned as the result of the comparison.
            For these purposes an empty string (which can only occur at
            the end of one or both version strings being compared) counts
            as zero.
          </p>

          <p>
            These two steps (comparing and removing initial non-digit
            strings and initial digit strings) are repeated until a
            difference is found or both strings are exhausted.
          </p>

          <p>
            Note that the purpose of epochs is to allow us to leave behind
            mistakes in version numbering, and to cope with situations
            where the version numbering scheme changes.  It is
            <em>not</em> intended to cope with version numbers containing
            strings of letters which the package management system cannot
            interpret (such as <tt>ALPHA</tt> or <tt>pre-</tt>), or with
            silly orderings (the author of this manual has heard of a
            package whose versions went <tt>1.1</tt>, <tt>1.2</tt>,
            <tt>1.3</tt>, <tt>1</tt>, <tt>2.1</tt>, <tt>2.2</tt>,
            <tt>2</tt> and so forth).
          </p>
        </sect1>

        <sect1 id="f-Description">
          <heading><tt>Description</tt></heading>

          <p>
            In a source or binary control file, the <tt>Description</tt>
            field contains a description of the binary package, consisting
            of two parts, the synopsis or the short description, and the
            long description. The field's format is as follows:
          </p>

          <p>
<example>
        Description: &lt;single line synopsis&gt;
         &lt;extended description over several lines&gt;
</example>
          </p>

          <p>
            The lines in the extended description can have these formats:
          </p>

          <p><list>

            <item>
              Those starting with a single space are part of a paragraph.
              Successive lines of this form will be word-wrapped when
              displayed. The leading space will usually be stripped off.
            </item>

            <item>
              Those starting with two or more spaces. These will be
              displayed verbatim. If the display cannot be panned
              horizontally, the displaying program will line wrap them "hard"
              (i.e., without taking account of word breaks). If it can they
              will be allowed to trail off to the right. None, one or two
              initial spaces may be deleted, but the number of spaces
              deleted from each line will be the same (so that you can have
              indenting work correctly, for example).
            </item>

            <item>
              Those containing a single space followed by a single full stop
              character. These are rendered as blank lines. This is the
              <em>only</em> way to get a blank line<footnote>
                Completely empty lines will not be rendered as blank lines.
                Instead, they will cause the parser to think you're starting
                a whole new record in the control file, and will therefore
                likely abort with an error.
              </footnote>.
            </item>

            <item>
              Those containing a space, a full stop and some more characters.
              These are for future expansion. Do not use them.
            </item>

          </list></p>

          <p>
            Do not use tab characters.  Their effect is not predictable.
          </p>

          <p>
            See <ref id="descriptions"> for further information on this.
          </p>

          <p>
            In a <file>.changes</file> file, the <tt>Description</tt> field
            contains a summary of the descriptions for the packages being
            uploaded.
          </p>

          <p>
            The part of the field before the first newline is empty;
            thereafter each line has the name of a binary package and
            the summary description line from that binary package.
            Each line is indented by one space.
          </p>

        </sect1>

        <sect1 id="f-Distribution">
          <heading><tt>Distribution</tt></heading>

          <p>
            In a <file>.changes</file> file or parsed changelog output
            this contains the (space-separated) name(s) of the
            distribution(s) where this version of the package should
            be installed.  Valid distributions are determined by the
            archive maintainers.<footnote>
                Current distribution names are:
                <taglist compact="compact">
                  <tag><em>stable</em></tag>
                  <item>
                      This is the current "released" version of Debian
                      GNU/Linux.  Once the distribution is
                      <em>stable</em> only security fixes and other
                      major bug fixes are allowed. When changes are
                      made to this distribution, the release number is
                      increased (for example: 2.2r1 becomes 2.2r2 then
                      2.2r3, etc).
                  </item>

                  <tag><em>unstable</em></tag>
                  <item>
                      This distribution value refers to the
                      <em>developmental</em> part of the Debian
                      distribution tree. New packages, new upstream
                      versions of packages and bug fixes go into the
                      <em>unstable</em> directory tree. Download from
                      this distribution at your own risk.
                  </item>

                  <tag><em>testing</em></tag>
                  <item>
                      This distribution value refers to the
                      <em>testing</em> part of the Debian distribution
                      tree.  It receives its packages from the
                      unstable distribution after a short time lag to
                      ensure that there are no major issues with the
                      unstable packages.  It is less prone to breakage
                      than unstable, but still risky.  It is not
                      possible to upload packages directly to
                      <em>testing</em>.
                  </item>

                  <tag><em>frozen</em></tag>
                  <item>
                      From time to time, the <em>testing</em>
                      distribution enters a state of "code-freeze" in
                      anticipation of release as a <em>stable</em>
                      version. During this period of testing only
                      fixes for existing or newly-discovered bugs will
                      be allowed.  The exact details of this stage are
                      determined by the Release Manager.
                  </item>

                  <tag><em>experimental</em></tag>
                  <item>
                      The packages with this distribution value are
                      deemed by their maintainers to be high
                      risk. Oftentimes they represent early beta or
                      developmental packages from various sources that
                      the maintainers want people to try, but are not
                      ready to be a part of the other parts of the
                      Debian distribution tree. Download at your own
                      risk.
                  </item>
                </taglist>

                <p>
                  You should list <em>all</em> distributions that the
                  package should be installed into.
                </p>

                <p>
                  More information is available in the Debian Developer's
                  Reference, section "The Debian archive".
                </p>
            </footnote>
          </p>
        </sect1>

        <sect1 id="f-Date">
          <heading><tt>Date</tt></heading>

          <p>
            This field includes the date the package was built or last edited.
          </p>

          <p>
            The value of this field is usually extracted from the
            <file>debian/changelog</file> file - see
            <ref id="dpkgchangelog">).
          </p>
        </sect1>

        <sect1 id="f-Format">
          <heading><tt>Format</tt></heading>

          <p>
            This field specifies a format revision for the file.
            The most current format described in the Policy Manual
            is version <strong>1.5</strong>.  The syntax of the
            format value is the same as that of a package version
            number except that no epoch or Debian revision is allowed
            - see <ref id="f-Version">.
          </p>
        </sect1>

        <sect1 id="f-Urgency">
          <heading><tt>Urgency</tt></heading>

          <p>
            This is a description of how important it is to upgrade to
            this version from previous ones.  It consists of a single
            keyword taking one of the values <tt>low</tt>,
            <tt>medium</tt>, <tt>high</tt>, <tt>emergency</tt>, or
            <tt>critical</tt><footnote>
              Other urgency values are supported with configuration
              changes in the archive software but are not used in Debian.
              The urgency affects how quickly a package will be considered
              for inclusion into the <tt>testing</tt> distribution and
              gives an indication of the importance of any fixes included
              in the upload.  <tt>Emergency</tt> and <tt>critical</tt> are
              treated as synonymous.
            </footnote> (not case-sensitive) followed by an optional
            commentary (separated by a space) which is usually in
            parentheses.  For example:

            <example>
  Urgency: low (HIGH for users of diversions)
            </example>

          </p>

          <p>
            The value of this field is usually extracted from the
            <file>debian/changelog</file> file - see
            <ref id="dpkgchangelog">.
          </p>
        </sect1>

        <sect1 id="f-Changes">
          <heading><tt>Changes</tt></heading>

          <p>
            This field contains the human-readable changes data, describing
            the differences between the last version and the current one.
          </p>

          <p>
            There should be nothing in this field before the first
            newline; all the subsequent lines must be indented by at
            least one space; blank lines must be represented by a line
            consisting only of a space and a full stop.
          </p>

          <p>
            The value of this field is usually extracted from the
            <file>debian/changelog</file> file - see
            <ref id="dpkgchangelog">).
          </p>

          <p>
            Each version's change information should be preceded by a
            "title" line giving at least the version, distribution(s)
            and urgency, in a human-readable way.
          </p>

          <p>
            If data from several versions is being returned the entry
            for the most recent version should be returned first, and
            entries should be separated by the representation of a
            blank line (the "title" line may also be followed by the
            representation of blank line).
          </p>
        </sect1>

        <sect1 id="f-Binary">
          <heading><tt>Binary</tt></heading>

          <p>
            This field is a list of binary packages.
          </p>

          <p>
            When it appears in the <file>.dsc</file> file it is the list
            of binary packages which a source package can produce.  It
            does not necessarily produce all of these binary packages
            for every architecture.  The source control file doesn't
            contain details of which architectures are appropriate for
            which of the binary packages.
          </p>

          <p>
            When it appears in a <file>.changes</file> file it lists the
            names of the binary packages actually being uploaded.
          </p>

          <p>
            The syntax is a list of binary packages separated by
            commas<footnote>
                A space after each comma is conventional.
            </footnote>. Currently the packages must be separated using
            only spaces in the <file>.changes</file> file.
          </p>
        </sect1>

        <sect1 id="f-Installed-Size">
          <heading><tt>Installed-Size</tt></heading>

          <p>
            This field appears in the control files of binary
            packages, and in the <file>Packages</file> files.  It gives
            the total amount of disk space required to install the
            named package.
          </p>

          <p>
            The disk space is represented in kilobytes as a simple
            decimal number.
          </p>
        </sect1>

        <sect1 id="f-Files">
          <heading><tt>Files</tt></heading>

          <p>
            This field contains a list of files with information about
            each one.  The exact information and syntax varies with
            the context.  In all cases the part of the field
            contents on the same line as the field name is empty.  The
            remainder of the field is one line per file, each line
            being indented by one space and containing a number of
            sub-fields separated by spaces.
          </p>

          <p>
            In the <file>.dsc</file> file, each line contains the MD5
            checksum, size and filename of the tar file and (if applicable)
            diff file which make up the remainder of the source
            package<footnote>
                That is, the parts which are not the <tt>.dsc</tt>.
            </footnote>.
            The exact forms of the filenames are described
            in <ref id="pkg-sourcearchives">.
          </p>

          <p>
            In the <file>.changes</file> file this contains one line per
            file being uploaded.  Each line contains the MD5 checksum,
            size, section and priority and the filename.
            The <qref id="f-Section">section</qref>
            and <qref id="f-Priority">priority</qref>
            are the values of the corresponding fields in
            the main source control file.  If no section or priority is
            specified then <tt>-</tt> should be used, though section
            and priority values must be specified for new packages to
            be installed properly.
          </p>

          <p>
            The special value <tt>byhand</tt> for the section in a
            <tt>.changes</tt> file indicates that the file in question
            is not an ordinary package file and must by installed by
            hand by the distribution maintainers.  If the section is
            <tt>byhand</tt> the priority should be <tt>-</tt>.
          </p>

          <p>
            If a new Debian revision of a package is being shipped and
            no new original source archive is being distributed the
            <tt>.dsc</tt> must still contain the <tt>Files</tt> field
            entry for the original source archive
            <file><var>package</var>-<var>upstream-version</var>.orig.tar.gz</file>,
            but the <file>.changes</file> file should leave it out.  In
            this case the original source archive on the distribution
            site must match exactly, byte-for-byte, the original
            source archive which was used to generate the
            <file>.dsc</file> file and diff which are being uploaded.</p>
        </sect1>

        <sect1 id="f-Closes">
          <heading><tt>Closes</tt></heading>

          <p>
            A space-separated list of bug report numbers that the upload
            governed by the .changes file closes.
          </p>
        </sect1>

      </sect>

      <sect>
        <heading>User-defined fields</heading>

        <p>
          Additional user-defined fields may be added to the
          source package control file.  Such fields will be
          ignored, and not copied to (for example) binary or
          source package control files or upload control files.
        </p>

        <p>
          If you wish to add additional unsupported fields to
          these output files you should use the mechanism
          described here.
        </p>

        <p>
          Fields in the main source control information file with
          names starting <tt>X</tt>, followed by one or more of
          the letters <tt>BCS</tt> and a hyphen <tt>-</tt>, will
          be copied to the output files.  Only the part of the
          field name after the hyphen will be used in the output
          file.  Where the letter <tt>B</tt> is used the field
          will appear in binary package control files, where the
          letter <tt>S</tt> is used in source package control
          files and where <tt>C</tt> is used in upload control
          (<tt>.changes</tt>) files.
        </p>

        <p>
          For example, if the main source information control file
          contains the field
          <example>
  XBS-Comment: I stand between the candle and the star.
          </example>
          then the binary and source package control files will contain the
          field
          <example>
  Comment: I stand between the candle and the star.
          </example>
        </p>

      </sect>

    </chapt>


    <chapt id="maintainerscripts">
      <heading>Package maintainer scripts and installation procedure</heading>

      <sect>
        <heading>Introduction to package maintainer scripts</heading>

        <p>
          It is possible to supply scripts as part of a package which
          the package management system will run for you when your
          package is installed, upgraded or removed.
        </p>

        <p>
          These scripts are the files <prgn>preinst</prgn>,
          <prgn>postinst</prgn>, <prgn>prerm</prgn> and
          <prgn>postrm</prgn> in the control area of the package.
          They must be proper executable files; if they are scripts
          (which is recommended), they must start with the usual
          <tt>#!</tt> convention.  They should be readable and
          executable by anyone, and must not be world-writable.
        </p>

        <p>
          The package management system looks at the exit status from
          these scripts.  It is important that they exit with a
          non-zero status if there is an error, so that the package
          management system can stop its processing.  For shell
          scripts this means that you <em>almost always</em> need to
          use <tt>set -e</tt> (this is usually true when writing shell
          scripts, in fact).  It is also important, of course, that
          they don't exit with a non-zero status if everything went
          well.
        </p>

        <p>
          Additionally, packages interacting with users using
          <tt>debconf</tt> in the <prgn>postinst</prgn> script should
          install a <prgn>config</prgn> script  in the control area,
          see <ref id="maintscriptprompt"> for details.
        </p>

        <p>
          When a package is upgraded a combination of the scripts from
          the old and new packages is called during the upgrade
          procedure.  If your scripts are going to be at all
          complicated you need to be aware of this, and may need to
          check the arguments to your scripts.
        </p>

        <p>
          Broadly speaking the <prgn>preinst</prgn> is called before
          (a particular version of) a package is installed, and the
          <prgn>postinst</prgn> afterwards; the <prgn>prerm</prgn>
          before (a version of) a package is removed and the
          <prgn>postrm</prgn> afterwards.
        </p>

        <p>
          Programs called from maintainer scripts should not normally
          have a path prepended to them. Before installation is
          started, the package management system checks to see if the
          programs <prgn>ldconfig</prgn>,
          <prgn>start-stop-daemon</prgn>, <prgn>install-info</prgn>,
          and <prgn>update-rc.d</prgn> can be found via the
          <tt>PATH</tt> environment variable. Those programs, and any
          other program that one would expect to be in the
          <tt>PATH</tt>, should thus be invoked without an absolute
          pathname. Maintainer scripts should also not reset the
          <tt>PATH</tt>, though they might choose to modify it by
          prepending or appending package-specific directories. These
          considerations really apply to all shell scripts.</p>
      </sect>

      <sect id="idempotency">
        <heading>Maintainer scripts idempotency</heading>

        <p>
          It is necessary for the error recovery procedures that the
          scripts be idempotent.  This means that if it is run
          successfully, and then it is called again, it doesn't bomb
          out or cause any harm, but just ensures that everything is
          the way it ought to be.  If the first call failed, or
          aborted half way through for some reason, the second call
          should merely do the things that were left undone the first
          time, if any, and exit with a success status if everything
          is OK.<footnote>
              This is so that if an error occurs, the user interrupts
              <prgn>dpkg</prgn> or some other unforeseen circumstance
              happens you don't leave the user with a badly-broken
              package when <prgn>dpkg</prgn> attempts to repeat the
              action.
          </footnote>
        </p>
      </sect>

      <sect id="controllingterminal">
        <heading>Controlling terminal for maintainer scripts</heading>

        <p>
          The maintainer scripts are guaranteed to run with a
          controlling terminal and can interact with the user.
          Because these scripts may be executed with standard output
          redirected into a pipe for logging purposes, Perl scripts
          should set unbuffered output by setting <tt>$|=1</tt> so
          that the output is printed immediately rather than being
          buffered.
        </p>
      </sect>
      <sect id="exitstatus">
        <heading>Exit status</heading>

        <p>
          Each script must return a zero exit status for
          success, or a nonzero one for failure, since the package
          management system looks for the exit status of these scripts
          and determines what action to take next based on that datum.
        </p>
      </sect>

      <sect id="mscriptsinstact"><heading>Summary of ways maintainer
          scripts are called
        </heading>

        <p>
          <list compact="compact">
            <item>
              <var>new-preinst</var> <tt>install</tt>
            </item>
            <item>
              <var>new-preinst</var> <tt>install</tt> <var>old-version</var>
            </item>
            <item>
                <var>new-preinst</var> <tt>upgrade</tt> <var>old-version</var>
            </item>
            <item>
                <var>old-preinst</var> <tt>abort-upgrade</tt>
                <var>new-version</var>
            </item>
          </list>

        <p>
          <list compact="compact">
            <item>
                <var>postinst</var> <tt>configure</tt>
                <var>most-recently-configured-version</var>
            </item>
            <item>
                <var>old-postinst</var> <tt>abort-upgrade</tt>
                <var>new-version</var>
            </item>
            <item>
                <var>conflictor's-postinst</var> <tt>abort-remove</tt>
                <tt>in-favour</tt> <var>package</var>
                <var>new-version</var>
            </item>
            <item>
                <var>postinst</var> <tt>abort-remove</tt>
            </item>
            <item>
                <var>deconfigured's-postinst</var>
                <tt>abort-deconfigure</tt> <tt>in-favour</tt>
                <var>failed-install-package</var> <var>version</var>
                [<tt>removing</tt> <var>conflicting-package</var>
                <var>version</var>]
            </item>
          </list>

        <p>
          <list compact="compact">
            <item>
                <var>prerm</var> <tt>remove</tt>
            </item>
            <item>
                <var>old-prerm</var> <tt>upgrade</tt>
                <var>new-version</var>
            </item>
            <item>
                <var>new-prerm</var> <tt>failed-upgrade</tt>
                <var>old-version</var>
            </item>
            <item>
                <var>conflictor's-prerm</var> <tt>remove</tt>
                <tt>in-favour</tt> <var>package</var>
                <var>new-version</var>
            </item>
            <item>
                <var>deconfigured's-prerm</var> <tt>deconfigure</tt>
                <tt>in-favour</tt> <var>package-being-installed</var>
                <var>version</var> [<tt>removing</tt>
                <var>conflicting-package</var>
                <var>version</var>]
            </item>
          </list>

        <p>
          <list compact="compact">
            <item>
                <var>postrm</var> <tt>remove</tt>
            </item>
            <item>
                <var>postrm</var> <tt>purge</tt>
            </item>
            <item>
                <var>old-postrm</var> <tt>upgrade</tt>
                <var>new-version</var>
            </item>
            <item>
                <var>new-postrm</var> <tt>failed-upgrade</tt>
                <var>old-version</var>
            </item>
            <item>
                <var>new-postrm</var> <tt>abort-install</tt>
            </item>
            <item>
                <var>new-postrm</var> <tt>abort-install</tt>
                <var>old-version</var>
            </item>
            <item>
                <var>new-postrm</var> <tt>abort-upgrade</tt>
                <var>old-version</var>
            </item>
            <item>
                <var>disappearer's-postrm</var> <tt>disappear</tt>
                <var>overwriter</var>
                <var>overwriter-version</var>
            </item>
          </list>
        </p>


      <sect id="unpackphase">
        <heading>Details of unpack phase of installation or upgrade</heading>

        <p>
          The procedure on installation/upgrade/overwrite/disappear
          (i.e., when running <tt>dpkg --unpack</tt>, or the unpack
          stage of <tt>dpkg --install</tt>) is as follows.  In each
          case, if a major error occurs (unless listed below) the
          actions are, in general, run backwards - this means that the
          maintainer scripts are run with different arguments in
          reverse order.  These are the "error unwind" calls listed
          below.

          <enumlist>
            <item>
                <enumlist>
                  <item>
                      If a version of the package is already installed, call
                      <example compact="compact">
<var>old-prerm</var> upgrade <var>new-version</var>
                      </example>
                  </item>
                  <item>
                      If the script runs but exits with a non-zero
                      exit status, <prgn>dpkg</prgn> will attempt:
                      <example compact="compact">
<var>new-prerm</var> failed-upgrade <var>old-version</var>
                      </example>
                      If this works, the upgrade continues. If this
                      does not work, the error unwind:
                      <example compact="compact">
<var>old-postinst</var> abort-upgrade <var>new-version</var>
                      </example>
                      If this works, then the old-version is
                      "Installed", if not, the old version is in a
                      "Failed-Config" state.
                  </item>
                </enumlist>
            </item>

            <item>
                If a "conflicting" package is being removed at the same time,
                or if any package will be broken (due to <tt>Breaks</tt>):
                <enumlist>
                  <item>
                      If <tt>--auto-deconfigure</tt> is
                      specified, call, for each package to be deconfigured
                      due to <tt>Breaks</tt>:
                      <example compact="compact">
<var>deconfigured's-prerm</var> deconfigure \
  in-favour <var>package-being-installed</var> <var>version</var>
                      </example>
                      Error unwind:
                      <example compact="compact">
<var>deconfigured's-postinst</var> abort-deconfigure \
  in-favour <var>package-being-installed-but-failed</var> <var>version</var>
                      </example>
                      The deconfigured packages are marked as
                      requiring configuration, so that if
                      <tt>--install</tt> is used they will be
                      configured again if possible.
                  </item>
                  <item>
                      If any packages depended on a conflicting
                      package being removed and <tt>--auto-deconfigure</tt> is
                      specified, call, for each such package:
                      <example compact="compact">
<var>deconfigured's-prerm</var> deconfigure \
  in-favour <var>package-being-installed</var> <var>version</var> \
    removing <var>conflicting-package</var> <var>version</var>
                      </example>
                      Error unwind:
                      <example compact="compact">
<var>deconfigured's-postinst</var> abort-deconfigure \
  in-favour <var>package-being-installed-but-failed</var> <var>version</var> \
    removing <var>conflicting-package</var> <var>version</var>
                      </example>
                      The deconfigured packages are marked as
                      requiring configuration, so that if
                      <tt>--install</tt> is used they will be
                      configured again if possible.
                  </item>
                  <item>
                      To prepare for removal of each conflicting package, call:
                      <example compact="compact">
<var>conflictor's-prerm</var> remove \
  in-favour <var>package</var> <var>new-version</var>
                      </example>
                      Error unwind:
                      <example compact="compact">
<var>conflictor's-postinst</var> abort-remove \
  in-favour <var>package</var> <var>new-version</var>
                      </example>
                  </item>
                </enumlist>
            </item>

            <item>
                <enumlist>
                  <item>
                      If the package is being upgraded, call:
                      <example compact="compact">
<var>new-preinst</var> upgrade <var>old-version</var>
                      </example>
                      If this fails, we call:
                      <example>
<var>new-postrm</var> abort-upgrade <var>old-version</var>
                      </example>
                      <enumlist>
                        <item>
                          <p>
                            If that works, then
                            <example>
<var>old-postinst</var> abort-upgrade <var>new-version</var>
                            </example>
                            is called. If this works, then the old version
                            is in an "Installed" state, or else it is left
                            in an "Unpacked" state.
                          </p>
                        </item>
                        <item>
                          <p>
                            If it fails, then the old version is left
                            in an "Half-Installed" state.
                          </p>
                        </item>
                      </enumlist>
                      
                  </item>
                  <item>
                      Otherwise, if the package had some configuration
                      files from a previous version installed (i.e., it
                      is in the "configuration files only" state):
                      <example compact="compact">
<var>new-preinst</var> install <var>old-version</var>
                      </example>
                      Error unwind:
                      <example>
<var>new-postrm</var> abort-install <var>old-version</var>
                      </example>
                      If this fails, the package is left in a
                      "Half-Installed" state, which requires a
                      reinstall. If it works, the packages is left in
                      a "Config Files" state.
                  </item>
                  <item>
                      Otherwise (i.e., the package was completely purged):
                      <example compact="compact">
<var>new-preinst</var> install
                      </example>
                      Error unwind:
                      <example compact="compact">
<var>new-postrm</var> abort-install
                      </example>
                      If the error-unwind fails, the package is in a
                      "Half Installed" phase, and requires a
                      reinstall. If the error unwind works, the
                      package is in a not installed state.
                  </item>
                </enumlist>
            </item>

            <item>
              <p>
                The new package's files are unpacked, overwriting any
                that may be on the system already, for example any
                from the old version of the same package or from
                another package.  Backups of the old files are kept
                temporarily, and if anything goes wrong the package
                management system will attempt to put them back as
                part of the error unwind.
              </p>

              <p>
                It is an error for a package to contain files which
                are on the system in another package, unless
                <tt>Replaces</tt> is used (see <ref id="replaces">).
                <!--
                The following paragraph is not currently the case:
                Currently the <tt>- - force-overwrite</tt> flag is
                enabled, downgrading it to a warning, but this may not
                always be the case.
                -->
              </p>

              <p>
                It is a more serious error for a package to contain a
                plain file or other kind of non-directory where another
                package has a directory (again, unless
                <tt>Replaces</tt> is used).  This error can be
                overridden if desired using
                <tt>--force-overwrite-dir</tt>, but this is not
                advisable.
              </p>

              <p>
                Packages which overwrite each other's files produce
                behavior which, though deterministic, is hard for the
                system administrator to understand.  It can easily
                lead to "missing" programs if, for example, a package
                is installed which overwrites a file from another
                package, and is then removed again.<footnote>
                    Part of the problem is due to what is arguably a
                    bug in <prgn>dpkg</prgn>.
                </footnote>
              </p>

              <p>
                A directory will never be replaced by a symbolic link
                to a directory or vice versa; instead, the existing
                state (symlink or not) will be left alone and
                <prgn>dpkg</prgn> will follow the symlink if there is
                one.
              </p>
            </item>

            <item>
              <p>
                <enumlist>
                  <item>
                      If the package is being upgraded, call
                      <example compact="compact">
<var>old-postrm</var> upgrade <var>new-version</var>
                      </example>
                  </item>
                  <item>
                      If this fails, <prgn>dpkg</prgn> will attempt:
                      <example compact="compact">
<var>new-postrm</var> failed-upgrade <var>old-version</var>
                      </example>
                      If this works, installation continues. If not, 
                      Error unwind:
                      <example compact="compact">
<var>old-preinst</var> abort-upgrade <var>new-version</var>
                      </example>
                      If this fails, the old version is left in an
                      "Half Installed" state. If it works, dpkg now
                      calls:
                      <example compact="compact">
<var>new-postrm</var> abort-upgrade <var>old-version</var>
                      </example>
                      If this fails, the old version is left in an
                      "Half Installed" state. If it works, dpkg now
                      calls:
                      <example compact="compact">
<var>old-postinst</var> abort-upgrade <var>new-version</var>
                      </example>
                      If this fails, the old version is in an
                      "Unpacked" state.
                  </item>
                </enumlist>
              </p>

              <p>
                This is the point of no return - if
                <prgn>dpkg</prgn> gets this far, it won't back off
                past this point if an error occurs.  This will
                leave the package in a fairly bad state, which
                will require a successful re-installation to clear
                up, but it's when <prgn>dpkg</prgn> starts doing
                things that are irreversible.
              </p>
            </item>

            <item>
                Any files which were in the old version of the package
                but not in the new are removed.
            </item>

            <item>
                The new file list replaces the old.
            </item>

            <item>
                The new maintainer scripts replace the old.
            </item>

            <item>
                Any packages all of whose files have been overwritten
                during the installation, and which aren't required for
                dependencies, are considered to have been removed.
                For each such package
                <enumlist>
                  <item>
                      <prgn>dpkg</prgn> calls:
                      <example compact="compact">
<var>disappearer's-postrm</var> disappear \
  <var>overwriter</var> <var>overwriter-version</var>
                      </example>
                  </item>
                  <item>
                      The package's maintainer scripts are removed.
                  </item>
                  <item>
                      It is noted in the status database as being in a
                      sane state, namely not installed (any conffiles
                      it may have are ignored, rather than being
                      removed by <prgn>dpkg</prgn>).  Note that
                      disappearing packages do not have their prerm
                      called, because <prgn>dpkg</prgn> doesn't know
                      in advance that the package is going to
                      vanish.
                  </item>
                </enumlist>
            </item>

            <item>
                Any files in the package we're unpacking that are also
                listed in the file lists of other packages are removed
                from those lists.  (This will lobotomize the file list
                of the "conflicting" package if there is one.)
            </item>

            <item>
                The backup files made during installation, above, are
                deleted.
            </item>

            <item>
              <p>
                The new package's status is now sane, and recorded as
                "unpacked".
              </p>

              <p>
                Here is another point of no return - if the
                conflicting package's removal fails we do not unwind
                the rest of the installation; the conflicting package
                is left in a half-removed limbo.
              </p>
            </item>

            <item>
                If there was a conflicting package we go and do the
                removal actions (described below), starting with the
                removal of the conflicting package's files (any that
                are also in the package being installed have already
                been removed from the conflicting package's file list,
                and so do not get removed now).
            </item>
          </enumlist>
        </p>
      </sect>

      <sect id="configdetails"><heading>Details of configuration</heading>

        <p>
          When we configure a package (this happens with <tt>dpkg
            --install</tt> and <tt>dpkg --configure</tt>), we first
          update any <tt>conffile</tt>s and then call:
          <example compact="compact">
<var>postinst</var> configure <var>most-recently-configured-version</var>
          </example>
        </p>

        <p>
          No attempt is made to unwind after errors during
          configuration. If the configuration fails, the package is in
          a "Failed Config" state, and an error message is generated.
        </p>

        <p>
          If there is no most recently configured version
          <prgn>dpkg</prgn> will pass a null argument.
          <footnote>
            <p>
              Historical note: Truly ancient (pre-1997) versions of
              <prgn>dpkg</prgn> passed <tt>&lt;unknown&gt;</tt>
              (including the angle brackets) in this case.  Even older
              ones did not pass a second argument at all, under any
              circumstance.  Note that upgrades using such an old dpkg
              version are unlikely to work for other reasons, even if
              this old argument behavior is handled by your postinst script.
            </p>
          </footnote>     
        </p>
      </sect>

      <sect id="removedetails"><heading>Details of removal and/or
      configuration purging</heading>

        <p>
          <enumlist>
            <item>
              <p>
                <example compact="compact">
<var>prerm</var> remove
                </example>
              </p>
              <p>
                If prerm fails during replacement due to conflict
                <example>
<var>conflictor's-postinst</var> abort-remove \
  in-favour <var>package</var> <var>new-version</var>
                </example>
                Or else we call:
                <example>
<var>postinst</var> abort-remove
                </example>
              </p>
              <p>
                If this fails, the package is in a "Failed-Config"
                state, or else it remains "Installed".
              </p>
            </item>
            <item>
                The package's files are removed (except <tt>conffile</tt>s).
            </item>
            <item>
                <example compact="compact">
<var>postrm</var> remove
                </example>

              <p>
                If it fails, there's no error unwind, and the package is in
                an "Half-Installed" state.
              </p>
            </item>
            <item>
              <p>
                All the maintainer scripts except the <prgn>postrm</prgn>
                are removed.
              </p>

              <p>
                If we aren't purging the package we stop here.  Note
                that packages which have no <prgn>postrm</prgn> and no
                <tt>conffile</tt>s are automatically purged when
                removed, as there is no difference except for the
                <prgn>dpkg</prgn> status.
              </p>
            </item>
            <item>
                The <tt>conffile</tt>s and any backup files
                (<tt>~</tt>-files, <tt>#*#</tt> files,
                <tt>%</tt>-files, <tt>.dpkg-{old,new,tmp}</tt>, etc.)
                are removed.
            </item>
            <item>
              <p>
                <example compact="compact">
<var>postrm</var> purge
                </example>
              </p>
              <p>
                If this fails, the package remains in a "Config-Files"
                state.
              </p>
            </item>
            <item>
                The package's file list is removed.
            </item>
          </enumlist>

        </p>
      </sect>
    </chapt>


    <chapt id="relationships">
      <heading>Declaring relationships between packages</heading>

      <sect id="depsyntax">
        <heading>Syntax of relationship fields</heading>

        <p>
          These fields all have a uniform syntax.  They are a list of
          package names separated by commas.
        </p>

        <p>
          In the <tt>Depends</tt>, <tt>Recommends</tt>,
          <tt>Suggests</tt>, <tt>Pre-Depends</tt>,
          <tt>Build-Depends</tt> and <tt>Build-Depends-Indep</tt>
          control file fields of the package, which declare
          dependencies on other packages, the package names listed may
          also include lists of alternative package names, separated
          by vertical bar (pipe) symbols <tt>|</tt>.  In such a case,
          if any one of the alternative packages is installed, that
          part of the dependency is considered to be satisfied.
        </p>

        <p>
          All of the fields except for <tt>Provides</tt> may restrict
          their applicability to particular versions of each named
          package.  This is done in parentheses after each individual
          package name; the parentheses should contain a relation from
          the list below followed by a version number, in the format
          described in <ref id="f-Version">.
        </p>

        <p>
          The relations allowed are <tt>&lt;&lt;</tt>, <tt>&lt;=</tt>,
          <tt>=</tt>, <tt>&gt;=</tt> and <tt>&gt;&gt;</tt> for
          strictly earlier, earlier or equal, exactly equal, later or
          equal and strictly later, respectively.  The deprecated
          forms <tt>&lt;</tt> and <tt>&gt;</tt> were used to mean
          earlier/later or equal, rather than strictly earlier/later,
          so they should not appear in new packages (though
          <prgn>dpkg</prgn> still supports them).
        </p>

        <p>
          Whitespace may appear at any point in the version
          specification subject to the rules in <ref
          id="controlsyntax">, and must appear where it's necessary to
          disambiguate; it is not otherwise significant.  All of the
          relationship fields may span multiple lines.  For
          consistency and in case of future changes to
          <prgn>dpkg</prgn> it is recommended that a single space be
          used after a version relationship and before a version
          number; it is also conventional to put a single space after
          each comma, on either side of each vertical bar, and before
          each open parenthesis.  When wrapping a relationship field, it
          is conventional to do so after a comma and before the space
          following that comma.
        </p>

        <p>
          For example, a list of dependencies might appear as:
          <example compact="compact">
Package: mutt
Version: 1.3.17-1
Depends: libc6 (>= 2.2.1), exim | mail-transport-agent
          </example>
        </p>

        <p>
          All fields that specify build-time relationships
          (<tt>Build-Depends</tt>, <tt>Build-Depends-Indep</tt>,
          <tt>Build-Conflicts</tt> and <tt>Build-Conflicts-Indep</tt>)
          may be restricted to a certain set of architectures.  This
          is indicated in brackets after each individual package name and
          the optional version specification.  The brackets enclose a
          list of Debian architecture names separated by whitespace.
          Exclamation marks may be prepended to each of the names.
          (It is not permitted for some names to be prepended with
          exclamation marks while others aren't.) If the current Debian
          host architecture is not in this list and there are no
          exclamation marks in the list, or it is in the list with a
          prepended exclamation mark, the package name and the
          associated version specification are ignored completely for
          the purposes of defining the relationships.
        </p>

        <p>
          For example:
          <example compact="compact">
Source: glibc
Build-Depends-Indep: texinfo
Build-Depends: kernel-headers-2.2.10 [!hurd-i386],
  hurd-dev [hurd-i386], gnumach-dev [hurd-i386]
          </example>
        </p>

        <p>
          Note that the binary package relationship fields such as
          <tt>Depends</tt> appear in one of the binary package
          sections of the control file, whereas the build-time
          relationships such as <tt>Build-Depends</tt> appear in the
          source package section of the control file (which is the
          first section).
        </p>
      </sect>

      <sect id="binarydeps">
        <heading>Binary Dependencies - <tt>Depends</tt>,
          <tt>Recommends</tt>, <tt>Suggests</tt>, <tt>Enhances</tt>,
          <tt>Pre-Depends</tt>
        </heading>

        <p>
          Packages can declare in their control file that they have
          certain relationships to other packages - for example, that
          they may not be installed at the same time as certain other
          packages, and/or that they depend on the presence of others.
        </p>

        <p>
          This is done using the <tt>Depends</tt>, <tt>Pre-Depends</tt>,
          <tt>Recommends</tt>, <tt>Suggests</tt>, <tt>Enhances</tt>,
          <tt>Breaks</tt> and <tt>Conflicts</tt> control file fields.
        </p>

        <p>
          These seven fields are used to declare a dependency
          relationship by one package on another.  Except for
          <tt>Enhances</tt> and <tt>Breaks</tt>, they appear in the
          depending (binary) package's control file.
          (<tt>Enhances</tt> appears in the recommending package's
          control file, and <tt>Breaks</tt> appears in the version of
          depended-on package which causes the named package to
          break).
        </p>

        <p>
          A <tt>Depends</tt> field takes effect <em>only</em> when a
          package is to be configured.  It does not prevent a package
          being on the system in an unconfigured state while its
          dependencies are unsatisfied, and it is possible to replace
          a package whose dependencies are satisfied and which is
          properly installed with a different version whose
          dependencies are not and cannot be satisfied; when this is
          done the depending package will be left unconfigured (since
          attempts to configure it will give errors) and will not
          function properly.  If it is necessary, a
          <tt>Pre-Depends</tt> field can be used, which has a partial
          effect even when a package is being unpacked, as explained
          in detail below.  (The other three dependency fields,
          <tt>Recommends</tt>, <tt>Suggests</tt> and
          <tt>Enhances</tt>, are only used by the various front-ends
          to <prgn>dpkg</prgn> such as <prgn>apt-get</prgn>,
          <prgn>aptitude</prgn>, and <prgn>dselect</prgn>.)
        </p>

        <p>
          For this reason packages in an installation run are usually
          all unpacked first and all configured later; this gives
          later versions of packages with dependencies on later
          versions of other packages the opportunity to have their
          dependencies satisfied.
        </p>

        <p>
          In case of circular dependencies, since installation or
          removal order honoring the dependency order can't be
          established, dependency loops are broken at some point
          (based on rules below), and some packages may not be able to
          rely on their dependencies being present when being
          installed or removed, depending on which side of the break
          of the circular dependency loop they happen to be on.  If one
          of the packages in the loop has no postinst script, then the
          cycle will be broken at that package, so as to ensure that
          all postinst scripts run with the dependencies properly
          configured if this is possible. Otherwise the breaking point
          is arbitrary.
        </p>

        <p>
          The <tt>Depends</tt> field thus allows package maintainers
          to impose an order in which packages should be configured.
        </p>

        <p>
          The meaning of the five dependency fields is as follows:
          <taglist>
            <tag><tt>Depends</tt></tag>
            <item>
              <p>
                This declares an absolute dependency.  A package will
                not be configured unless all of the packages listed in
                its <tt>Depends</tt> field have been correctly
                configured.
              </p>

              <p>
                The <tt>Depends</tt> field should be used if the
                depended-on package is required for the depending
                package to provide a significant amount of
                functionality.
              </p>

              <p>
                The <tt>Depends</tt> field should also be used if the
                <prgn>postinst</prgn>, <prgn>prerm</prgn> or
                <prgn>postrm</prgn> scripts require the package to be
                present in order to run.  Note, however, that the
                <prgn>postrm</prgn> cannot rely on any non-essential
                packages to be present during the <tt>purge</tt>
                phase.
            </item>

            <tag><tt>Recommends</tt></tag>
            <item>
              <p>
                This declares a strong, but not absolute, dependency.
              </p>

              <p>
                The <tt>Recommends</tt> field should list packages
                that would be found together with this one in all but
                unusual installations.
              </p>
            </item>

            <tag><tt>Suggests</tt></tag>
            <item>
                This is used to declare that one package may be more
                useful with one or more others.  Using this field
                tells the packaging system and the user that the
                listed packages are related to this one and can
                perhaps enhance its usefulness, but that installing
                this one without them is perfectly reasonable.
            </item>

            <tag><tt>Enhances</tt></tag>
            <item>
                This field is similar to Suggests but works in the
                opposite direction. It is used to declare that a
                package can enhance the functionality of another
                package.
            </item>

            <tag><tt>Pre-Depends</tt></tag>
            <item>
              <p>
                This field is like <tt>Depends</tt>, except that it
                also forces <prgn>dpkg</prgn> to complete installation
                of the packages named before even starting the
                installation of the package which declares the
                pre-dependency, as follows:
              </p>

              <p>
                When a package declaring a pre-dependency is about to
                be <em>unpacked</em> the pre-dependency can be
                satisfied if the depended-on package is either fully
                configured, <em>or even if</em> the depended-on
                package(s) are only unpacked or half-configured,
                provided that they have been configured correctly at
                some point in the past (and not removed or partially
                removed since).  In this case, both the
                previously-configured and currently unpacked or
                half-configured versions must satisfy any version
                clause in the <tt>Pre-Depends</tt> field.
              </p>

              <p>
                When the package declaring a pre-dependency is about
                to be <em>configured</em>, the pre-dependency will be
                treated as a normal <tt>Depends</tt>, that is, it will
                be considered satisfied only if the depended-on
                package has been correctly configured.
              </p>

              <p>
                <tt>Pre-Depends</tt> should be used sparingly,
                preferably only by packages whose premature upgrade or
                installation would hamper the ability of the system to
                continue with any upgrade that might be in progress.
              </p>

              <p>
                <tt>Pre-Depends</tt> are also required if the
                <prgn>preinst</prgn> script depends on the named
                package.  It is best to avoid this situation if
                possible.
              </p>
            </item>
          </taglist>
        </p>

        <p>
          When selecting which level of dependency to use you should
          consider how important the depended-on package is to the
          functionality of the one declaring the dependency.  Some
          packages are composed of components of varying degrees of
          importance.  Such a package should list using
          <tt>Depends</tt> the package(s) which are required by the
          more important components.  The other components'
          requirements may be mentioned as Suggestions or
          Recommendations, as appropriate to the components' relative
          importance.
        </p>
      </sect>

      <sect id="breaks">
        <heading>Packages which break other packages - <tt>Breaks</tt></heading>

        <p>
          Using <tt>Breaks</tt> may cause problems for upgrades from older
          versions of Debian and should not be used until the stable
          release of Debian supports <tt>Breaks</tt>.
        </p>

        <p>
          When one binary package declares that it breaks another,
          <prgn>dpkg</prgn> will refuse to allow the package which
          declares <tt>Breaks</tt> be installed unless the broken
          package is deconfigured first, and it will refuse to
          allow the broken package to be reconfigured.
        </p>

        <p>
          A package will not be regarded as causing breakage merely
          because its configuration files are still installed; it must
          be at least half-installed.
        </p>

        <p>
          A special exception is made for packages which declare that
          they break their own package name or a virtual package which
          they provide (see below): this does not count as a real
          breakage.
        </p>

        <p>
          Normally a <tt>Breaks</tt> entry will have an "earlier than"
          version clause; such a <tt>Breaks</tt> is introduced in the
          version of an (implicit or explicit) dependency which
          violates an assumption or reveals a bug in earlier versions
          of the broken package.  This use of <tt>Breaks</tt> will
          inform higher-level package management tools that broken
          package must be upgraded before the new one.
        </p>

        <p>
          If the breaking package also overwrites some files from the
          older package, it should use <tt>Replaces</tt> (not
          <tt>Conflicts</tt>) to ensure this goes smoothly.
        </p>
      </sect>

      <sect id="conflicts">
        <heading>Conflicting binary packages - <tt>Conflicts</tt></heading>

        <p>
          When one binary package declares a conflict with another
          using a <tt>Conflicts</tt> field, <prgn>dpkg</prgn> will
          refuse to allow them to be installed on the system at the
          same time.
        </p>

        <p>
          If one package is to be installed, the other must be removed
          first - if the package being installed is marked as
          replacing (see <ref id="replaces">) the one on the system,
          or the one on the system is marked as deselected, or both
          packages are marked <tt>Essential</tt>, then
          <prgn>dpkg</prgn> will automatically remove the package
          which is causing the conflict, otherwise it will halt the
          installation of the new package with an error.  This
          mechanism is specifically designed to produce an error when
          the installed package is <tt>Essential</tt>, but the new
          package is not.
        </p>

        <p>
          A package will not cause a conflict merely because its
          configuration files are still installed; it must be at least
          half-installed.
        </p>

        <p>
          A special exception is made for packages which declare a
          conflict with their own package name, or with a virtual
          package which they provide (see below): this does not
          prevent their installation, and allows a package to conflict
          with others providing a replacement for it.  You use this
          feature when you want the package in question to be the only
          package providing some feature.
        </p>

        <p>
          A <tt>Conflicts</tt> entry should almost never have an
          "earlier than" version clause.  This would prevent
          <prgn>dpkg</prgn> from upgrading or installing the package
          which declared such a conflict until the upgrade or removal
          of the conflicted-with package had been completed.  Instead,
          <tt>Breaks</tt> may be used (once <tt>Breaks</tt> is supported
          by the stable release of Debian).
        </p>
      </sect>

      <sect id="virtual"><heading>Virtual packages - <tt>Provides</tt>
        </heading>

        <p>
          As well as the names of actual ("concrete") packages, the
          package relationship fields <tt>Depends</tt>,
          <tt>Recommends</tt>, <tt>Suggests</tt>, <tt>Enhances</tt>,
          <tt>Pre-Depends</tt>, <tt>Breaks</tt>, <tt>Conflicts</tt>,
          <tt>Build-Depends</tt>, <tt>Build-Depends-Indep</tt>,
          <tt>Build-Conflicts</tt> and <tt>Build-Conflicts-Indep</tt>
          may mention "virtual packages".
        </p>

        <p>
          A <em>virtual package</em> is one which appears in the
          <tt>Provides</tt> control file field of another package.
          The effect is as if the package(s) which provide a
          particular virtual package name had been listed by name
          everywhere the virtual package name appears. (See also <ref
            id="virtual_pkg">)
        </p>

        <p>
          If there are both concrete and virtual packages of the same
          name, then the dependency may be satisfied (or the conflict
          caused) by either the concrete package with the name in
          question or any other concrete package which provides the
          virtual package with the name in question.  This is so that,
          for example, supposing we have
          <example compact="compact">
Package: foo
Depends: bar
          </example> and someone else releases an enhanced version of
          the <tt>bar</tt> package they can say:
          <example compact="compact">
Package: bar-plus
Provides: bar
          </example>
          and the <tt>bar-plus</tt> package will now also satisfy the
          dependency for the <tt>foo</tt> package.
        </p>

        <p>
          If a relationship field has a version number attached
          then only real packages will be considered to see whether
          the relationship is satisfied (or the prohibition violated,
          for a conflict or breakage) - it is assumed that a real
          package which provides the virtual package is not of the
          "right" version.  So, a <tt>Provides</tt> field may not
          contain version numbers, and the version number of the
          concrete package which provides a particular virtual package
          will not be looked at when considering a dependency on or
          conflict with the virtual package name.
        </p>

        <p>
          It is likely that the ability will be added in a future
          release of <prgn>dpkg</prgn> to specify a version number for
          each virtual package it provides.  This feature is not yet
          present, however, and is expected to be used only
          infrequently.
        </p>

        <p>
          If you want to specify which of a set of real packages
          should be the default to satisfy a particular dependency on
          a virtual package, you should list the real package as an
          alternative before the virtual one.
        </p>
      </sect>


      <sect id="replaces"><heading>Overwriting files and replacing
          packages - <tt>Replaces</tt></heading>

        <p>
          Packages can declare in their control file that they should
          overwrite files in certain other packages, or completely
          replace other packages. The <tt>Replaces</tt> control file
          field has these two distinct purposes.
        </p>

        <sect1><heading>Overwriting files in other packages</heading>

          <p>
            Firstly, as mentioned before, it is usually an error for a
            package to contain files which are on the system in
            another package.
          </p>

          <p>
            However, if the overwriting package declares that it
            <tt>Replaces</tt> the one containing the file being
            overwritten, then <prgn>dpkg</prgn> will replace the file
            from the old package with that from the new.  The file
            will no longer be listed as "owned" by the old package.
          </p>

          <p>
            If a package is completely replaced in this way, so that
            <prgn>dpkg</prgn> does not know of any files it still
            contains, it is considered to have "disappeared".  It will
            be marked as not wanted on the system (selected for
            removal) and not installed.  Any <tt>conffile</tt>s
            details noted for the package will be ignored, as they
            will have been taken over by the overwriting package.  The
            package's <prgn>postrm</prgn> script will be run with a
            special argument to allow the package to do any final
            cleanup required.  See <ref id="mscriptsinstact">.
            <footnote>
              <p>
                Replaces is a one way relationship -- you have to              
                install the replacing package after the replaced
                package.
              </p>
            </footnote>
          </p>

          <p>
            For this usage of <tt>Replaces</tt>, virtual packages (see
            <ref id="virtual">) are not considered when looking at a
            <tt>Replaces</tt> field - the packages declared as being
            replaced must be mentioned by their real names.
          </p>

          <p>
            Furthermore, this usage of <tt>Replaces</tt> only takes
            effect when both packages are at least partially on the
            system at once, so that it can only happen if they do not
            conflict or if the conflict has been overridden.
          </p>

        </sect1>

        <sect1><heading>Replacing whole packages, forcing their
            removal</heading>

          <p>
            Secondly, <tt>Replaces</tt> allows the packaging system to
            resolve which package should be removed when there is a
            conflict - see <ref id="conflicts">.  This usage only
            takes effect when the two packages <em>do</em> conflict,
            so that the two usages of this field do not interfere with
            each other.
          </p>

          <p>
            In this situation, the package declared as being replaced
            can be a virtual package, so for example, all mail
            transport agents (MTAs) would have the following fields in
            their control files:
            <example compact="compact">
Provides: mail-transport-agent
Conflicts: mail-transport-agent
Replaces: mail-transport-agent
            </example>
            ensuring that only one MTA can be installed at any one
            time.
        </sect1>
      </sect>

      <sect id="sourcebinarydeps">
        <heading>Relationships between source and binary packages -
          <tt>Build-Depends</tt>, <tt>Build-Depends-Indep</tt>,
          <tt>Build-Conflicts</tt>, <tt>Build-Conflicts-Indep</tt>
        </heading>

        <p>
          Source packages that require certain binary packages to be
          installed or absent at the time of building the package
          can declare relationships to those binary packages.
        </p>

        <p>
          This is done using the <tt>Build-Depends</tt>,
          <tt>Build-Depends-Indep</tt>, <tt>Build-Conflicts</tt> and
          <tt>Build-Conflicts-Indep</tt> control file fields.
        </p>

        <p>
          Build-dependencies on "build-essential" binary packages can be
          omitted. Please see <ref id="pkg-relations"> for more information.
        </p>

        <p>
          The dependencies and conflicts they define must be satisfied
          (as defined earlier for binary packages) in order to invoke
          the targets in <tt>debian/rules</tt>, as follows:<footnote>
            <p>
              If you make "build-arch" or "binary-arch", you need
              Build-Depends.  If you make "build-indep" or
              "binary-indep", you need Build-Depends and
              Build-Depends-Indep.  If you make "build" or "binary",
              you need both.
            </p>
            <p>
              There is no Build-Depends-Arch; this role is essentially
              met with Build-Depends.  Anyone building the
              <tt>build-indep</tt> and binary-indep<tt></tt> targets
              is basically assumed to be building the whole package
              anyway and so installs all build dependencies.  The
              autobuilders use <tt>dpkg-buildpackage -B</tt>, which
              calls <tt>build</tt> (not <tt>build-arch</tt>, since it
              does not yet know how to check for its existence) and
              <tt>binary-arch</tt>.
            </p>
            <p>
              The purpose of the original split, I recall, was so that
              the autobuilders wouldn't need to install extra packages
              needed only for the binary-indep targets.  But without a
              build-arch/build-indep split, this didn't work, since
              most of the work is done in the build target, not in the
              binary target.
            </p>
          </footnote>

          <taglist>
            <tag><tt>Build-Depends</tt>, <tt>Build-Conflicts</tt></tag>
            <item>
                The <tt>Build-Depends</tt> and
                <tt>Build-Conflicts</tt> fields must be satisfied when
                any of the following targets is invoked:
                <tt>build</tt>, <tt>clean</tt>, <tt>binary</tt>,
                <tt>binary-arch</tt>, <tt>build-arch</tt>,
                <tt>build-indep</tt> and <tt>binary-indep</tt>.
            </item>
            <tag><tt>Build-Depends-Indep</tt>,
              <tt>Build-Conflicts-Indep</tt></tag>
            <item>
                The <tt>Build-Depends-Indep</tt> and
                <tt>Build-Conflicts-Indep</tt> fields must be
                satisfied when any of the following targets is
                invoked: <tt>build</tt>, <tt>build-indep</tt>,
                <tt>binary</tt> and <tt>binary-indep</tt>.
            </item>
          </taglist>
        </p>

      </sect>

    </chapt>


    <chapt id="sharedlibs"><heading>Shared libraries</heading>

      <p>
        Packages containing shared libraries must be constructed with
        a little care to make sure that the shared library is always
        available.  This is especially important for packages whose
        shared libraries are vitally important, such as the C library
        (currently <tt>libc6</tt>).
      </p>

      <p>
        Packages involving shared libraries should be split up into
        several binary packages. This section mostly deals with how
        this separation is to be accomplished; rules for files within
        the shared library packages are in <ref id="libraries"> instead.
      </p>

      <sect id="sharedlibs-runtime">
        <heading>Run-time shared libraries</heading>

      <p>
        The run-time shared library needs to be placed in a package
        whose name changes whenever the shared object version
        changes.<footnote>
            <p>
              Since it is common place to install several versions of a
              package that just provides shared libraries, it is a
              good idea that the library package should not
              contain any extraneous non-versioned files, unless they
              happen to be in versioned directories.</p>
          </footnote>
          The most common mechanism is to place it in a package
        called
        <package><var>libraryname</var><var>soversion</var></package>,
        where <file><var>soversion</var></file> is the version number
        in the soname of the shared library<footnote>
              The soname is the shared object name: it's the thing
              that has to match exactly between building an executable
              and running it for the dynamic linker to be able run the
              program.  For example, if the soname of the library is
              <file>libfoo.so.6</file>, the library package would be
              called <file>libfoo6</file>.
          </footnote>.
        Alternatively, if it would be confusing to directly append
        <var>soversion</var> to <var>libraryname</var> (e.g. because
        <var>libraryname</var> itself ends in a number), you may use
        <package><var>libraryname</var>-<var>soversion</var></package> and
        <package><var>libraryname</var>-<var>soversion</var>-dev</package>
        instead.
      </p>

        <p>                                                                 
          If your package includes run-time support programs that            
          do not need to be invoked manually by users, but are               
          nevertheless required  for the package to function, then it        
          is recommended that these programs are placed                      
          (if they are binary) in a subdirectory of                          
          <file>/usr/lib</file>, preferably under                            
          <file>/usr/lib/</file><var>package-name</var>.                     
          If the program is architecture independent, the                    
          recommendation is for it to be placed in a subdirectory of         
          <file>/usr/share</file> instead, preferably under                  
          <file>/usr/share/</file><var>package-name</var>.                     
        </p>                                                                 
                                                                            

      <p>
        If you have several shared libraries built from the same
        source tree you may lump them all together into a single
        shared library package, provided that you change all of
        their sonames at once (so that you don't get filename
        clashes if you try to install different versions of the
        combined shared libraries package).
      </p>

      <p>
        The package should install the shared libraries under
        their normal names.  For example, the <package>libgdbm3</package>
        package should install <file>libgdbm.so.3.0.0</file> as
        <file>/usr/lib/libgdbm.so.3.0.0</file>.  The files should not be
        renamed or re-linked by any <prgn>prerm</prgn> or
        <prgn>postrm</prgn> scripts; <prgn>dpkg</prgn> will take care
        of renaming things safely without affecting running programs,
        and attempts to interfere with this are likely to lead to
        problems.
      </p>

      <p>
        Shared libraries should not be installed executable, since
        the dynamic linker does not require this and trying to
        execute a shared library usually results in a core dump.
      </p>

      <p>
        The run-time library package should include the symbolic link that
        <prgn>ldconfig</prgn> would create for the shared libraries.
        For example, the <package>libgdbm3</package> package should include
        a symbolic link from <file>/usr/lib/libgdbm.so.3</file> to
        <file>libgdbm.so.3.0.0</file>.  This is needed so that the dynamic
        linker (for example <prgn>ld.so</prgn> or
        <prgn>ld-linux.so.*</prgn>) can find the library between the
        time that <prgn>dpkg</prgn> installs it and the time that
        <prgn>ldconfig</prgn> is run in the <prgn>postinst</prgn>
        script.<footnote>
            The package management system requires the library to be
            placed before the symbolic link pointing to it in the
            <file>.deb</file> file.  This is so that when
            <prgn>dpkg</prgn> comes to install the symlink
            (overwriting the previous symlink pointing at an older
            version of the library), the new shared library is already
            in place.  In the past, this was achieved by creating the
            library in the temporary packaging directory before
            creating the symlink.  Unfortunately, this was not always
            effective, since the building of the tar file in the
            <file>.deb</file> depended on the behavior of the underlying
            file system.  Some file systems (such as reiserfs) reorder
            the files so that the order of creation is forgotten.
            Since version 1.7.0, <prgn>dpkg</prgn>
            reorders the files itself as necessary when building a
            package.  Thus it is no longer important to concern
            oneself with the order of file creation.
        </footnote>
      </p>

        <sect1 id="ldconfig">
          <heading><tt>ldconfig</tt></heading>

        <p>
          Any package installing shared libraries in one of the default
          library directories of the dynamic linker (which are currently
          <file>/usr/lib</file> and <file>/lib</file>) or a directory that is
          listed in <file>/etc/ld.so.conf</file><footnote>
            These are currently
            <list compact="compact">
              <item>/usr/local/lib</item>
              <item>/usr/lib/libc5-compat</item>
              <item>/lib/libc5-compat</item>
            </list>
          </footnote>
          must use <prgn>ldconfig</prgn> to update the shared library
          system.
        </p>

        <p>
            The package maintainer scripts must only call
            <prgn>ldconfig</prgn> under these circumstances:
            <list compact="compact">
              <item>When the <prgn>postinst</prgn> script is run with a
                  first argument of <tt>configure</tt>, the script must call
                  <prgn>ldconfig</prgn>, and may optionally invoke
                  <prgn>ldconfig</prgn> at other times.
              </item>
              <item>When the <prgn>postrm</prgn> script is run with a
                  first argument of <tt>remove</tt>, the script should call
                  <prgn>ldconfig</prgn>.
              </item>
            </list>
         <footnote>
            <p>
              During install or upgrade, the preinst is called before
              the new files are installed, so calling "ldconfig" is
              pointless.  The preinst of an existing package can also be
              called if an upgrade fails.  However, this happens during
              the critical time when a shared libs may exist on-disk
              under a temporary name.  Thus, it is dangerous and
              forbidden by current policy to call "ldconfig" at this
              time.
            </p>

            <p>
              When a package is installed or upgraded, "postinst
              configure" runs after the new files are safely on-disk.
              Since it is perfectly safe to invoke ldconfig
              unconditionally in a postinst, it is OK for a package to
              simply put ldconfig in its postinst without checking the
              argument.  The postinst can also be called to recover from
              a failed upgrade.  This happens before any new files are
              unpacked, so there is no reason to call "ldconfig" at this
              point.
            </p>

            <p>
              For a package that is being removed, prerm is
              called with all the files intact, so calling ldconfig is
              useless.  The other calls to "prerm" happen in the case of
              upgrade at a time when all the files of the old package
              are on-disk, so again calling "ldconfig" is pointless.
            </p>

            <p>
              postrm, on the other hand, is called with the "remove"
              argument just after the files are removed, so this is
              the proper time to call "ldconfig" to notify the system
              of the fact that the shared libraries from the package
              are removed.  The postrm can be called at several other
              times.  At the time of "postrm purge", "postrm
              abort-install", or "postrm abort-upgrade", calling
              "ldconfig" is useless because the shared lib files are
              not on-disk.  However, when "postrm" is invoked with
              arguments "upgrade", "failed-upgrade", or "disappear", a
              shared lib may exist on-disk under a temporary filename.
            </p>
          </footnote>
        </p>
        </sect1>

      </sect>

      <sect id="sharedlibs-runtime-progs">
        <heading>Run-time support programs</heading>

      <p>
        If your package has some run-time support programs which use
        the shared library you must not put them in the shared
        library package.  If you do that then you won't be able to
        install several versions of the shared library without
        getting filename clashes.
      </p>

      <p>
        Instead, either create another package for the runtime binaries
        (this package might typically be named
        <package><var>libraryname</var>-runtime</package>; note the absence
        of the <var>soversion</var> in the package name), or if the
        development package is small, include them in there.
      </p>
      </sect>

      <sect id="sharedlibs-static">
        <heading>Static libraries</heading>

      <p>
        The static library (<file><var>libraryname.a</var></file>)
        is usually provided in addition to the shared version.
        It is placed into the development package (see below).
      </p>

      <p>
        In some cases, it is acceptable for a library to be
        available in static form only; these cases include:
        <list>
          <item>libraries for languages whose shared library support
                is immature or unstable</item>
          <item>libraries whose interfaces are in flux or under
                development (commonly the case when the library's
                major version number is zero, or where the ABI breaks
                across patchlevels)</item>
          <item>libraries which are explicitly intended to be
                available only in static form by their upstream
                author(s)</item>
        </list>
      </p>

      <sect id="sharedlibs-dev">
        <heading>Development files</heading>

      <p>
        The development files associated to a shared library need to be
        placed in a package called
        <package><var>libraryname</var><var>soversion</var>-dev</package>,
        or if you prefer only to support one development version at a
        time, <package><var>libraryname</var>-dev</package>.
      </p>

      <p>
        In case several development versions of a library exist, you may
        need to use <prgn>dpkg</prgn>'s Conflicts mechanism (see
        <ref id="conflicts">) to ensure that the user only installs one
        development version at a time (as different development versions are
        likely to have the same header files in them, which would cause a
        filename clash if both were installed).
      </p>

      <p>
        The development package should contain a symlink for the associated
        shared library without a version number. For example, the
        <package>libgdbm-dev</package> package should include a symlink
        from <file>/usr/lib/libgdbm.so</file> to
        <file>libgdbm.so.3.0.0</file>.  This symlink is needed by the linker
        (<prgn>ld</prgn>) when compiling packages, as it will only look for
        <file>libgdbm.so</file> when compiling dynamically.
      </p>
      </sect>

      <sect id="sharedlibs-intradeps">
        <heading>Dependencies between the packages of the same library</heading>

        <p>
          Typically the development version should have an exact
          version dependency on the runtime library, to make sure that
          compilation and linking happens correctly.  The
          <tt>${binary:Version}</tt> substitution variable can be
          useful for this purpose.
          <footnote>
            Previously, <tt>${Source-Version}</tt> was used, but its name
            was confusing and it has been deprecated since dpkg 1.13.19.
          </footnote>
        </p>
      </sect>

      <sect id="sharedlibs-shlibdeps">
        <heading>Dependencies between the library and other packages -
        the <tt>shlibs</tt> system</heading>

        <p>
          If a package contains a binary or library which links to a
          shared library, we must ensure that when the package is
          installed on the system, all of the libraries needed are
          also installed.  This requirement led to the creation of the
          <tt>shlibs</tt> system, which is very simple in its design:
          any package which <em>provides</em> a shared library also
          provides information on the package dependencies required to
          ensure the presence of this library, and any package which
          <em>uses</em> a shared library uses this information to
          determine the dependencies it requires.  The files which
          contain the mapping from shared libraries to the necessary
          dependency information are called <file>shlibs</file> files.
        </p>

        <p>
          Thus, when a package is built which contains any shared
          libraries, it must provide a <file>shlibs</file> file for other
          packages to use, and when a package is built which contains
          any shared libraries or compiled binaries, it must run
          <qref id="pkg-dpkg-shlibdeps"><prgn>dpkg-shlibdeps</prgn></qref>
          on these to determine the libraries used and hence the
          dependencies needed by this package.<footnote>
            <p>
              In the past, the shared libraries linked to were
              determined by calling <prgn>ldd</prgn>, but now
              <prgn>objdump</prgn> is used to do this.  The only
              change this makes to package building is that
              <prgn>dpkg-shlibdeps</prgn> must also be run on shared
              libraries, whereas in the past this was unnecessary.
              The rest of this footnote explains the advantage that
              this method gives.
            </p>

            <p>
              We say that a binary <tt>foo</tt> <em>directly</em> uses
              a library <tt>libbar</tt> if it is explicitly linked
              with that library (that is, it uses the flag
              <tt>-lbar</tt> during the linking stage).  Other
              libraries that are needed by <tt>libbar</tt> are linked
              <em>indirectly</em> to <tt>foo</tt>, and the dynamic
              linker will load them automatically when it loads
              <tt>libbar</tt>.  A package should depend on
              the libraries it directly uses, and the dependencies for
              those libraries should automatically pull in the other
              libraries.
            </p>

            <p>
              Unfortunately, the <prgn>ldd</prgn> program shows both
              the directly and indirectly used libraries, meaning that
              the dependencies determined included both direct and
              indirect dependencies.  The use of <prgn>objdump</prgn>
              avoids this problem by determining only the directly
              used libraries.
            </p>

            <p>
              A good example of where this helps is the following.  We
              could update <tt>libimlib</tt> with a new version that
              supports a new graphics format called dgf (but retaining
              the same major version number).  If we used the old
              <prgn>ldd</prgn> method, every package that uses
              <tt>libimlib</tt> would need to be recompiled so it
              would also depend on <tt>libdgf</tt> or it wouldn't run
              due to missing symbols.  However with the new system,
              packages using <tt>libimlib</tt> can rely on
              <tt>libimlib</tt> itself having the dependency on
              <tt>libdgf</tt> and so they would not need rebuilding.
            </p>
          </footnote>
        </p>

        <p>
          In the following sections, we will first describe where the
          various <tt>shlibs</tt> files are to be found, then how to
          use <prgn>dpkg-shlibdeps</prgn>, and finally the <tt>shlibs</tt>
          file format and how to create them if your package contains a
          shared library.
        </p>

      <sect1>
        <heading>The <tt>shlibs</tt> files present on the system</heading>

        <p>
          There are several places where <tt>shlibs</tt> files are
          found.  The following list gives them in the order in which
          they are read by
          <qref id="pkg-dpkg-shlibdeps"><prgn>dpkg-shlibdeps</prgn></qref>.
          (The first one which gives the required information is used.)
        </p>

        <p>
          <list>
            <item>
              <p><file>debian/shlibs.local</file></p>

              <p>
                This lists overrides for this package.  Its use is
                described below (see <ref id="shlibslocal">).
              </p>
            </item>

            <item>
              <p><file>/etc/dpkg/shlibs.override</file></p>

              <p>
                This lists global overrides.  This list is normally
                empty.  It is maintained by the local system
                administrator.
              </p>
            </item>

            <item>
              <p><file>DEBIAN/shlibs</file> files in the "build directory"</p>

              <p>
                When packages are being built, any
                <file>debian/shlibs</file> files are copied into the
                control file area of the temporary build directory and
                given the name <file>shlibs</file>.  These files give
                details of any shared libraries included in the
                package.<footnote>
                    An example may help here.  Let us say that the
                    source package <tt>foo</tt> generates two binary
                    packages, <tt>libfoo2</tt> and
                    <tt>foo-runtime</tt>.  When building the binary
                    packages, the two packages are created in the
                    directories <file>debian/libfoo2</file> and
                    <file>debian/foo-runtime</file> respectively.
                    (<file>debian/tmp</file> could be used instead of one
                    of these.)  Since <tt>libfoo2</tt> provides the
                    <tt>libfoo</tt> shared library, it will require a
                    <tt>shlibs</tt> file, which will be installed in
                    <file>debian/libfoo2/DEBIAN/shlibs</file>, eventually
                    to become
                    <file>/var/lib/dpkg/info/libfoo2.shlibs</file>.  Then
                    when <prgn>dpkg-shlibdeps</prgn> is run on the
                    executable
                    <file>debian/foo-runtime/usr/bin/foo-prog</file>, it
                    will examine the
                    <file>debian/libfoo2/DEBIAN/shlibs</file> file to
                    determine whether <tt>foo-prog</tt>'s library
                    dependencies are satisfied by any of the libraries
                    provided by <tt>libfoo2</tt>.  For this reason,
                    <prgn>dpkg-shlibdeps</prgn> must only be run once
                    all of the individual binary packages'
                    <tt>shlibs</tt> files have been installed into the
                    build directory.
                </footnote>
              </p>
            </item>

            <item>
              <p><file>/var/lib/dpkg/info/*.shlibs</file></p>

              <p>
                These are the <file>shlibs</file> files corresponding to
                all of the packages installed on the system, and are
                maintained by the relevant package maintainers.
              </p>
            </item>

            <item>
              <p><file>/etc/dpkg/shlibs.default</file></p>

              <p>
                This file lists any shared libraries whose packages
                have failed to provide correct <file>shlibs</file> files.
                It was used when the <file>shlibs</file> setup was first
                introduced, but it is now normally empty.  It is
                maintained by the <tt>dpkg</tt> maintainer.
              </p>
            </item>
          </list>
        </p>
      </sect1>

      <sect1>
        <heading>How to use <prgn>dpkg-shlibdeps</prgn> and the
            <file>shlibs</file> files</heading>

        <p>
          Put a call to
          <qref id="pkg-dpkg-shlibdeps"><prgn>dpkg-shlibdeps</prgn></qref>
          into your <file>debian/rules</file> file.  If your package
          contains only compiled binaries and libraries (but no scripts),
          you can use a command such as:
          <example compact="compact">
dpkg-shlibdeps debian/tmp/usr/bin/* debian/tmp/usr/sbin/* \
  debian/tmp/usr/lib/*
          </example>
          Otherwise, you will need to explicitly list the compiled
          binaries and libraries.<footnote>
              If you are using <tt>debhelper</tt>, the
              <prgn>dh_shlibdeps</prgn> program will do this work for
              you.  It will also correctly handle multi-binary
              packages.
          </footnote>
        </p>

        <p>
          This command puts the dependency information into the
          <file>debian/substvars</file> file, which is then used by
          <prgn>dpkg-gencontrol</prgn>.  You will need to place a
          <tt>${shlibs:Depends}</tt> variable in the <tt>Depends</tt>
          field in the control file for this to work.
        </p>

        <p>
          If <prgn>dpkg-shlibdeps</prgn> doesn't complain, you're
          done.  If it does complain you might need to create your own
          <file>debian/shlibs.local</file> file, as explained below (see
          <ref id="shlibslocal">).
        </p>

        <p>
          If you have multiple binary packages, you will need to call
          <prgn>dpkg-shlibdeps</prgn> on each one which contains
          compiled libraries or binaries.  In such a case, you will
          need to use the <tt>-T</tt> option to the <tt>dpkg</tt>
          utilities to specify a different <file>substvars</file> file.
        </p>

        <p>
          If you are creating a udeb for use in the Debian Installer, you
          will need to specify that <prgn>dpkg-shlibdeps</prgn> should use
          the dependency line of type <tt>udeb</tt> by adding
          <tt>-tudeb</tt> as option<footnote>
              <prgn>dh_shlibdeps</prgn> from the <tt>debhelper</tt> suite
              will automatically add this option if it knows it is
              processing a udeb.
          </footnote>. If there is no dependency line of type <tt>udeb</tt>
          in the <file>shlibs</file> file, <prgn>dpkg-shlibdeps</prgn> will
          fall back to the regular dependency line.
        </p>

        <p>
          For more details on dpkg-shlibdeps, please see
          <ref id="pkg-dpkg-shlibdeps"> and
          <manref name="dpkg-shlibdeps" section="1">.
        </p>
      </sect1>

      <sect1 id="shlibs">
        <heading>The <file>shlibs</file> File Format</heading>

        <p>
          Each <file>shlibs</file> file has the same format.  Lines
          beginning with <tt>#</tt> are considered to be comments and
          are ignored.  Each line is of the form:
          <example compact="compact">
[<var>type</var>: ]<var>library-name</var> <var>soname-version</var> <var>dependencies ...</var>
          </example>
        </p>

        <p>
          We will explain this by reference to the example of the
          <tt>zlib1g</tt> package, which (at the time of writing)
          installs the shared library <file>/usr/lib/libz.so.1.1.3</file>.
        </p>

        <p>
          <var>type</var> is an optional element that indicates the type
          of package for which the line is valid. The only type currently
          in use is <tt>udeb</tt>. The colon and space after the type are
          required.
        </p>

        <p>
          <var>library-name</var> is the name of the shared library,
          in this case <tt>libz</tt>.  (This must match the name part
          of the soname, see below.)
        </p>

        <p>
          <var>soname-version</var> is the version part of the soname of
          the library.  The soname is the thing that must exactly match
          for the library to be recognized by the dynamic linker, and is
          usually of the form
          <tt><var>name</var>.so.<var>major-version</var></tt>, in our
          example, <tt>libz.so.1</tt>.<footnote>
              This can be determined using the command
              <example compact="compact">
objdump -p /usr/lib/libz.so.1.1.3 | grep SONAME
              </example>
          </footnote>
          The version part is the part which comes after
          <tt>.so.</tt>, so in our case, it is <tt>1</tt>.
        </p>

        <p>
          <var>dependencies</var> has the same syntax as a dependency
          field in a binary package control file.  It should give
          details of which packages are required to satisfy a binary
          built against the version of the library contained in the
          package.  See <ref id="depsyntax"> for details.
        </p>

        <p>
          In our example, if the first version of the <tt>zlib1g</tt>
          package which contained a minor number of at least
          <tt>1.3</tt> was <var>1:1.1.3-1</var>, then the
          <tt>shlibs</tt> entry for this library could say:
          <example compact="compact">
libz 1 zlib1g (>= 1:1.1.3)
          </example>
          The version-specific dependency is to avoid warnings from
          the dynamic linker about using older shared libraries with
          newer binaries.
        </p>

        <p>
          As zlib1g also provides a udeb containing the shared library,
          there would also be a second line:
          <example compact="compact">
udeb: libz 1 zlib1g-udeb (>= 1:1.1.3)
          </example>
        </p>
      </sect1>

      <sect1>
        <heading>Providing a <file>shlibs</file> file</heading>

        <p>
          If your package provides a shared library, you need to create
          a <file>shlibs</file> file following the format described above.
          It is usual to call this file <file>debian/shlibs</file> (but if
          you have multiple binary packages, you might want to call it
          <file>debian/shlibs.<var>package</var></file> instead).  Then
          let <file>debian/rules</file> install it in the control area:
          <example compact="compact">
install -m644 debian/shlibs debian/tmp/DEBIAN
          </example>
          or, in the case of a multi-binary package:
          <example compact="compact">
install -m644 debian/shlibs.<var>package</var> debian/<var>package</var>/DEBIAN/shlibs
          </example>
          An alternative way of doing this is to create the
          <file>shlibs</file> file in the control area directly from
          <file>debian/rules</file> without using a <file>debian/shlibs</file>
          file at all,<footnote>
              This is what <prgn>dh_makeshlibs</prgn> in the
              <tt>debhelper</tt> suite does. If your package also has a udeb
              that provides a shared library, <prgn>dh_makeshlibs</prgn> can
              automatically generate the <tt>udeb:</tt> lines if you specify
              the name of the udeb with the <tt>--add-udeb</tt> option.
          </footnote>
          since the <file>debian/shlibs</file> file itself is ignored by
          <prgn>dpkg-shlibdeps</prgn>.
        </p>

        <p>
          As <prgn>dpkg-shlibdeps</prgn> reads the
          <file>DEBIAN/shlibs</file> files in all of the binary packages
          being built from this source package, all of the
          <file>DEBIAN/shlibs</file> files should be installed before
          <prgn>dpkg-shlibdeps</prgn> is called on any of the binary
          packages.
        </p>
      </sect1>

      <sect1 id="shlibslocal">
        <heading>Writing the <file>debian/shlibs.local</file> file</heading>

        <p>
          This file is intended only as a <em>temporary</em> fix if
          your binaries or libraries depend on a library whose package
          does not yet provide a correct <file>shlibs</file> file.
        </p>

        <p>
          We will assume that you are trying to package a binary
          <tt>foo</tt>.  When you try running
          <prgn>dpkg-shlibdeps</prgn> you get the following error
          message (<tt>-O</tt> displays the dependency information on
          <tt>stdout</tt> instead of writing it to
          <tt>debian/substvars</tt>, and the lines have been wrapped
          for ease of reading):
          <example compact="compact">
$ dpkg-shlibdeps -O debian/tmp/usr/bin/foo
dpkg-shlibdeps: warning: unable to find dependency
  information for shared library libbar (soname 1,
  path /usr/lib/libbar.so.1, dependency field Depends)
shlibs:Depends=libc6 (>= 2.2.2-2)
          </example>
          You can then run <prgn>ldd</prgn> on the binary to find the
          full location of the library concerned:
          <example compact="compact">
$ ldd foo
libbar.so.1 => /usr/lib/libbar.so.1 (0x4001e000)
libc.so.6 => /lib/libc.so.6 (0x40032000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x40000000)
          </example>
          So the <prgn>foo</prgn> binary depends on the
          <prgn>libbar</prgn> shared library, but no package seems to
          provide a <file>*.shlibs</file> file handling
          <file>libbar.so.1</file> in <file>/var/lib/dpkg/info/</file>.  Let's
          determine the package responsible:
          <example compact="compact">
$ dpkg -S /usr/lib/libbar.so.1
bar1: /usr/lib/libbar.so.1
$ dpkg -s bar1 | grep Version
Version: 1.0-1
          </example>
          This tells us that the <tt>bar1</tt> package, version 1.0-1,
          is the one we are using.  Now we can file a bug against the
          <tt>bar1</tt> package and create our own
          <file>debian/shlibs.local</file> to locally fix the problem.
          Including the following line into your
          <file>debian/shlibs.local</file> file:
          <example compact="compact">
libbar 1 bar1 (>= 1.0-1)
          </example>
          should allow the package build to work.
        </p>

        <p>
          As soon as the maintainer of <tt>bar1</tt> provides a
          correct <file>shlibs</file> file, you should remove this line
          from your <file>debian/shlibs.local</file> file.  (You should
          probably also then have a versioned <tt>Build-Depends</tt>
          on <tt>bar1</tt> to help ensure that others do not have the
          same problem building your package.)
        </p>
      </sect1>

      </sect>

    </chapt>


    <chapt id="opersys"><heading>The Operating System</heading>

      <sect>
        <heading>File system hierarchy</heading>


        <sect1 id="fhs">
          <heading>File system Structure</heading>

          <p>
            The location of all installed files and directories must
            comply with the File system Hierarchy Standard (FHS),
            version 2.3, with the exceptions noted below, and except
            where doing so would violate other terms of Debian
            Policy.  The following exceptions to the FHS apply:

            <enumlist>
              <item>
                <p>
                  Legacy XFree86 servers are permitted to retain the
                  configuration file location 
                  <file>/etc/X11/XF86Config-4</file>.
                </p>
              </item>
              <item>
                <p>
                  The optional rules related to user specific
                  configuration files for applications are stored in
                  the user's home directory are relaxed.  It is
                  recommended that such files start with the
                  '<tt>.</tt>' character (a "dot file"), and if an
                  application needs to create more than one dot file
                  then the preferred placement is in a subdirectory
                  with a name starting with a '.' character, (a "dot
                  directory"). In this case it is recommended the
                  configuration files not start with the '.'
                  character.
                </p>
              </item>
              <item>
                <p>
                  The requirement for amd64 to use <file>/lib64</file>
                  for 64 bit binaries is removed.
                </p>
              </item>
              <item>
                <p>
                  The requirement that
                  <file>/usr/local/share/man</file> be "synonymous"
                  with <file>/usr/local/man</file> is relaxed to a
                  recommendation</p>
              </item>
              <item>
                <p>
                  The requirement that windowmanagers with a single
                  configuration file call it <file>system.*wmrc</file>
                  is removed, as is the restriction that the window
                  manager subdirectory be named identically to the
                  window manager name itself.
                </p>
              </item>
              <item>
                <p>
                  The requirement that boot manager configuration
                  files live in <file>/etc</file>, or at least are
                  symlinked there, is relaxed to a recommendation.
                </p>
              </item>
            </enumlist>

          </p>
          <p>
            The version of this document referred here can be
            found in the <tt>debian-policy</tt> package or on <url
            id="http://www.debian.org/doc/packaging-manuals/fhs/"
              name="FHS (Debian copy)"> alongside this manual (or, if
            you have the <package>debian-policy</package> installed,
            you can try <url
              id="file:///usr/share/doc/debian-policy/fhs/" name="FHS
              (local copy)">). The
            latest version, which may be a more recent version, may
            be found on
            <url id="http://www.pathname.com/fhs/" name="FHS (upstream)">.
            Specific questions about following the standard may be
            asked on the <tt>debian-devel</tt> mailing list, or
            referred to the FHS mailing list (see the
            <url id="http://www.pathname.com/fhs/" name="FHS web site"> for
            more information).
          </p>
        </sect1>

        <sect1>
          <heading>Site-specific programs</heading>

          <p>
            As mandated by the FHS, packages must not place any
            files in <file>/usr/local</file>, either by putting them in
            the file system archive to be unpacked by <prgn>dpkg</prgn>
            or by manipulating them in their maintainer scripts.
          </p>

          <p>
            However, the package may create empty directories below
            <file>/usr/local</file> so that the system administrator knows
            where to place site-specific files.  These are not
            directories <em>in</em> <file>/usr/local</file>, but are
            children of directories in <file>/usr/local</file>.  These
            directories (<file>/usr/local/*/dir/</file>)
            should be removed on package removal if they are
            empty.
          </p>

          <p>
            Note, that this applies only to directories <em>below</em>
            <file>/usr/local</file>, not <em>in</em> <file>/usr/local</file>.
            Packages must not create sub-directories in the directory
            <file>/usr/local</file> itself, except those listed in FHS,
            section 4.5.  However, you may create directories below
            them as you wish. You must not remove any of the
            directories listed in 4.5, even if you created them.
          </p>

          <p>
            Since <file>/usr/local</file> can be mounted read-only from a
            remote server, these directories must be created and
            removed by the <prgn>postinst</prgn> and <prgn>prerm</prgn>
            maintainer scripts and not be included in the
            <file>.deb</file> archive.  These scripts must not fail if
            either of these operations fail.
          </p>

          <p>
            For example, the <tt>emacsen-common</tt> package could
            contain something like
            <example compact="compact">
if [ ! -e /usr/local/share/emacs ]
then
  if mkdir /usr/local/share/emacs 2>/dev/null
  then
    chown root:staff /usr/local/share/emacs
    chmod 2775 /usr/local/share/emacs
  fi
fi
            </example>
            in its <prgn>postinst</prgn> script, and
            <example compact="compact">
rmdir /usr/local/share/emacs/site-lisp 2>/dev/null || true
rmdir /usr/local/share/emacs 2>/dev/null || true
            </example>
            in the <prgn>prerm</prgn> script.  (Note that this form is
            used to ensure that if the script is interrupted, the
            directory <file>/usr/local/share/emacs</file> will still be
            removed.)
          </p>

          <p>
            If you do create a directory in <file>/usr/local</file> for
            local additions to a package, you should ensure that
            settings in <file>/usr/local</file> take precedence over the
            equivalents in <file>/usr</file>.
          </p>

          <p>
            However, because <file>/usr/local</file> and its contents are
            for exclusive use of the local administrator, a package
            must not rely on the presence or absence of files or
            directories in <file>/usr/local</file> for normal operation.
          </p>

          <p>
            The <file>/usr/local</file> directory itself and all the
            subdirectories created by the package should (by default) have
            permissions 2775 (group-writable and set-group-id) and be
            owned by <tt>root.staff</tt>.
          </p>
        </sect1>

        <sect1>
          <heading>The system-wide mail directory</heading>
          <p>
            The system-wide mail directory is <file>/var/mail</file>. This
            directory is part of the base system and should not owned
            by any particular mail agents.  The use of the old
            location <file>/var/spool/mail</file> is deprecated, even
            though the spool may still be physically located there.
            To maintain partial upgrade compatibility for systems
            which have <file>/var/spool/mail</file> as their physical mail
            spool, packages using <file>/var/mail</file> must depend on
            either <package>libc6</package> (&gt;= 2.1.3-13), or on
            <package>base-files</package> (&gt;= 2.2.0), or on later
            versions of either one of these packages.
          </p>
        </sect1>
      </sect>

      <sect>
        <heading>Users and groups</heading>

        <sect1>
          <heading>Introduction</heading>
          <p>
            The Debian system can be configured to use either plain or
            shadow passwords.
          </p>

          <p>
            Some user ids (UIDs) and group ids (GIDs) are reserved
            globally for use by certain packages.  Because some
            packages need to include files which are owned by these
            users or groups, or need the ids compiled into binaries,
            these ids must be used on any Debian system only for the
            purpose for which they are allocated. This is a serious
            restriction, and we should avoid getting in the way of
            local administration policies. In particular, many sites
            allocate users and/or local system groups starting at 100.
          </p>

          <p>
            Apart from this we should have dynamically allocated ids,
            which should by default be arranged in some sensible
            order, but the behavior should be configurable.
          </p>

          <p>
            Packages other than <tt>base-passwd</tt> must not modify
            <file>/etc/passwd</file>, <file>/etc/shadow</file>,
            <file>/etc/group</file> or <file>/etc/gshadow</file>.
          </p>
        </sect1>

        <sect1>
          <heading>UID and GID classes</heading>
          <p>
            The UID and GID numbers are divided into classes as
            follows:
            <taglist>
              <tag>0-99:</tag>
              <item>
                <p>
                  Globally allocated by the Debian project, the same
                  on every Debian system.  These ids will appear in
                  the <file>passwd</file> and <file>group</file> files of all
                  Debian systems, new ids in this range being added
                  automatically as the <tt>base-passwd</tt> package is
                  updated.
                </p>

                <p>
                  Packages which need a single statically allocated
                  uid or gid should use one of these; their
                  maintainers should ask the <tt>base-passwd</tt>
                  maintainer for ids.
                </p>
              </item>

              <tag>100-999:</tag>
              <item>
                <p>
                  Dynamically allocated system users and groups.
                  Packages which need a user or group, but can have
                  this user or group allocated dynamically and
                  differently on each system, should use <tt>adduser
                  --system</tt> to create the group and/or user.
                  <prgn>adduser</prgn> will check for the existence of
                  the user or group, and if necessary choose an unused
                  id based on the ranges specified in
                  <file>adduser.conf</file>.
                </p>
              </item>

              <tag>1000-29999:</tag>
              <item>
                <p>
                  Dynamically allocated user accounts.  By default
                  <prgn>adduser</prgn> will choose UIDs and GIDs for
                  user accounts in this range, though
                  <file>adduser.conf</file> may be used to modify this
                  behavior.
                </p>
              </item>

              <tag>30000-59999:</tag>
              <item>
                <p>Reserved.</p>
              </item>

              <tag>60000-64999:</tag>
              <item>
                <p>
                  Globally allocated by the Debian project, but only
                  created on demand. The ids are allocated centrally
                  and statically, but the actual accounts are only
                  created on users' systems on demand.
                </p>

                <p>
                  These ids are for packages which are obscure or
                  which require many statically-allocated ids.  These
                  packages should check for and create the accounts in
                  <file>/etc/passwd</file> or <file>/etc/group</file> (using
                  <prgn>adduser</prgn> if it has this facility) if
                  necessary.  Packages which are likely to require
                  further allocations should have a "hole" left after
                  them in the allocation, to give them room to
                  grow.
                </p>
              </item>

              <tag>65000-65533:</tag>
              <item>
                <p>Reserved.</p>
              </item>

              <tag>65534:</tag>
              <item>
                <p>
                  User <tt>nobody</tt>. The corresponding gid refers
                  to the group <tt>nogroup</tt>.
                </p>
              </item>

              <tag>65535:</tag>
              <item>
                <p>
                  <tt>(uid_t)(-1) == (gid_t)(-1)</tt> <em>must
                  not</em> be used, because it is the error return
                  sentinel value.
                </p>
              </item>
            </taglist>
          </p>
        </sect1>
      </sect>

      <sect id="sysvinit">
        <heading>System run levels and <file>init.d</file> scripts</heading>

        <sect1 id="/etc/init.d">
          <heading>Introduction</heading>

          <p>
            The <file>/etc/init.d</file> directory contains the scripts
            executed by <prgn>init</prgn> at boot time and when the
            init state (or "runlevel") is changed (see <manref
            name="init" section="8">).
          </p>

          <p>
            There are at least two different, yet functionally
            equivalent, ways of handling these scripts.  For the sake
            of simplicity, this document describes only the symbolic
            link method. However, it must not be assumed by maintainer
            scripts that this method is being used, and any automated
            manipulation of the various runlevel behaviors by
            maintainer scripts must be performed using
            <prgn>update-rc.d</prgn> as described below and not by
            manually installing or removing symlinks.  For information
            on the implementation details of the other method,
            implemented in the <tt>file-rc</tt> package, please refer
            to the documentation of that package.
          </p>

          <p>
            These scripts are referenced by symbolic links in the
            <file>/etc/rc<var>n</var>.d</file> directories.  When changing
            runlevels, <prgn>init</prgn> looks in the directory
            <file>/etc/rc<var>n</var>.d</file> for the scripts it should
            execute, where <tt><var>n</var></tt> is the runlevel that
            is being changed to, or <tt>S</tt> for the boot-up
            scripts.
          </p>

          <p>
            The names of the links all have the form
            <file>S<var>mm</var><var>script</var></file> or
            <file>K<var>mm</var><var>script</var></file> where
            <var>mm</var> is a two-digit number and <var>script</var>
            is the name of the script (this should be the same as the
            name of the actual script in <file>/etc/init.d</file>).
          </p>

          <p>
            When <prgn>init</prgn> changes runlevel first the targets
            of the links whose names start with a <tt>K</tt> are
            executed, each with the single argument <tt>stop</tt>,
            followed by the scripts prefixed with an <tt>S</tt>, each
            with the single argument <tt>start</tt>.  (The links are
            those in the <file>/etc/rc<var>n</var>.d</file> directory
            corresponding to the new runlevel.)  The <tt>K</tt> links
            are responsible for killing services and the <tt>S</tt>
            link for starting services upon entering the runlevel.
          </p>

          <p>
            For example, if we are changing from runlevel 2 to
            runlevel 3, init will first execute all of the <tt>K</tt>
            prefixed scripts it finds in <file>/etc/rc3.d</file>, and then
            all of the <tt>S</tt> prefixed scripts in that directory.
            The links starting with <tt>K</tt> will cause the
            referred-to file to be executed with an argument of
            <tt>stop</tt>, and the <tt>S</tt> links with an argument
            of <tt>start</tt>.
          </p>

          <p>
            The two-digit number <var>mm</var> is used to determine
            the order in which to run the scripts: low-numbered links
            have their scripts run first.  For example, the
            <tt>K20</tt> scripts will be executed before the
            <tt>K30</tt> scripts.  This is used when a certain service
            must be started before another.  For example, the name
            server <prgn>bind</prgn> might need to be started before
            the news server <prgn>inn</prgn> so that <prgn>inn</prgn>
            can set up its access lists.  In this case, the script
            that starts <prgn>bind</prgn> would have a lower number
            than the script that starts <prgn>inn</prgn> so that it
            runs first:
            <example compact="compact">
/etc/rc2.d/S17bind
/etc/rc2.d/S70inn
            </example>
          </p>

          <p>
            The two runlevels 0 (halt) and 6 (reboot) are slightly
            different.  In these runlevels, the links with an
            <tt>S</tt> prefix are still called after those with a
            <tt>K</tt> prefix, but they too are called with the single
            argument <tt>stop</tt>.
          </p>

          <p>
            Also, if the script name ends in <tt>.sh</tt>, the script
            will be sourced in runlevel <tt>S</tt> rather than being
            run in a forked subprocess, but will be explicitly run by
            <prgn>sh</prgn> in all other runlevels.
          </p>
        </sect1>

        <sect1>
          <heading>Writing the scripts</heading>

          <p>
            Packages that include daemons for system services should
            place scripts in <file>/etc/init.d</file> to start or stop
            services at boot time or during a change of runlevel.
            These scripts should be named
            <file>/etc/init.d/<var>package</var></file>, and they should
            accept one argument, saying what to do:

            <taglist>
              <tag><tt>start</tt></tag>
              <item>start the service,</item>

              <tag><tt>stop</tt></tag>
              <item>stop the service,</item>

              <tag><tt>restart</tt></tag>
              <item>stop and restart the service if it's already running,
                  otherwise start the service</item>

              <tag><tt>reload</tt></tag>
              <item><p>cause the configuration of the service to be
                  reloaded without actually stopping and restarting
                  the service,</item>

              <tag><tt>force-reload</tt></tag>
              <item>cause the configuration to be reloaded if the
                  service supports this, otherwise restart the
                  service.</item>
            </taglist>

            The <tt>start</tt>, <tt>stop</tt>, <tt>restart</tt>, and
            <tt>force-reload</tt> options should be supported by all
            scripts in <file>/etc/init.d</file>, the <tt>reload</tt>
            option is optional.
          </p>

          <p>
            The <file>init.d</file> scripts must ensure that they will
            behave sensibly if invoked with <tt>start</tt> when the
            service is already running, or with <tt>stop</tt> when it
            isn't, and that they don't kill unfortunately-named user
            processes.  The best way to achieve this is usually to use
            <prgn>start-stop-daemon</prgn>.
          </p>

          <p>
            If a service reloads its configuration automatically (as
            in the case of <prgn>cron</prgn>, for example), the
            <tt>reload</tt> option of the <file>init.d</file> script
            should behave as if the configuration has been reloaded
            successfully.
          </p>

          <p>
            The <file>/etc/init.d</file> scripts must be treated as
            configuration files, either (if they are present in the
            package, that is, in the .deb file) by marking them as
            <tt>conffile</tt>s, or, (if they do not exist in the .deb)
            by managing them correctly in the maintainer scripts (see
            <ref id="config-files">).  This is important since we want
            to give the local system administrator the chance to adapt
            the scripts to the local system, e.g., to disable a
            service without de-installing the package, or to specify
            some special command line options when starting a service,
            while making sure their changes aren't lost during the next
            package upgrade.
          </p>

          <p>
            These scripts should not fail obscurely when the
            configuration files remain but the package has been
            removed, as configuration files remain on the system after
            the package has been removed.  Only when <prgn>dpkg</prgn>
            is executed with the <tt>--purge</tt> option will
            configuration files be removed.  In particular, as the
            <file>/etc/init.d/<var>package</var></file> script itself is
            usually a <tt>conffile</tt>, it will remain on the system
            if the package is removed but not purged.  Therefore, you
            should include a <tt>test</tt> statement at the top of the
            script, like this:
            <example compact="compact">
test -f <var>program-executed-later-in-script</var> || exit 0
            </example>
          </p>

          <p>
            Often there are some variables in the <file>init.d</file>
            scripts whose values control the behavior of the scripts,
            and which a system administrator is likely to want to
            change.  As the scripts themselves are frequently
            <tt>conffile</tt>s, modifying them requires that the
            administrator merge in their changes each time the package
            is upgraded and the <tt>conffile</tt> changes.  To ease
            the burden on the system administrator, such configurable
            values should not be placed directly in the script.
            Instead, they should be placed in a file in
            <file>/etc/default</file>, which typically will have the same
            base name as the <file>init.d</file> script.  This extra file
            should be sourced by the script when the script runs.  It
            must contain only variable settings and comments in SUSv3
            <prgn>sh</prgn> format.  It may either be a
            <tt>conffile</tt> or a configuration file maintained by
            the package maintainer scripts.  See <ref id="config-files">
            for more details.
          </p>

          <p>
            To ensure that vital configurable values are always
            available, the <file>init.d</file> script should set default
            values for each of the shell variables it uses, either
            before sourcing the <file>/etc/default/</file> file or
            afterwards using something like the <tt>:
            ${VAR:=default}</tt> syntax.  Also, the <file>init.d</file>
            script must behave sensibly and not fail if the
            <file>/etc/default</file> file is deleted.
          </p>
        </sect1>

        <sect1>
          <heading>Interfacing with the initscript system</heading>

          <p>
            Maintainers should use the abstraction layer provided by
            the <prgn>update-rc.d</prgn> and <prgn>invoke-rc.d</prgn>
            programs to deal with initscripts in their packages'
            scripts such as <prgn>postinst</prgn>, <prgn>prerm</prgn>
            and <prgn>postrm</prgn>.
          </p>

          <p>
            Directly managing the /etc/rc?.d links and directly
            invoking the <file>/etc/init.d/</file> initscripts should
            be done only by packages providing the initscript
            subsystem (such as <prgn>sysv-rc</prgn> and
            <prgn>file-rc</prgn>).
          </p>

          <sect2>
            <heading>Managing the links</heading>

            <p>
              The program <prgn>update-rc.d</prgn> is provided for
              package maintainers to arrange for the proper creation and
              removal of <file>/etc/rc<var>n</var>.d</file> symbolic links,
              or their functional equivalent if another method is being
              used.  This may be used by maintainers in their packages'
              <prgn>postinst</prgn> and <prgn>postrm</prgn> scripts.
            </p>

            <p>
              You must not include any <file>/etc/rc<var>n</var>.d</file>
              symbolic links in the actual archive or manually create or
              remove the symbolic links in maintainer scripts; you must
              use the <prgn>update-rc.d</prgn> program instead.  (The
              former will fail if an alternative method of maintaining
              runlevel information is being used.)  You must not include
              the <file>/etc/rc<var>n</var>.d</file> directories themselves
              in the archive either.  (Only the <tt>sysvinit</tt>
              package may do so.)
            </p>

            <p>
              By default <prgn>update-rc.d</prgn> will start services in
              each of the multi-user state runlevels (2, 3, 4, and 5)
              and stop them in the halt runlevel (0), the single-user
              runlevel (1) and the reboot runlevel (6).  The system
              administrator will have the opportunity to customize
              runlevels by simply adding, moving, or removing the
              symbolic links in <file>/etc/rc<var>n</var>.d</file> if
              symbolic links are being used, or by modifying
              <file>/etc/runlevel.conf</file> if the <tt>file-rc</tt> method
              is being used.
            </p>

            <p>
              To get the default behavior for your package, put in your
              <prgn>postinst</prgn> script
              <example compact="compact">
                update-rc.d <var>package</var> defaults
              </example>
              and in your <prgn>postrm</prgn>
              <example compact="compact">
                if [ "$1" = purge ]; then
                update-rc.d <var>package</var> remove
                fi
              </example>. Note that if your package changes runlevels
              or priority, you may have to remove and recreate the links,
              since otherwise the old links may persist. Refer to the
              documentation of <prgn>update-rc.d</prgn>.
            </p>

            <p>
              This will use a default sequence number of 20.  If it does
              not matter when or in which order the <file>init.d</file>
              script is run, use this default.  If it does, then you
              should talk to the maintainer of the <prgn>sysvinit</prgn>
              package or post to <tt>debian-devel</tt>, and they will
              help you choose a number.
            </p>

            <p>
              For more information about using <tt>update-rc.d</tt>,
              please consult its man page <manref name="update-rc.d"
                section="8">.
            </p>
          </sect2>

          <sect2>
            <heading>Running initscripts</heading>
            <p>
              The program <prgn>invoke-rc.d</prgn> is provided to make
              it easier for package maintainers to properly invoke an
              initscript, obeying runlevel and other locally-defined
              constraints that might limit a package's right to start,
              stop and otherwise manage services. This program may be
              used by maintainers in their packages' scripts.
            </p>

            <p>
              The package maintainer scripts must use
              <prgn>invoke-rc.d</prgn> to invoke the
              <file>/etc/init.d/*</file> initscripts, instead of
              calling them directly.
            </p>

            <p>
              By default, <prgn>invoke-rc.d</prgn> will pass any
              action requests (start, stop, reload, restart...) to the
              <file>/etc/init.d</file> script, filtering out requests
              to start or restart a service out of its intended
              runlevels.
            </p>

            <p>
              Most packages will simply need to change:
              <example compact="compact">/etc/init.d/&lt;package&gt;
              &lt;action&gt;</example> in their <prgn>postinst</prgn>
              and <prgn>prerm</prgn> scripts to:
              <example compact="compact">
        if which invoke-rc.d >/dev/null 2>&1; then
                invoke-rc.d <var>package</var> &lt;action&gt;
        else
                /etc/init.d/<var>package</var> &lt;action&gt;
        fi
              </example>
            </p>

            <p>
              A package should register its initscript services using
              <prgn>update-rc.d</prgn> before it tries to invoke them
              using <prgn>invoke-rc.d</prgn>. Invocation of
              unregistered services may fail.
            </p>

            <p>
              For more information about using
              <prgn>invoke-rc.d</prgn>, please consult its man page
              <manref name="invoke-rc.d" section="8">.
            </p>
          </sect2>
        </sect1>

        <sect1>
          <heading>Boot-time initialization</heading>

          <p>
            There used to be another directory, <file>/etc/rc.boot</file>,
            which contained scripts which were run once per machine
            boot. This has been deprecated in favour of links from
            <file>/etc/rcS.d</file> to files in <file>/etc/init.d</file> as
            described in <ref id="/etc/init.d">.  Packages must not
            place files in <file>/etc/rc.boot</file>.
          </p>
        </sect1>

        <sect1>
          <heading>Example</heading>

          <p>
            An example on which you can base your
            <file>/etc/init.d</file> scripts is found in
            <file>/etc/init.d/skeleton</file>.
          </p>

        </sect1>
      </sect>

      <sect>
        <heading>Console messages from <file>init.d</file> scripts</heading>

        <p>
          This section describes the formats to be used for messages
          written to standard output by the <file>/etc/init.d</file>
          scripts.  The intent is to improve the consistency of
          Debian's startup and shutdown look and feel.  For this
          reason, please look very carefully at the details.  We want
          the messages to have the same format in terms of wording,
          spaces, punctuation and case of letters.
        </p>

        <p>
          Here is a list of overall rules that should be used for
          messages generated by <file>/etc/init.d</file> scripts.  
        </p>

        <p>
          <list>
            <item>
                The message should fit in one line (fewer than 80
                characters), start with a capital letter and end with
                a period (<tt>.</tt>) and line feed (<tt>"\n"</tt>).
            </item>

            <item>
              If the script is performing some time consuming task in
              the background (not merely starting or stopping a
              program, for instance), an ellipsis (three dots:
              <tt>...</tt>) should be output to the screen, with no
              leading or tailing whitespace or line feeds.
            </item>

            <item>
              The messages should appear as if the computer is telling
              the user what it is doing (politely :-), but should not
                mention "it" directly.  For example, instead of:
                <example compact="compact">
I'm starting network daemons: nfsd mountd.
                </example>
                the message should say
                <example compact="compact">
Starting network daemons: nfsd mountd.
                </example>
            </item>
          </list>
        </p>

        <p>
          <tt>init.d</tt> script should use the following  standard
          message formats for the situations enumerated below.
        </p>

        <p>
          <list>
            <item>
              <p>When daemons are started</p>

              <p>
                If the script starts one or more daemons, the output
                should look like this (a single line, no leading
                spaces):
                <example compact="compact">
Starting <var>description</var>: <var>daemon-1</var> ... <var>daemon-n</var>.
                </example>
                The <var>description</var> should describe the
                subsystem the daemon or set of daemons are part of,
                while <var>daemon-1</var> up to <var>daemon-n</var>
                denote each daemon's name (typically the file name of
                the program).
              </p>

              <p>
                For example, the output of <file>/etc/init.d/lpd</file>
                would look like:
                <example compact="compact">
Starting printer spooler: lpd.
                </example>
              </p>

              <p>
                This can be achieved by saying
                <example compact="compact">
echo -n "Starting printer spooler: lpd"
start-stop-daemon --start --quiet --exec /usr/sbin/lpd
echo "."
                </example>
                in the script. If there are more than one daemon to
                start, the output should look like this:
                <example compact="compact">
echo -n "Starting remote file system services:"
echo -n " nfsd"; start-stop-daemon --start --quiet nfsd
echo -n " mountd"; start-stop-daemon --start --quiet mountd
echo -n " ugidd"; start-stop-daemon --start --quiet ugidd
echo "."
                </example>
                This makes it possible for the user to see what is
                happening and when the final daemon has been started.
                Care should be taken in the placement of white spaces:
                in the example above the system administrators can
                easily comment out a line if they don't want to start
                a specific daemon, while the displayed message still
                looks good.
              </p>
            </item>

            <item>
              <p>When a system parameter is being set</p>

              <p>
                If you have to set up different system parameters
                during the system boot, you should use this format:
                <example compact="compact">
Setting <var>parameter</var> to "<var>value</var>".
                </example>
              </p>

              <p>
                You can use a statement such as the following to get
                the quotes right:
                <example compact="compact">
echo "Setting DNS domainname to \"$domainname\"."
                </example>
              </p>

              <p>
                Note that the same symbol (<tt>"</tt>) is used for the left
                and right quotation marks.  A grave accent (<tt>`</tt>) is
                not a quote character; neither is an apostrophe
                (<tt>'</tt>).
              </p>
            </item>

            <item>
              <p>When a daemon is stopped or restarted</p>

              <p>
                When you stop or restart a daemon, you should issue a
                message identical to the startup message, except that
                <tt>Starting</tt> is replaced with <tt>Stopping</tt>
                or <tt>Restarting</tt> respectively.
              </p>

              <p>
                For example, stopping the printer daemon will look like
                this:
                <example compact="compact">
Stopping printer spooler: lpd.
                </example>
              </p>
            </item>

            <item>
              <p>When something is executed</p>

              <p>
                There are several examples where you have to run a
                program at system startup or shutdown to perform a
                specific task, for example, setting the system's clock
                using <prgn>netdate</prgn> or killing all processes
                when the system shuts down.  Your message should look
                like this:
                <example compact="compact">
Doing something very useful...done.
                </example>
                You should print the <tt>done.</tt> immediately after
                the job has been completed, so that the user is
                informed why they have to wait.  You can get this
                behavior by saying
                <example compact="compact">
echo -n "Doing something very useful..."
do_something
echo "done."
                </example>
                in your script.
              </p>
            </item>

            <item>
              <p>When the configuration is reloaded</p>

              <p>
                When a daemon is forced to reload its configuration
                files you should use the following format:
                <example compact="compact">
Reloading <var>description</var> configuration...done.
                </example>
                where <var>description</var> is the same as in the
                daemon starting message.
              </p>
            </item>
          </list>
        </p>
      </sect>

      <sect>
        <heading>Cron jobs</heading>

        <p>
          Packages must not modify the configuration file
          <file>/etc/crontab</file>, and they must not modify the files in
          <file>/var/spool/cron/crontabs</file>.</p>

        <p>
          If a package wants to install a job that has to be executed
          via cron, it should place a file with the name of the
          package in one or more of the following directories:
          <example compact="compact">
/etc/cron.daily
/etc/cron.weekly
/etc/cron.monthly
          </example>
          As these directory names imply, the files within them are
          executed on a daily, weekly, or monthly basis,
          respectively. The exact times are listed in
          <file>/etc/crontab</file>.</p>

        <p>
          All files installed in any of these directories must be
          scripts (e.g., shell scripts or Perl scripts) so that they
          can easily be modified by the local system administrator.
          In addition, they should be treated as configuration
          files.
        </p>

        <p>
          If a certain job has to be executed more frequently than
          daily, the package should install a file
          <file>/etc/cron.d/<var>package</var></file>. This file uses the
          same syntax as <file>/etc/crontab</file> and is processed by
          <prgn>cron</prgn> automatically. The file must also be
          treated as a configuration file. (Note that entries in the
          <file>/etc/cron.d</file> directory are not handled by
          <prgn>anacron</prgn>. Thus, you should only use this
          directory for jobs which may be skipped if the system is not
          running.)</p>

        <p>
          The scripts or crontab entries in these directories should
          check if all necessary programs are installed before they
          try to execute them. Otherwise, problems will arise when a
          package was removed but not purged since configuration files
          are kept on the system in this situation.</p>
      </sect>

      <sect id="menus">
        <heading>Menus</heading>

        <p>
          The Debian <tt>menu</tt> package provides a standard
          interface between packages providing applications and
          <em>menu programs</em> (either X window managers or
          text-based menu programs such as <prgn>pdmenu</prgn>).
        </p>

        <p>
          All packages that provide applications that need not be
          passed any special command line arguments for normal
          operation should register a menu entry for those
          applications, so that users of the <tt>menu</tt> package
          will automatically get menu entries in their window
          managers, as well in shells like <tt>pdmenu</tt>.
        </p>

        <p>
          Menu entries should follow the current menu policy.
        </p>

        <p>
          The menu policy can be found in the <tt>menu-policy</tt>
          files in the <tt>debian-policy</tt> package.
          It is also available from the Debian web mirrors at
          <tt><url name="/doc/packaging-manuals/menu-policy/"
                id="http://www.debian.org/doc/packaging-manuals/menu-policy/"></tt>.
        </p>

        <p>
          Please also refer to the <em>Debian Menu System</em>
          documentation that comes with the <package>menu</package>
          package for information about how to register your
          applications.
        </p>
      </sect>

      <sect id="mime">
        <heading>Multimedia handlers</heading>

        <p>
          MIME (Multipurpose Internet Mail Extensions, RFCs 2045-2049)
          is a mechanism for encoding files and data streams and
          providing meta-information about them, in particular their
          type (e.g.  audio or video) and format (e.g. PNG, HTML,
          MP3).
        </p>

        <p>
          Registration of MIME type handlers allows programs like mail
          user agents and web browsers to invoke these handlers to
          view, edit or display MIME types they don't support directly.
        </p>

        <p>
          Packages which provide the ability to view/show/play,
          compose, edit or print MIME types should register themselves
          as such following the current MIME support policy.
        </p>

        <p>
          The MIME support policy can be found in the <tt>mime-policy</tt>
          files in the <tt>debian-policy</tt> package.
          It is also available from the Debian web mirrors at
          <tt><url name="/doc/packaging-manuals/mime-policy/"
                id="http://www.debian.org/doc/packaging-manuals/mime-policy/"></tt>.
        </p>

      </sect>

      <sect>
        <heading>Keyboard configuration</heading>

        <p>
          To achieve a consistent keyboard configuration so that all
          applications interpret a keyboard event the same way, all
          programs in the Debian distribution must be configured to
          comply with the following guidelines.
        </p>

        <p>
          The following keys must have the specified interpretations:

          <taglist>
            <tag><tt>&lt;--</tt></tag>
            <item>delete the character to the left of the cursor</item>

            <tag><tt>Delete</tt></tag>
            <item>delete the character to the right of the cursor</item>

            <tag><tt>Control+H</tt></tag>
            <item>emacs: the help prefix</item>
          </taglist>

          The interpretation of any keyboard events should be
          independent of the terminal that is used, be it a virtual
          console, an X terminal emulator, an rlogin/telnet session,
          etc.
        </p>

        <p>
          The following list explains how the different programs
          should be set up to achieve this:
        </p>

        <p>
          <list>
            <item>
                <tt>&lt;--</tt> generates <tt>KB_BackSpace</tt> in X.
            </item>

            <item>
                <tt>Delete</tt> generates <tt>KB_Delete</tt> in X.
            </item>

            <item>
                X translations are set up to make
                <tt>KB_Backspace</tt> generate ASCII DEL, and to make
                <tt>KB_Delete</tt> generate <tt>ESC [ 3 ~</tt> (this
                is the vt220 escape code for the "delete character"
                key).  This must be done by loading the X resources
                using <prgn>xrdb</prgn> on all local X displays, not
                using the application defaults, so that the
                translation resources used correspond to the
                <prgn>xmodmap</prgn> settings.
            </item>

            <item>
                The Linux console is configured to make
                <tt>&lt;--</tt> generate DEL, and <tt>Delete</tt>
                generate <tt>ESC [ 3 ~</tt>.
            </item>

            <item>
                X applications are configured so that <tt>&lt;</tt>
                deletes left, and <tt>Delete</tt> deletes right.  Motif
                applications already work like this.
            </item>

            <item>
                Terminals should have <tt>stty erase ^?</tt> .
            </item>

            <item>
                The <tt>xterm</tt> terminfo entry should have <tt>ESC
                [ 3 ~</tt> for <tt>kdch1</tt>, just as for
                <tt>TERM=linux</tt> and <tt>TERM=vt220</tt>.
            </item>

            <item>
                Emacs is programmed to map <tt>KB_Backspace</tt> or
                the <tt>stty erase</tt> character to
                <tt>delete-backward-char</tt>, and <tt>KB_Delete</tt>
                or <tt>kdch1</tt> to <tt>delete-forward-char</tt>, and
                <tt>^H</tt> to <tt>help</tt> as always.
            </item>

            <item>
                Other applications use the <tt>stty erase</tt>
                character and <tt>kdch1</tt> for the two delete keys,
                with ASCII DEL being "delete previous character" and
                <tt>kdch1</tt> being "delete character under
                cursor".
            </item>

          </list>
        </p>

        <p>
          This will solve the problem except for the following
          cases:
        </p>

        <p>
          <list>
            <item>
                Some terminals have a <tt>&lt;--</tt> key that cannot
                be made to produce anything except <tt>^H</tt>.  On
                these terminals Emacs help will be unavailable on
                <tt>^H</tt> (assuming that the <tt>stty erase</tt>
                character takes precedence in Emacs, and has been set
                correctly).  <tt>M-x help</tt> or <tt>F1</tt> (if
                available) can be used instead.
            </item>

            <item>
                Some operating systems use <tt>^H</tt> for <tt>stty
                erase</tt>.  However, modern telnet versions and all
                rlogin versions propagate <tt>stty</tt> settings, and
                almost all UNIX versions honour <tt>stty erase</tt>.
                Where the <tt>stty</tt> settings are not propagated
                correctly, things can be made to work by using
                <tt>stty</tt> manually.
            </item>

            <item>
                Some systems (including previous Debian versions) use
                <prgn>xmodmap</prgn> to arrange for both
                <tt>&lt;--</tt> and <tt>Delete</tt> to generate
                <tt>KB_Delete</tt>.  We can change the behavior of
                their X clients using the same X resources that we use
                to do it for our own clients, or configure our clients
                using their resources when things are the other way
                around.  On displays configured like this
                <tt>Delete</tt> will not work, but <tt>&lt;--</tt>
                will.
            </item>

            <item>
                Some operating systems have different <tt>kdch1</tt>
                settings in their <tt>terminfo</tt> database for
                <tt>xterm</tt> and others.  On these systems the
                <tt>Delete</tt> key will not work correctly when you
                log in from a system conforming to our policy, but
                <tt>&lt;--</tt> will.
            </item>
          </list>
        </p>
      </sect>

      <sect>
        <heading>Environment variables</heading>

        <p>
          A program must not depend on environment variables to get
          reasonable defaults.  (That's because these environment
          variables would have to be set in a system-wide
          configuration file like <file>/etc/profile</file>, which is not
          supported by all shells.)
        </p>

        <p>
          If a program usually depends on environment variables for its
          configuration, the program should be changed to fall back to
          a reasonable default configuration if these environment
          variables are not present. If this cannot be done easily
          (e.g., if the source code of a non-free program is not
          available), the program must be replaced by a small
          "wrapper" shell script which sets the environment variables
          if they are not already defined, and calls the original program.
        </p>

        <p>
          Here is an example of a wrapper script for this purpose:

          <example compact="compact">
#!/bin/sh
BAR=${BAR:-/var/lib/fubar}
export BAR
exec /usr/lib/foo/foo "$@"
          </example>
        </p>

        <p>
          Furthermore, as <file>/etc/profile</file> is a configuration
          file of the <prgn>base-files</prgn> package, other packages must
          not put any environment variables or other commands into that
          file.
        </p>
      </sect>

      <sect id="doc-base">
        <heading>Registering Documents using doc-base</heading>

        <p>
          The <package>doc-base</package> package implements a
          flexible mechanism for handling and presenting
          documentation. The recommended practice is for every Debian
          package that provides online documentation (other than just
          manual pages) to register these documents with
          <package>doc-base</package> by installing a
          <package>doc-base</package> control file via the
          <prgn/install-docs/ script at installation time and
          de-register the manuals again when the package is removed.
        </p> 
        <p>
          Please refer to the documentation that comes with the
          <package>doc-base</package>  package for information and
          details. 
        </p>
      </sect>

    </chapt>


    <chapt id="files">
      <heading>Files</heading>

      <sect>
        <heading>Binaries</heading>

        <p>
          Two different packages must not install programs with
          different functionality but with the same filenames.  (The
          case of two programs having the same functionality but
          different implementations is handled via "alternatives" or
          the "Conflicts" mechanism.  See <ref id="maintscripts"> and
          <ref id="conflicts"> respectively.)  If this case happens,
          one of the programs must be renamed.  The maintainers should
          report this to the <tt>debian-devel</tt> mailing list and
          try to find a consensus about which program will have to be
          renamed.  If a consensus cannot be reached, <em>both</em>
          programs must be renamed.
        </p>

        <p>
         By default, when a package is being built, any binaries
         created should include debugging information, as well as
         being compiled with optimization.  You should also turn on
         as many reasonable compilation warnings as possible; this
         makes life easier for porters, who can then look at build
         logs for possible problems.  For the C programming language,
         this means the following compilation parameters should be
         used:
          <example compact="compact">
CC = gcc
CFLAGS = -O2 -g -Wall # sane warning options vary between programs
LDFLAGS = # none
INSTALL = install -s # (or use strip on the files in debian/tmp)
          </example>
        </p>

        <p>
          Note that by default all installed binaries should be stripped,
          either by using the <tt>-s</tt> flag to
          <prgn>install</prgn>, or by calling <prgn>strip</prgn> on
          the binaries after they have been copied into
          <file>debian/tmp</file> but before the tree is made into a
          package.
        </p>

        <p>
          Although binaries in the build tree should be compiled with
          debugging information by default, it can often be difficult
          to debug programs if they are also subjected to compiler
          optimization.  For this reason, it is recommended to support
          the standardized environment
          variable <tt>DEB_BUILD_OPTIONS</tt>.  This variable can
          contain several flags to change how a package is compiled
          and built.
        </p>

        <p>
          <taglist>
            <tag>noopt</tag>
            <item>
                The presence of this string means that the package
                should be compiled with a minimum of optimization.
                For C programs, it is best to add <tt>-O0</tt>
                to <tt>CFLAGS</tt> (although this is usually the
                default).  Some programs might fail to build or run at
                this level of optimization; it may be necessary to
                use <tt>-O1</tt>, for example.
            </item>
            <tag>nostrip</tag>
            <item>
                This string means that the debugging symbols should
                not be stripped from the binary during installation,
                so that debugging information may be included in the package.
            </item>
          </taglist>
        </p>

        <p>
          The following makefile snippet is an example of how one may
          implement the build options; you will probably have to
          massage this example in order to make it work for your
          package.
          <example compact="compact">
CFLAGS = -Wall -g
INSTALL = install
INSTALL_FILE    = $(INSTALL) -p    -o root -g root  -m  644
INSTALL_PROGRAM = $(INSTALL) -p    -o root -g root  -m  755
INSTALL_SCRIPT  = $(INSTALL) -p    -o root -g root  -m  755
INSTALL_DIR     = $(INSTALL) -p -d -o root -g root  -m  755

ifneq (,$(findstring noopt,$(DEB_BUILD_OPTIONS)))
CFLAGS += -O0
else
CFLAGS += -O2
endif
ifeq (,$(findstring nostrip,$(DEB_BUILD_OPTIONS)))
INSTALL_PROGRAM += -s
endif
          </example>
        </p>

        <p>
          It is up to the package maintainer to decide what
          compilation options are best for the package.  Certain
          binaries (such as computationally-intensive programs) will
          function better with certain flags (<tt>-O3</tt>, for
          example); feel free to use them.  Please use good judgment
          here.  Don't use flags for the sake of it; only use them
          if there is good reason to do so.  Feel free to override
          the upstream author's ideas about which compilation
          options are best: they are often inappropriate for our
          environment.
        </p>
      </sect>


      <sect id="libraries">
        <heading>Libraries</heading>

        <p>
          If the package is <strong>architecture: any</strong>, then
          the shared library compilation and linking flags must have
          <tt>-fPIC</tt>, or the package shall not build on some of
          the supported architectures<footnote>
            <p>
              If you are using GCC, <tt>-fPIC</tt> produces code with
              relocatable position independent code, which is required for
              most architectures to create a shared library, with i386 and
              perhaps some others where non position independent code is
              permitted in a shared library.
            </p>
            <p>
              Position independent code may have a performance penalty,
              especially on <tt>i386</tt>. However, in most cases the
              speed penalty must be measured against the memory wasted on
              the few architectures where non position independent code is
              even possible.
            </p>
          </footnote>. Any exception to this rule must be discussed on
          the mailing list <em>debian-devel@lists.debian.org</em>, and
          a rough consensus obtained.  The reasons for not compiling
          with <tt>-fPIC</tt> flag must be recorded in the file
          <tt>README.Debian</tt>, and care must be taken to either
          restrict the architecture or arrange for <tt>-fPIC</tt> to
          be used on architectures where it is required.<footnote>
            <p>
              Some of the reasons why this might be required is if the
              library contains hand crafted assembly code that is not
              relocatable, the speed penalty is excessive for compute
              intensive libs, and similar reasons.
            </p>
          </footnote>
        </p>
        <p>
          As to the static libraries, the common case is not to have
          relocatable code, since there is no benefit, unless in specific
          cases; therefore the static version must not be compiled
          with the <tt>-fPIC</tt> flag. Any exception to this rule
          should be discussed on the mailing list
          <em>debian-devel@lists.debian.org</em>,  and the reasons for
          compiling with the <tt>-fPIC</tt> flag must be recorded in
          the file <tt>README.Debian</tt>. <footnote>
            <p>
              Some of the reasons for linking static libraries with
              the <tt>-fPIC</tt> flag are if, for example, one needs a
              Perl API for a library that is under rapid development,
              and has an unstable API, so shared libraries are
              pointless at this phase of the library's development. In
              that case, since Perl needs a library with relocatable
              code, it may make sense to create a static library with
              relocatable code. Another reason cited is if you are
              distilling various libraries into a common shared
              library, like <tt>mklibs</tt> does in the Debian
              installer project.
            </p>
          </footnote>
        </p>
        <p>
          In other words, if both a shared and a static library is
          being built, each source unit (<tt>*.c</tt>, for example,
          for C files) will need to be compiled twice, for the normal
          case. 
        </p>
        <p>
          You must specify the gcc option <tt>-D_REENTRANT</tt>
          when building a library (either static or shared) to make
          the library compatible with LinuxThreads.
        </p>

        <p>
          Although not enforced by the build tools, shared libraries
          must be linked against all libraries that they use symbols from
          in the same way that binaries are.  This ensures the correct
          functioning of the <qref id="sharedlibs-shlibdeps">shlibs</qref>
          system and guarantees that all libraries can be safely opened
          with <tt>dlopen()</tt>.  Packagers may wish to use the gcc
          option <tt>-Wl,-z,defs</tt> when building a shared library.
          Since this option enforces symbol resolution at build time,
          a missing library reference will be caught early as a fatal
          build error.
        </p>

        <p>
          All installed shared libraries should be stripped with
          <example compact="compact">
strip --strip-unneeded <var>your-lib</var>
          </example>
          (The option <tt>--strip-unneeded</tt> makes
          <prgn>strip</prgn> remove only the symbols which aren't
          needed for relocation processing.)  Shared libraries can
          function perfectly well when stripped, since the symbols for
          dynamic linking are in a separate part of the ELF object
          file.<footnote>
              You might also want to use the options
              <tt>--remove-section=.comment</tt> and
              <tt>--remove-section=.note</tt> on both shared libraries
              and executables, and <tt>--strip-debug</tt> on static
              libraries.
          </footnote>
        </p>

        <p>
          Note that under some circumstances it may be useful to
          install a shared library unstripped, for example when
          building a separate package to support debugging.
        </p>

        <p>
          Shared object files (often <file>.so</file> files) that are not
          public libraries, that is, they are not meant to be linked
          to by third party executables (binaries of other packages),
          should be installed in subdirectories of the
          <file>/usr/lib</file> directory.  Such files are exempt from the
          rules that govern ordinary shared libraries, except that
          they must not be installed executable and should be
          stripped.<footnote>
              A common example are the so-called "plug-ins",
              internal shared objects that are dynamically loaded by
              programs using <manref name="dlopen" section="3">.
          </footnote>
        </p>

        <p>
          Packages containing shared libraries that may be linked to
          by other packages' binaries, but which for some
          <em>compelling</em> reason can not be installed in
          <file>/usr/lib</file> directory, may install the shared library
          files in subdirectories of the <file>/usr/lib</file> directory,
          in which case they should arrange to add that directory in
          <file>/etc/ld.so.conf</file> in the package's post-installation
          script, and remove it in the package's post-removal script.
        </p>

        <p>
          An ever increasing number of packages are using
          <prgn>libtool</prgn> to do their linking.  The latest GNU
          libtools (>= 1.3a) can take advantage of the metadata in the
          installed <prgn>libtool</prgn> archive files (<file>*.la</file>
          files).  The main advantage of <prgn>libtool</prgn>'s
          <file>.la</file> files is that it allows <prgn>libtool</prgn> to
          store and subsequently access metadata with respect to the
          libraries it builds.  <prgn>libtool</prgn> will search for
          those files, which contain a lot of useful information about
          a library (such as library dependency information for static
          linking).  Also, they're <em>essential</em> for programs
          using <tt>libltdl</tt>.<footnote>
              Although <prgn>libtool</prgn> is fully capable of
              linking against shared libraries which don't have
              <tt>.la</tt> files, as it is a mere shell script it can
              add considerably to the build time of a
              <prgn>libtool</prgn>-using package if that shell script
              has to derive all this information from first principles
              for each library every time it is linked.  With the
              advent of <prgn>libtool</prgn> version 1.4 (and to a
              lesser extent <prgn>libtool</prgn> version 1.3), the
              <file>.la</file> files also store information about
              inter-library dependencies which cannot necessarily be
              derived after the <file>.la</file> file is deleted.
          </footnote>
        </p>

        <p>
          Packages that use <prgn>libtool</prgn> to create shared
          libraries should include the <file>.la</file> files in the
          <tt>-dev</tt> package, unless the package relies on
          <tt>libtool</tt>'s <tt>libltdl</tt> library, in which case
          the <tt>.la</tt> files must go in the run-time library
          package.
        </p>

        <p>
          You must make sure that you use only released versions of
          shared libraries to build your packages; otherwise other
          users will not be able to run your binaries
          properly. Producing source packages that depend on
          unreleased compilers is also usually a bad
          idea.
        </p>
      </sect>


      <sect>
        <heading>Shared libraries</heading>
        <p>
          This section has moved to <ref id="sharedlibs">.
        </p>
      </sect>


      <sect id="scripts">
        <heading>Scripts</heading>

        <p>
          All command scripts, including the package maintainer
          scripts inside the package and used by <prgn>dpkg</prgn>,
          should have a <tt>#!</tt> line naming the shell to be used
          to interpret them.
        </p>

        <p>
          In the case of Perl scripts this should be
          <tt>#!/usr/bin/perl</tt>.
        </p>

        <p>
          When scripts are installed into a directory in the system
          PATH, the script name should not include an extension such
          as <tt>.sh</tt> or <tt>.pl</tt> that denotes the scripting
          language currently used to implement it.
        </p>
        <p>
          Shell scripts (<prgn>sh</prgn> and <prgn>bash</prgn>)
          should almost certainly start with <tt>set -e</tt> so that
          errors are detected.  Every script should use
          <tt>set -e</tt> or check the exit status of <em>every</em>
          command.
        </p>

        <p>
          Scripts may assume that <file>/bin/sh</file> implements the
          SUSv3 Shell Command Language<footnote>
            Single UNIX Specification, version 3, which is also IEEE
            1003.1-2004 (POSIX), and is available on the World Wide Web
            from <url id="http://www.unix.org/version3/online.html"
                      name="The Open Group"> after free
            registration.</footnote>
          plus the following additional features not mandated by
          SUSv3:<footnote>
            These features are in widespread use in the Linux community
            and are implemented in all of bash, dash, and ksh, the most
            common shells users may wish to use as <file>/bin/sh</file>.
          </footnote>
          <list>
            <item><tt>echo -n</tt>, if implemented as a shell built-in,
              must not generate a newline.</item>
            <item><tt>test</tt>, if implemented as a shell built-in, must
              support <tt>-a</tt> and <tt>-o</tt> as binary logical
              operators.</item>
            <item><tt>local</tt> to create a scoped variable must be
              supported; however, <tt>local</tt> may or may not preserve
              the variable value from an outer scope and may or may not
              support arguments more complex than simple variables.  Only
              uses such as:
<example compact>
fname () {
    local a
    a=''
    # ... use a ...
}
</example>
              must be supported.
            </item>
          </list>
          If a shell script requires non-SUSv3 features from the shell
          interpreter other than those listed above, the appropriate shell
          must be specified in the first line of the script (e.g.,
          <tt>#!/bin/bash</tt>) and the package must depend on the package
          providing the shell (unless the shell package is marked
          "Essential", as in the case of <prgn>bash</prgn>).
        </p>

        <p>
          You may wish to restrict your script to SUSv3 features plus the
          above set when possible so that it may use <file>/bin/sh</file>
          as its interpreter. If your script works with <prgn>dash</prgn>
          (originally called <prgn>ash</prgn>), it probably complies with
          the above requirements, but if you are in doubt, use
          <file>/bin/bash</file>.
        </p>

        <p>
          Perl scripts should check for errors when making any
          system calls, including <tt>open</tt>, <tt>print</tt>,
          <tt>close</tt>, <tt>rename</tt> and <tt>system</tt>.
        </p>

        <p>
          <prgn>csh</prgn> and <prgn>tcsh</prgn> should be avoided as
          scripting languages.  See <em>Csh Programming Considered
          Harmful</em>, one of the <tt>comp.unix.*</tt> FAQs, which
          can be found at <url id="http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/">.
          If an upstream package comes with <prgn>csh</prgn> scripts
          then you must make sure that they start with
          <tt>#!/bin/csh</tt> and make your package depend on the
          <prgn>c-shell</prgn> virtual package.
        </p>

        <p>
          Any scripts which create files in world-writeable
          directories (e.g., in <file>/tmp</file>) must use a
          mechanism which will fail atomically if a file with the same
          name already exists.
        </p>

        <p>
          The Debian base system provides the <prgn>tempfile</prgn>
          and <prgn>mktemp</prgn> utilities for use by scripts for
          this purpose.
        </p>
      </sect>


      <sect>
        <heading>Symbolic links</heading>

        <p>
          In general, symbolic links within a top-level directory
          should be relative, and symbolic links pointing from one
          top-level directory into another should be absolute. (A
          top-level directory is a sub-directory of the root
          directory <file>/</file>.)
        </p>

        <p>
          In addition, symbolic links should be specified as short as
          possible, i.e., link targets like <file>foo/../bar</file> are
          deprecated.
        </p>

        <p>
          Note that when creating a relative link using
          <prgn>ln</prgn> it is not necessary for the target of the
          link to exist relative to the working directory you're
          running <prgn>ln</prgn> from, nor is it necessary to change
          directory to the directory where the link is to be made.
          Simply include the string that should appear as the target
          of the link (this will be a pathname relative to the
          directory in which the link resides) as the first argument
          to <prgn>ln</prgn>.
        </p>

        <p>
          For example, in your <prgn>Makefile</prgn> or
          <file>debian/rules</file>, you can do things like:
          <example compact="compact">
ln -fs gcc $(prefix)/bin/cc
ln -fs gcc debian/tmp/usr/bin/cc
ln -fs ../sbin/sendmail $(prefix)/bin/runq
ln -fs ../sbin/sendmail debian/tmp/usr/bin/runq
          </example>
        </p>

        <p>
          A symbolic link pointing to a compressed file should always
          have the same file extension as the referenced file. (For
          example, if a file <file>foo.gz</file> is referenced by a
          symbolic link, the filename of the link has to end with
          "<file>.gz</file>" too, as in <file>bar.gz</file>.)
        </p>
      </sect>

      <sect>
        <heading>Device files</heading>

        <p>
          Packages must not include device files in the package file
          tree.
        </p>

        <p>
          If a package needs any special device files that are not
          included in the base system, it must call
          <prgn>MAKEDEV</prgn> in the <prgn>postinst</prgn> script,
          after notifying the user<footnote>
              This notification could be done via a (low-priority)
              debconf message, or an echo (printf) statement.
          </footnote>.
        </p>

        <p>
          Packages must not remove any device files in the
          <prgn>postrm</prgn> or any other script. This is left to the
          system administrator.
        </p>

        <p>
          Debian uses the serial devices
          <file>/dev/ttyS*</file>. Programs using the old
          <file>/dev/cu*</file> devices should be changed to use
          <file>/dev/ttyS*</file>.
        </p>
      </sect>

      <sect id="config-files">
        <heading>Configuration files</heading>

        <sect1>
          <heading>Definitions</heading>

          <p>
            <taglist>
              <tag>configuration file</tag>
              <item>
                  A file that affects the operation of a program, or
                  provides site- or host-specific information, or
                  otherwise customizes the behavior of a program.
                  Typically, configuration files are intended to be
                  modified by the system administrator (if needed or
                  desired) to conform to local policy or to provide
                  more useful site-specific behavior.
              </item>

              <tag><tt>conffile</tt></tag>
              <item>
                  A file listed in a package's <tt>conffiles</tt>
                  file, and is treated specially by <prgn>dpkg</prgn>
                  (see <ref id="configdetails">).
              </item>
            </taglist>
          </p>

          <p>
            The distinction between these two is important; they are
            not interchangeable concepts. Almost all
            <tt>conffile</tt>s are configuration files, but many
            configuration files are not <tt>conffiles</tt>.
          </p>

          <p>
            Note that a script that embeds configuration information
            (such as most of the files in <file>/etc/default</file> and
            <file>/etc/cron.{daily,weekly,monthly}</file>) is de-facto a
            configuration file and should be treated as such.
          </p>
        </sect1>

        <sect1>
          <heading>Location</heading>

          <p>
            Any configuration files created or used by your package
            must reside in <file>/etc</file>. If there are several,
            consider creating a subdirectory of <file>/etc</file>
            named after your package.
          </p>

          <p>
            If your package creates or uses configuration files
            outside of <file>/etc</file>, and it is not feasible to modify
            the package to use <file>/etc</file> directly, put the files
            in <file>/etc</file> and create symbolic links to those files
            from the location that the package requires.
          </p>
        </sect1>

        <sect1>
          <heading>Behavior</heading>

          <p>
            Configuration file handling must conform to the following
            behavior:
            <list compact="compact">
              <item>
                  local changes must be preserved during a package
                  upgrade, and
              </item>
              <item>
                  configuration files must be preserved when the
                  package is removed, and only deleted when the
                  package is purged.
              </item>
            </list>
          </p>

          <p>
            The easy way to achieve this behavior is to make the
            configuration file a <tt>conffile</tt>. This is
            appropriate only if it is possible to distribute a default
            version that will work for most installations, although
            some system administrators may choose to modify it. This
            implies that the default version will be part of the
            package distribution, and must not be modified by the
            maintainer scripts during installation (or at any other
            time).
          </p>

          <p>
            In order to ensure that local changes are preserved
            correctly, no package may contain or make hard links to
            conffiles.<footnote>
                Rationale: There are two problems with hard links.
                The first is that some editors break the link while
                editing one of the files, so that the two files may
                unwittingly become unlinked and different.  The second
                is that <prgn>dpkg</prgn> might break the hard link
                while upgrading <tt>conffile</tt>s.
            </footnote>
          </p>

          <p>
            The other way to do it is via the maintainer scripts.  In
            this case, the configuration file must not be listed as a
            <tt>conffile</tt> and must not be part of the package
            distribution. If the existence of a file is required for
            the package to be sensibly configured it is the
            responsibility of the package maintainer to provide
            maintainer scripts which correctly create, update and
            maintain the file and remove it on purge.  (See <ref
            id="maintainerscripts"> for more information.)  These
            scripts must be idempotent (i.e., must work correctly if
            <prgn>dpkg</prgn> needs to re-run them due to errors
            during installation or removal), must cope with all the
            variety of ways <prgn>dpkg</prgn> can call maintainer
            scripts, must not overwrite or otherwise mangle the user's
            configuration without asking, must not ask unnecessary
            questions (particularly during upgrades), and must
            otherwise be good citizens.
          </p>

          <p>
            The scripts are not required to configure every possible
            option for the package, but only those necessary to get
            the package running on a given system. Ideally the
            sysadmin should not have to do any configuration other
            than that done (semi-)automatically by the
            <prgn>postinst</prgn> script.
          </p>

          <p>
            A common practice is to create a script called
            <file><var>package</var>-configure</file> and have the
            package's <prgn>postinst</prgn> call it if and only if the
            configuration file does not already exist.  In certain
            cases it is useful for there to be an example or template
            file which the maintainer scripts use.  Such files should
            be in <file>/usr/share/<var>package</var></file> or
            <file>/usr/lib/<var>package</var></file> (depending on whether
            they are architecture-independent or not).  There should
            be symbolic links to them from
            <file>/usr/share/doc/<var>package</var>/examples</file> if
            they are examples, and should be perfectly ordinary
            <prgn>dpkg</prgn>-handled files (<em>not</em>
            configuration files).
          </p>

          <p>
            These two styles of configuration file handling must
            not be mixed, for that way lies madness:
            <prgn>dpkg</prgn> will ask about overwriting the file
            every time the package is upgraded.
          </p>
        </sect1>

        <sect1>
          <heading>Sharing configuration files</heading>

          <p>
            Packages which specify the same file as a
            <tt>conffile</tt> must be tagged as <em>conflicting</em>
            with each other.  (This is an instance of the general rule
            about not sharing files.  Note that neither alternatives
            nor diversions are likely to be appropriate in this case;
            in particular, <prgn>dpkg</prgn> does not handle diverted
            <tt>conffile</tt>s well.)
          </p>

          <p>
            The maintainer scripts must not alter a <tt>conffile</tt>
            of <em>any</em> package, including the one the scripts
            belong to.
          </p>

          <p>
            If two or more packages use the same configuration file
            and it is reasonable for both to be installed at the same
            time, one of these packages must be defined as
            <em>owner</em> of the configuration file, i.e., it will be
            the package which handles that file as a configuration
            file.  Other packages that use the configuration file must
            depend on the owning package if they require the
            configuration file to operate. If the other package will
            use the configuration file if present, but is capable of
            operating without it, no dependency need be declared.
          </p>

          <p>
            If it is desirable for two or more related packages to
            share a configuration file <em>and</em> for all of the
            related packages to be able to modify that configuration
            file, then the following should be done:
            <enumlist compact="compact">
              <item>
                  One of the related packages (the "owning" package)
                  will manage the configuration file with maintainer
                  scripts as described in the previous section.
              </item>
              <item>
                  The owning package should also provide a program
                  that the other packages may use to modify the
                  configuration file.
              </item>
              <item>
                  The related packages must use the provided program
                  to make any desired modifications to the
                  configuration file.  They should either depend on
                  the core package to guarantee that the configuration
                  modifier program is available or accept gracefully
                  that they cannot modify the configuration file if it
                  is not.  (This is in addition to the fact that the
                  configuration file may not even be present in the
                  latter scenario.)
              </item>
            </enumlist>
          </p>

          <p>
            Sometimes it's appropriate to create a new package which
            provides the basic infrastructure for the other packages
            and which manages the shared configuration files.  (The
            <tt>sgml-base</tt> package is a good example.)
          </p>
        </sect1>

        <sect1>
          <heading>User configuration files ("dotfiles")</heading>

          <p>
            The files in <file>/etc/skel</file> will automatically be
            copied into new user accounts by <prgn>adduser</prgn>.
            No other program should reference the files in
            <file>/etc/skel</file>.
          </p>

          <p>
            Therefore, if a program needs a dotfile to exist in
            advance in <file>$HOME</file> to work sensibly, that dotfile
            should be installed in <file>/etc/skel</file> and treated as a
            configuration file.
          </p>

          <p>
            However, programs that require dotfiles in order to
            operate sensibly are a bad thing, unless they do create
            the dotfiles themselves automatically.
          </p>

          <p>
            Furthermore, programs should be configured by the Debian
            default installation to behave as closely to the upstream
            default behavior as possible.
          </p>

          <p>
            Therefore, if a program in a Debian package needs to be
            configured in some way in order to operate sensibly, that
            should be done using a site-wide configuration file placed
            in <file>/etc</file>.  Only if the program doesn't support a
            site-wide default configuration and the package maintainer
            doesn't have time to add it may a default per-user file be
            placed in <file>/etc/skel</file>.
          </p>

          <p>
            <file>/etc/skel</file> should be as empty as we can make it.
            This is particularly true because there is no easy (or
            necessarily desirable) mechanism for ensuring that the
            appropriate dotfiles are copied into the accounts of
            existing users when a package is installed.
          </p>
        </sect1>
      </sect>

      <sect>
        <heading>Log files</heading>
        <p>
          Log files should usually be named
          <file>/var/log/<var>package</var>.log</file>.  If you have many
          log files, or need a separate directory for permission
          reasons (<file>/var/log</file> is writable only by
          <file>root</file>), you should usually create a directory named
          <file>/var/log/<var>package</var></file> and place your log
          files there.
        </p>

        <p>
          Log files must be rotated occasionally so that they don't
          grow indefinitely; the best way to do this is to drop a log
          rotation configuration file into the directory
          <file>/etc/logrotate.d</file> and use the facilities provided by
          logrotate.<footnote>
            <p>
              The traditional approach to log files has been to set up
              <em>ad hoc</em> log rotation schemes using simple shell
              scripts and cron.  While this approach is highly
              customizable, it requires quite a lot of sysadmin work.
              Even though the original Debian system helped a little
              by automatically installing a system which can be used
              as a template, this was deemed not enough.
            </p>

            <p>
              The use of <prgn>logrotate</prgn>, a program developed
              by Red Hat, is better, as it centralizes log management.
              It has both a configuration file
              (<file>/etc/logrotate.conf</file>) and a directory where
              packages can drop their individual log rotation
              configurations (<file>/etc/logrotate.d</file>).
            </p>
          </footnote>
          Here is a good example for a logrotate config
          file (for more information see <manref name="logrotate"
            section="8">):
          <example compact="compact">
/var/log/foo/*.log {
rotate 12
weekly
compress
postrotate
/etc/init.d/foo force-reload
endscript
}
          </example>
          This rotates all files under <file>/var/log/foo</file>, saves 12
          compressed generations, and forces the daemon to reload its
          configuration information after the log rotation.
        </p>

        <p>
          Log files should be removed when the package is
          purged (but not when it is only removed).  This should be
          done by the <prgn>postrm</prgn> script when it is called
          with the argument <tt>purge</tt> (see <ref
          id="removedetails">).
        </p>
      </sect>

      <sect>
        <heading>Permissions and owners</heading>

        <p>
          The rules in this section are guidelines for general use.
          If necessary you may deviate from the details below.
          However, if you do so you must make sure that what is done
          is secure and you should try to be as consistent as possible
          with the rest of the system.  You should probably also
          discuss it on <prgn>debian-devel</prgn> first.
        </p>

        <p>
          Files should be owned by <tt>root.root</tt>, and made
          writable only by the owner and universally readable (and
          executable, if appropriate), that is mode 644 or 755.
        </p>

        <p>
          Directories should be mode 755 or (for group-writability)
          mode 2775.  The ownership of the directory should be
          consistent with its mode: if a directory is mode 2775, it
          should be owned by the group that needs write access to
          it.<footnote>
            <p>
              When a package is upgraded, and the owner or permissions
              of a file included in the package has changed, dpkg
              arranges for the ownership and permissions to be
              correctly set upon installation. However, this does not
              extend to directories; the permissions and ownership of
              directories already on the system does not change on
              install or upgrade of packages.  This makes sense, since
              otherwise common directories like <tt>/usr</tt> would
              always be in flux.  To correctly change permissions of a
              directory the package owns, explicit action is required,
              usually in the <tt>postinst</tt> script. Care must be
              taken to handle downgrades as well, in that case.
            </p>
          </footnote>
        </p>


        <p>
          Setuid and setgid executables should be mode 4755 or 2755
          respectively, and owned by the appropriate user or group.
          They should not be made unreadable (modes like 4711 or
          2711 or even 4111); doing so achieves no extra security,
          because anyone can find the binary in the freely available
          Debian package; it is merely inconvenient.  For the same
          reason you should not restrict read or execute permissions
          on non-set-id executables.
        </p>

        <p>
          Some setuid programs need to be restricted to particular
          sets of users, using file permissions.  In this case they
          should be owned by the uid to which they are set-id, and by
          the group which should be allowed to execute them.  They
          should have mode 4754; again there is no point in making
          them unreadable to those users who must not be allowed to
          execute them.
        </p>

        <p>
          It is possible to arrange that the system administrator can
          reconfigure the package to correspond to their local
          security policy by changing the permissions on a binary:
          they can do this by using <prgn>dpkg-statoverride</prgn>, as
          described below.<footnote>
              Ordinary files installed by <prgn>dpkg</prgn> (as
              opposed to <tt>conffile</tt>s and other similar objects)
              normally have their permissions reset to the distributed
              permissions when the package is reinstalled.  However,
              the use of <prgn>dpkg-statoverride</prgn> overrides this
              default behavior.  If you use this method, you should
              remember to describe <prgn>dpkg-statoverride</prgn> in
              the package documentation; being a relatively new
              addition to Debian, it is probably not yet well-known.
          </footnote>
          Another method you should consider is to create a group for
          people allowed to use the program(s) and make any setuid
          executables executable only by that group.
        </p>

        <p>
          If you need to create a new user or group for your package
          there are two possibilities.  Firstly, you may need to
          make some files in the binary package be owned by this
          user or group, or you may need to compile the user or
          group id (rather than just the name) into the binary
          (though this latter should be avoided if possible, as in
          this case you need a statically allocated id).</p>

        <p>
          If you need a statically allocated id, you must ask for a
          user or group id from the <tt>base-passwd</tt> maintainer,
          and must not release the package until you have been
          allocated one.  Once you have been allocated one you must
          either make the package depend on a version of the
          <tt>base-passwd</tt> package with the id present in
          <file>/etc/passwd</file> or <file>/etc/group</file>, or arrange for
          your package to create the user or group itself with the
          correct id (using <tt>adduser</tt>) in its
          <prgn>preinst</prgn> or <prgn>postinst</prgn>.  (Doing it in
          the <prgn>postinst</prgn> is to be preferred if it is
          possible, otherwise a pre-dependency will be needed on the
          <tt>adduser</tt> package.)
        </p>

        <p>
          On the other hand, the program might be able to determine
          the uid or gid from the user or group name at runtime, so
          that a dynamically allocated id can be used.  In this case
          you should choose an appropriate user or group name,
          discussing this on <prgn>debian-devel</prgn> and checking
          with the <package/base-passwd/ maintainer that it is unique and that
          they do not wish you to use a statically allocated id
          instead.  When this has been checked you must arrange for
          your package to create the user or group if necessary using
          <prgn>adduser</prgn> in the <prgn>preinst</prgn> or
          <prgn>postinst</prgn> script (again, the latter is to be
          preferred if it is possible).
        </p>

        <p>
          Note that changing the numeric value of an id associated
          with a name is very difficult, and involves searching the
          file system for all appropriate files.  You need to think
          carefully whether a static or dynamic id is required, since
          changing your mind later will cause problems.
        </p>

        <sect1><heading>The use of <prgn>dpkg-statoverride</prgn></heading>
          <p>
            This section is not intended as policy, but as a
            description of the use of <prgn>dpkg-statoverride</prgn>.
          </p>

          <p>
            If a system administrator wishes to have a file (or
            directory or other such thing) installed with owner and
            permissions different from those in the distributed Debian
            package, they can use the <prgn>dpkg-statoverride</prgn>
            program to instruct <prgn>dpkg</prgn> to use the different
            settings every time the file is installed.  Thus the
            package maintainer should distribute the files with their
            normal permissions, and leave it for the system
            administrator to make any desired changes.  For example, a
            daemon which is normally required to be setuid root, but
            in certain situations could be used without being setuid,
            should be installed setuid in the <tt>.deb</tt>.  Then the
            local system administrator can change this if they wish.
            If there are two standard ways of doing it, the package
            maintainer can use <tt>debconf</tt> to find out the
            preference, and call <prgn>dpkg-statoverride</prgn> in the
            maintainer script if necessary to accommodate the system
            administrator's choice. Care must be taken during
            upgrades to not override an existing setting.
          </p>

          <p>
            Given the above, <prgn>dpkg-statoverride</prgn> is
            essentially a tool for system administrators and would not
            normally be needed in the maintainer scripts.  There is
            one type of situation, though, where calls to
            <prgn>dpkg-statoverride</prgn> would be needed in the
            maintainer scripts, and that involves packages which use
            dynamically allocated user or group ids.  In such a
            situation, something like the following idiom can be very
            helpful in the package's <prgn>postinst</prgn>, where
            <tt>sysuser</tt> is a dynamically allocated id:
            <example>
for i in /usr/bin/foo /usr/sbin/bar
do
  # only do something when no setting exists
  if ! dpkg-statoverride --list $i >/dev/null 2>&1
  then
    #include: debconf processing, question about foo and bar
    if [ "$RET" = "true" ] ; then
      dpkg-statoverride --update --add sysuser root 4755 $i
    fi
  fi
done
            </example>
            The corresponding <tt>dpkg-statoverride --remove</tt>
            calls can then be made unconditionally when the package is
            purged.
          </p>
        </sect1>
      </sect>
    </chapt>


    <chapt id="customized-programs">
      <heading>Customized programs</heading>

      <sect id="arch-spec">
        <heading>Architecture specification strings</heading>

        <p>
          If a program needs to specify an <em>architecture specification
            string</em> in some place, it should select one of the
          strings provided by <tt>dpkg-architecture -L</tt>. The
          strings are in the format
          <tt><var>os</var>-<var>arch</var></tt>, though the OS part
          is sometimes elided, as when the OS is Linux.<footnote>
            <p>Currently, the strings are:
              i386 ia64 alpha amd64 armeb arm hppa m32r m68k mips
              mipsel powerpc ppc64 s390 s390x sh3 sh3eb sh4 sh4eb
              sparc darwin-i386 darwin-ia64 darwin-alpha darwin-amd64
              darwin-armeb darwin-arm darwin-hppa darwin-m32r
              darwin-m68k darwin-mips darwin-mipsel darwin-powerpc
              darwin-ppc64 darwin-s390 darwin-s390x darwin-sh3
              darwin-sh3eb darwin-sh4 darwin-sh4eb darwin-sparc
              freebsd-i386 freebsd-ia64 freebsd-alpha freebsd-amd64
              freebsd-armeb freebsd-arm freebsd-hppa freebsd-m32r
              freebsd-m68k freebsd-mips freebsd-mipsel freebsd-powerpc
              freebsd-ppc64 freebsd-s390 freebsd-s390x freebsd-sh3
              freebsd-sh3eb freebsd-sh4 freebsd-sh4eb freebsd-sparc
              kfreebsd-i386 kfreebsd-ia64 kfreebsd-alpha
              kfreebsd-amd64 kfreebsd-armeb kfreebsd-arm kfreebsd-hppa
              kfreebsd-m32r kfreebsd-m68k kfreebsd-mips
              kfreebsd-mipsel kfreebsd-powerpc kfreebsd-ppc64
              kfreebsd-s390 kfreebsd-s390x kfreebsd-sh3 kfreebsd-sh3eb
              kfreebsd-sh4 kfreebsd-sh4eb kfreebsd-sparc knetbsd-i386
              knetbsd-ia64 knetbsd-alpha knetbsd-amd64 knetbsd-armeb
              knetbsd-arm knetbsd-hppa knetbsd-m32r knetbsd-m68k
              knetbsd-mips knetbsd-mipsel knetbsd-powerpc
              knetbsd-ppc64 knetbsd-s390 knetbsd-s390x knetbsd-sh3
              knetbsd-sh3eb knetbsd-sh4 knetbsd-sh4eb knetbsd-sparc
              netbsd-i386 netbsd-ia64 netbsd-alpha netbsd-amd64
              netbsd-armeb netbsd-arm netbsd-hppa netbsd-m32r
              netbsd-m68k netbsd-mips netbsd-mipsel netbsd-powerpc
              netbsd-ppc64 netbsd-s390 netbsd-s390x netbsd-sh3
              netbsd-sh3eb netbsd-sh4 netbsd-sh4eb netbsd-sparc
              openbsd-i386 openbsd-ia64 openbsd-alpha openbsd-amd64
              openbsd-armeb openbsd-arm openbsd-hppa openbsd-m32r
              openbsd-m68k openbsd-mips openbsd-mipsel openbsd-powerpc
              openbsd-ppc64 openbsd-s390 openbsd-s390x openbsd-sh3
              openbsd-sh3eb openbsd-sh4 openbsd-sh4eb openbsd-sparc
              hurd-i386 hurd-ia64 hurd-alpha hurd-amd64 hurd-armeb
              hurd-arm hurd-hppa hurd-m32r hurd-m68k hurd-mips
              hurd-mipsel hurd-powerpc hurd-ppc64 hurd-s390 hurd-s390x
              hurd-sh3 hurd-sh3eb hurd-sh4 hurd-sh4eb hurd-sparc
            </p>
          </footnote>
        </p>

        <p>
          Note that we don't want to use
          <tt><var>arch</var>-debian-linux</tt> to apply to the rule
          <tt><var>architecture</var>-<var>vendor</var>-<var>os</var></tt>
          since this would make our programs incompatible with other
          Linux distributions.  We also don't use something like
          <tt><var>arch</var>-unknown-linux</tt>, since the
          <tt>unknown</tt> does not look very good.
        </p>
      </sect>

      <sect>
        <heading>Daemons</heading>

        <p>
          The configuration files <file>/etc/services</file>,
          <file>/etc/protocols</file>, and <file>/etc/rpc</file> are managed
          by the <prgn>netbase</prgn> package and must not be modified
          by other packages.
        </p>

        <p>
          If a package requires a new entry in one of these files, the
          maintainer should get in contact with the
          <prgn>netbase</prgn> maintainer, who will add the entries
          and release a new version of the <prgn>netbase</prgn>
          package.
        </p>

        <p>
          The configuration file <file>/etc/inetd.conf</file> must not be
          modified by the package's scripts except via the
          <prgn>update-inetd</prgn> script or the
          <file>DebianNet.pm</file> Perl module.  See their documentation
          for details on how to add entries.
        </p>

        <p>
          If a package wants to install an example entry into
          <file>/etc/inetd.conf</file>, the entry must be preceded with
          exactly one hash character (<tt>#</tt>). Such lines are
          treated as "commented out by user" by the
          <prgn>update-inetd</prgn> script and are not changed or
          activated during package updates.
        </p>
      </sect>

      <sect>
        <heading>Using pseudo-ttys and modifying wtmp, utmp and
        lastlog</heading>

        <p>
          Some programs need to create pseudo-ttys. This should be done
          using Unix98 ptys if the C library supports it. The resulting
          program must not be installed setuid root, unless that
          is required for other functionality.
        </p>

        <p>
          The files <file>/var/run/utmp</file>, <file>/var/log/wtmp</file> and
          <file>/var/log/lastlog</file> must be installed writable by
          group <tt>utmp</tt>.  Programs which need to modify those
          files must be installed setgid <tt>utmp</tt>.
        </p>
      </sect>

      <sect>
        <heading>Editors and pagers</heading>

        <p>
          Some programs have the ability to launch an editor or pager
          program to edit or display a text document.  Since there are
          lots of different editors and pagers available in the Debian
          distribution, the system administrator and each user should
          have the possibility to choose their preferred editor and
          pager.
        </p>

        <p>
          In addition, every program should choose a good default
          editor/pager if none is selected by the user or system
          administrator.
        </p>

        <p>
          Thus, every program that launches an editor or pager must
          use the EDITOR or PAGER environment variable to determine
          the editor or pager the user wishes to use.  If these
          variables are not set, the programs <file>/usr/bin/editor</file>
          and <file>/usr/bin/pager</file> should be used, respectively.
        </p>

        <p>
          These two files are managed through the <prgn>dpkg</prgn>
          "alternatives" mechanism.  Thus every package providing an
          editor or pager must call the
          <prgn>update-alternatives</prgn> script to register these
          programs.
        </p>

        <p>
          If it is very hard to adapt a program to make use of the
          EDITOR or PAGER variables, that program may be configured to
          use <file>/usr/bin/sensible-editor</file> and
          <file>/usr/bin/sensible-pager</file> as the editor or pager
          program respectively.  These are two scripts provided in the
          Debian base system that check the EDITOR and PAGER variables
          and launch the appropriate program, and fall back to
          <file>/usr/bin/editor</file> and <file>/usr/bin/pager</file> if the
          variable is not set.
        </p>

        <p>
          A program may also use the VISUAL environment variable to
          determine the user's choice of editor.  If it exists, it
          should take precedence over EDITOR.  This is in fact what
          <file>/usr/bin/sensible-editor</file> does.
        </p>

        <p>
          It is not required for a package to depend on
          <tt>editor</tt> and <tt>pager</tt>, nor is it required for a
          package to provide such virtual packages.<footnote>
              The Debian base system already provides an editor and a
              pager program.
          </footnote>
        </p>
      </sect>

      <sect id="web-appl">
        <heading>Web servers and applications</heading>

        <p>
          This section describes the locations and URLs that should
          be used by all web servers and web applications in the
          Debian system.
        </p>

        <p>
          <enumlist>
            <item>
                Cgi-bin executable files are installed in the
                directory
                <example compact="compact">
/usr/lib/cgi-bin/<var>cgi-bin-name</var>
                </example>
                and should be referred to as
                <example compact="compact">
http://localhost/cgi-bin/<var>cgi-bin-name</var>
                </example>

            </item>

            <item>
              <p>Access to HTML documents</p>

              <p>
                HTML documents for a package are stored in
                <file>/usr/share/doc/<var>package</var></file>
                and can be referred to as
                <example compact="compact">
http://localhost/doc/<var>package</var>/<var>filename</var>
                </example>
              </p>

              <p>
                The web server should restrict access to the document
                tree so that only clients on the same host can read
                the documents. If the web server does not support such
                access controls, then it should not provide access at
                all, or ask about providing access during installation.
              </p>
            </item>

            <item>
              <p>Access to images</p>
              <p>
                It is recommended that images for a package be stored
                in <tt>/usr/share/images/<var>package</var></tt> and
                may be referred to through an alias <tt>/images/</tt>
                as
                <example>
                  http://localhost/images/&lt;package&gt;/&lt;filename&gt;     
                </example>
                
              </p>
            </item>

            <item>
              <p>Web Document Root</p>

              <p>
                Web Applications should try to avoid storing files in
                the Web Document Root.  Instead they should use the
                /usr/share/doc/<var>package</var> directory for
                documents and register the Web Application via the
                <package>doc-base</package> package.  If access to the
                web document root is unavoidable then use
                <example compact="compact">
/var/www
                </example>
                as the Document Root.  This might be just a symbolic
                link to the location where the system administrator
                has put the real document root.
              </p>
            </item>
            <item><p>Providing httpd and/or httpd-cgi</p>
              <p>
                All web servers should provide the virtual package
                <tt>httpd</tt>. If a web server has CGI support it should
                provide <tt>httpd-cgi</tt> additionally.
              </p>
              <p>
                All web applications which do not contain CGI scripts should
                depend on <tt>httpd</tt>, all those web applications which
                <tt>do</tt> contain CGI scripts, should depend on
                <tt>httpd-cgi</tt>.
              </p>
            </item>
          </enumlist>
        </p>
      </sect>

      <sect id="mail-transport-agents">
        <heading>Mail transport, delivery and user agents</heading>

        <p>
          Debian packages which process electronic mail, whether mail
          user agents (MUAs) or mail transport agents (MTAs), must
          ensure that they are compatible with the configuration
          decisions below.  Failure to do this may result in lost
          mail, broken <tt>From:</tt> lines, and other serious brain
          damage!
        </p>

        <p>
          The mail spool is <file>/var/mail</file> and the interface to
          send a mail message is <file>/usr/sbin/sendmail</file> (as per
          the FHS).  On older systems, the mail spool may be
          physically located in <file>/var/spool/mail</file>, but all
          access to the mail spool should be via the
          <file>/var/mail</file> symlink.  The mail spool is part of the
          base system and not part of the MTA package.
        </p>

        <p>
          All Debian MUAs, MTAs, MDAs and other mailbox accessing
          programs (such as IMAP daemons) must lock the mailbox in an
          NFS-safe way. This means that <tt>fcntl()</tt> locking must
          be combined with dot locking.  To avoid deadlocks, a program
          should use <tt>fcntl()</tt> first and dot locking after
          this, or alternatively implement the two locking methods in
          a non blocking way<footnote>
              If it is not possible to establish both locks, the
              system shouldn't wait for the second lock to be
              established, but remove the first lock, wait a (random)
              time, and start over locking again.
          </footnote>. Using the functions <tt>maillock</tt> and
          <tt>mailunlock</tt> provided by the
          <tt>liblockfile*</tt><footnote>
              You will need to depend on <tt>liblockfile1 (&gt;&gt;1.01)</tt>
              to use these functions.
          </footnote> packages is the recommended way to realize this.
        </p>

        <p>
          Mailboxes are generally mode 660
          <tt><var>user</var>.mail</tt> unless the system
          administrator has chosen otherwise.  A MUA may remove a
          mailbox (unless it has nonstandard permissions) in which
          case the MTA or another MUA must recreate it if needed.
          Mailboxes must be writable by group mail.
        </p>

        <p>
          The mail spool is 2775 <tt>root.mail</tt>, and MUAs should
          be setgid mail to do the locking mentioned above (and
          must obviously avoid accessing other users' mailboxes
          using this privilege).</p>

        <p>
          <file>/etc/aliases</file> is the source file for the system mail
          aliases (e.g., postmaster, usenet, etc.), it is the one
          which the sysadmin and <prgn>postinst</prgn> scripts may
          edit.  After <file>/etc/aliases</file> is edited the program or
          human editing it must call <prgn>newaliases</prgn>.  All MTA
          packages must come with a <prgn>newaliases</prgn> program,
          even if it does nothing, but older MTA packages did not do
          this so programs should not fail if <prgn>newaliases</prgn>
          cannot be found.  Note that because of this, all MTA
          packages must have <tt>Provides</tt>, <tt>Conflicts</tt> and
          <tt>Replaces: mail-transport-agent</tt> control file
          fields.
        </p>

        <p>
          The convention of writing <tt>forward to
            <var>address</var></tt> in the mailbox itself is not
          supported.  Use a <tt>.forward</tt> file instead.</p>

        <p>
          The <prgn>rmail</prgn> program used by UUCP
          for incoming mail should be  <file>/usr/sbin/rmail</file>.
          Likewise, <prgn>rsmtp</prgn>, for receiving
          batch-SMTP-over-UUCP, should be <file>/usr/sbin/rsmtp</file> if it
          is supported.</p>

        <p>
          If your package needs to know what hostname to use on (for
          example) outgoing news and mail messages which are generated
          locally, you should use the file <file>/etc/mailname</file>.  It
          will contain the portion after the username and <tt>@</tt>
          (at) sign for email addresses of users on the machine
          (followed by a newline).
        </p>

        <p>
          Such a package should check for the existence of this file
          when it is being configured.  If it exists, it should be
          used without comment, although an MTA's configuration script
          may wish to prompt the user even if it finds that this file
          exists.  If the file does not exist, the package should
          prompt the user for the value (preferably using
          <prgn>debconf</prgn>) and store it in <file>/etc/mailname</file>
          as well as using it in the package's configuration.  The
          prompt should make it clear that the name will not just be
          used by that package.  For example, in this situation the
          <tt>inn</tt> package could say something like:
          <example compact="compact">
Please enter the "mail name" of your system.  This is the
hostname portion of the address to be shown on outgoing
news and mail messages.  The default is
<var>syshostname</var>, your system's host name.  Mail
name ["<var>syshostname</var>"]:
          </example>
          where <var>syshostname</var> is the output of <tt>hostname
            --fqdn</tt>.
        </p>
      </sect>

      <sect>
        <heading>News system configuration</heading>

        <p>
          All the configuration files related to the NNTP (news)
          servers and clients should be located under
          <file>/etc/news</file>.</p>

        <p>
          There are some configuration issues that apply to a number
          of news clients and server packages on the machine. These
          are:

          <taglist>
            <tag><file>/etc/news/organization</file></tag>
            <item>
                A string which should appear as the
                organization header for all messages posted
                by NNTP clients on the machine
            </item>

            <tag><file>/etc/news/server</file></tag>
            <item>
                Contains the FQDN of the upstream NNTP
                server, or localhost if the local machine is
                an NNTP server.
            </item>
          </taglist>

          Other global files may be added as required for cross-package news
          configuration.
        </p>
      </sect>


      <sect>
        <heading>Programs for the X Window System</heading>

        <sect1>
          <heading>Providing X support and package priorities</heading>

          <p>
            Programs that can be configured with support for the X
            Window System must be configured to do so and must declare
            any package dependencies necessary to satisfy their
            runtime requirements when using the X Window System.  If
            such a package is of higher priority than the X packages
            on which it depends, it is required that either the
            X-specific components be split into a separate package, or
            that an alternative version of the package, which includes
            X support, be provided, or that the package's priority be
            lowered.
          </p>
        </sect1>

        <sect1>
          <heading>Packages providing an X server</heading>

          <p>
            Packages that provide an X server that, directly or
            indirectly, communicates with real input and display
            hardware should declare in their control data that they
            provide the virtual package <tt>xserver</tt>.<footnote>
                This implements current practice, and provides an
                actual policy for usage of the <tt>xserver</tt>
                virtual package which appears in the virtual packages
                list.  In a nutshell, X servers that interface
                directly with the display and input hardware or via
                another subsystem (e.g., GGI) should provide
                <tt>xserver</tt>.  Things like <tt>Xvfb</tt>,
                <tt>Xnest</tt>, and <tt>Xprt</tt> should not.
            </footnote>
          </p>
        </sect1>

        <sect1>
          <heading>Packages providing a terminal emulator</heading>

          <p>
            Packages that provide a terminal emulator for the X Window
            System which meet the criteria listed below should declare
            in their control data that they provide the virtual
            package <tt>x-terminal-emulator</tt>.  They should also
            register themselves as an alternative for
            <file>/usr/bin/x-terminal-emulator</file>, with a priority of
            20.
          </p>

          <p>
            To be an <tt>x-terminal-emulator</tt>, a program must:
            <list compact="compact">
              <item>
                  Be able to emulate a DEC VT100 terminal, or a
                  compatible terminal.
              </item>

              <item>
                  Support the command-line option <tt>-e
                    <var>command</var></tt>, which creates a new
                  terminal window<footnote>
                      "New terminal window" does not necessarily mean
                      a new top-level X window directly parented by
                      the window manager; it could, if the terminal
                      emulator application were so coded, be a new
                      "view" in a multiple-document interface (MDI).
                  </footnote>
                  and runs the specified <var>command</var>,
                  interpreting the entirety of the rest of the command
                  line as a command to pass straight to exec, in the
                  manner that <tt>xterm</tt> does.
              </item>

              <item>
                  Support the command-line option <tt>-T
                    <var>title</var></tt>, which creates a new terminal
                  window with the window title <var>title</var>.
              </item>
            </list>
          </p>
        </sect1>

        <sect1>
          <heading>Packages providing a window manager</heading>

          <p>
            Packages that provide a window manager should declare in
            their control data that they provide the virtual package
            <tt>x-window-manager</tt>.  They should also register
            themselves as an alternative for
            <file>/usr/bin/x-window-manager</file>, with a priority
            calculated as follows:
            <list compact="compact">
              <item>
                  Start with a priority of 20.
              </item>

              <item>
                  If the window manager supports the Debian menu
                  system, add 20 points if this support is available
                  in the package's default configuration (i.e., no
                  configuration files belonging to the system or user
                  have to be edited to activate the feature); if
                  configuration files must be modified, add only 10
                  points.
                </p>
              </item>

              <item>
                  If the window manager complies with <url
                    id="http://www.freedesktop.org/Standards/wm-spec"
                    name="The Window Manager Specification Project">,
                  written by the <url id="http://www.freedesktop.org/"
                    name="Free Desktop Group">, add 40 points.
              </item>

              <item>
                  If the window manager permits the X session to be
                  restarted using a <em>different</em> window manager
                  (without killing the X server) in its default
                  configuration, add 10 points; otherwise add none.
              </item>
            </list>
          </p>
        </sect1>

        <sect1>
          <heading>Packages providing fonts</heading>

          <p>
            Packages that provide fonts for the X Window
            System<footnote>
                For the purposes of Debian Policy, a "font for the X
                Window System" is one which is accessed via X protocol
                requests.  Fonts for the Linux console, for PostScript
                renderer, or any other purpose, do not fit this
                definition.  Any tool which makes such fonts available
                to the X Window System, however, must abide by this
                font policy.
            </footnote>
            must do a number of things to ensure that they are both
            available without modification of the X or font server
            configuration, and that they do not corrupt files used by
            other font packages to register information about
            themselves.
            <enumlist>
              <item>
                  Fonts of any type supported by the X Window System
                  must be in a separate binary package from any
                  executables, libraries, or documentation (except
                  that specific to the fonts shipped, such as their
                  license information).  If one or more of the fonts
                  so packaged are necessary for proper operation of
                  the package with which they are associated the font
                  package may be Recommended; if the fonts merely
                  provide an enhancement, a Suggests relationship may
                  be used.  Packages must not Depend on font
                  packages.<footnote>
                      This is because the X server may retrieve fonts
                      from the local file system or over the network
                      from an X font server; the Debian package system
                      is empowered to deal only with the local
                      file system.
                  </footnote>
              </item>

              <item>
                  BDF fonts must be converted to PCF fonts with the
                  <prgn>bdftopcf</prgn> utility (available in the
                  <tt>xfonts-utils</tt> package, <prgn>gzip</prgn>ped, and
                  placed in a directory that corresponds to their
                  resolution:
                  <list compact="compact">
                    <item>
                        100 dpi fonts must be placed in
                        <file>/usr/share/fonts/X11/100dpi/</file>.
                    </item>

                    <item>
                        75 dpi fonts must be placed in
                        <file>/usr/share/fonts/X11/75dpi/</file>.
                    </item>

                    <item>
                        Character-cell fonts, cursor fonts, and other
                        low-resolution fonts must be placed in
                        <file>/usr/share/fonts/X11/misc/</file>.
                    </item>
                  </list>
              </item>

              <item>
                  Speedo fonts must be placed in
                  <file>/usr/share/fonts/X11/Speedo/</file>.
              </item>

              <item>
                  Type 1 fonts must be placed in
                  <file>/usr/share/fonts/X11/Type1/</file>.  If font
                  metric files are available, they must be placed here
                  as well.
              </item>

              <item>
                  Subdirectories of <file>/usr/share/fonts/X11/</file>
                  other than those listed above must be neither
                  created nor used.  (The <file>PEX</file>, <file>CID</file>,
                  and <file>cyrillic</file> directories are excepted for
                  historical reasons, but installation of files into
                  these directories remains discouraged.)
              </item>

              <item>
                  Font packages may, instead of placing files directly
                  in the X font directories listed above, provide
                  symbolic links in that font directory pointing to
                  the files' actual location in the filesystem.  Such
                  a location must comply with the FHS.
              </item>

              <item>
                  Font packages should not contain both 75dpi and
                  100dpi versions of a font.  If both are available,
                  they should be provided in separate binary packages
                  with <tt>-75dpi</tt> or <tt>-100dpi</tt> appended to
                  the names of the packages containing the
                  corresponding fonts.
              </item>

              <item>
                  Fonts destined for the <file>misc</file> subdirectory
                  should not be included in the same package as 75dpi
                  or 100dpi fonts; instead, they should be provided in
                  a separate package with <tt>-misc</tt> appended to
                  its name.
              </item>

              <item>
                  Font packages must not provide the files
                  <file>fonts.dir</file>, <file>fonts.alias</file>, or
                  <file>fonts.scale</file> in a font directory:
                  <list>
                    <item>
                        <file>fonts.dir</file> files must not be provided at all.
                    </item>

                    <item>
                        <file>fonts.alias</file> and <file>fonts.scale</file>
                        files, if needed, should be provided in the
                        directory
                        <file>/etc/X11/fonts/<var>fontdir</var>/<var>package</var>.<var>extension</var></file>,
                        where <var>fontdir</var> is the name of the
                        subdirectory of
                        <file>/usr/share/fonts/X11/</file> where the
                        package's corresponding fonts are stored
                        (e.g., <tt>75dpi</tt> or <tt>misc</tt>),
                        <var>package</var> is the name of the package
                        that provides these fonts, and
                        <var>extension</var> is either <tt>scale</tt>
                        or <tt>alias</tt>, whichever corresponds to
                        the file contents.
                    </item>
                  </list>
              </item>

              <item>
                  Font packages must declare a dependency on
                  <tt>xfonts-utils</tt> in their control
                  data.
              </item>

              <item>
                  Font packages that provide one or more
                  <file>fonts.scale</file> files as described above must
                  invoke <prgn>update-fonts-scale</prgn> on each
                  directory into which they installed fonts
                  <em>before</em> invoking
                  <prgn>update-fonts-dir</prgn> on that directory.
                  This invocation must occur in both the
                  <prgn>postinst</prgn> (for all arguments) and
                  <prgn>postrm</prgn> (for all arguments except
                  <tt>upgrade</tt>) scripts.
              </item>

              <item>
                  Font packages that provide one or more
                  <file>fonts.alias</file> files as described above must
                  invoke <prgn>update-fonts-alias</prgn> on each
                  directory into which they installed fonts.  This
                  invocation must occur in both the
                  <prgn>postinst</prgn> (for all arguments) and
                  <prgn>postrm</prgn> (for all arguments except
                  <tt>upgrade</tt>) scripts.
              </item>

              <item>
                  Font packages must invoke
                  <prgn>update-fonts-dir</prgn> on each directory into
                  which they installed fonts.  This invocation must
                  occur in both the <prgn>postinst</prgn> (for all
                  arguments) and <prgn>postrm</prgn> (for all
                  arguments except <tt>upgrade</tt>) scripts.
              </item>

              <item>
                  Font packages must not provide alias names for the
                  fonts they include which collide with alias names
                  already in use by fonts already packaged.
              </item>

              <item>
                  Font packages must not provide fonts with the same
                  XLFD registry name as another font already packaged.
              </item>
            </enumlist>
          </p>
        </sect1>

        <sect1>
          <heading>Application defaults files</heading>

          <p>
            Application defaults files must be installed in the
            directory <file>/etc/X11/app-defaults/</file> (use of a
            localized subdirectory of <file>/etc/X11/</file> as described
            in the <em>X Toolkit Intrinsics - C Language
            Interface</em> manual is also permitted).  They must be
            registered as <tt>conffile</tt>s or handled as
            configuration files.  Packages must not provide the
            directory <file>/usr/X11R6/lib/X11/app-defaults/</file>.
          </p>

          <p>
            Customization of programs' X resources may also be
            supported with the provision of a file with the same name
            as that of the package placed in the
            <file>/etc/X11/Xresources/</file> directory, which must
            registered as a <tt>conffile</tt> or handled as a
            configuration file.<footnote>
                Note that this mechanism is not the same as using
                app-defaults; app-defaults are tied to the client
                binary on the local file system, whereas X resources
                are stored in the X server and affect all connecting
                clients.
            </footnote>
            <em>Important:</em> packages that install files into the
            <file>/etc/X11/Xresources/</file> directory must conflict with
            <tt>xbase (&lt;&lt; 3.3.2.3a-2)</tt>; if this is not done
            it is possible for the installing package to destroy a
            previously-existing <file>/etc/X11/Xresources</file> file
            which had been customized by the system administrator.
          </p>
        </sect1>

        <sect1>
          <heading>Installation directory issues</heading>

          <p>
            Packages using the X Window System should not be
            configured to install files under the
            <file>/usr/X11R6/</file> directory. The
            <file>/usr/X11R6/</file> directory hierarchy should be
            regarded as obsolete.
          </p>

          <p>
            Programs that use GNU <prgn>autoconf</prgn> and
            <prgn>automake</prgn> are usually easily configured at
            compile time to use <file>/usr/</file> instead of
            <file>/usr/X11R6/</file>, and this should be done whenever
            possible.  Configuration files for window managers and
            display managers should be placed in a subdirectory of
            <file>/etc/X11/</file> corresponding to the package name due
            to these programs' tight integration with the mechanisms
            of the X Window System.  Application-level programs should
            use the <file>/etc/</file> directory unless otherwise mandated
            by policy.
          </p>

          <p>
            The installation of files into subdirectories
            of <file>/usr/X11R6/include/X11/</file> and
            <file>/usr/X11R6/lib/X11/</file> is now prohibited;
            package maintainers should determine if subdirectories of
            <file>/usr/lib/</file> and <file>/usr/share/</file> can be used
            instead. 
          </p>

          <p>
            Packages should install any relevant files into the
            directories <file>/usr/include/X11/</file> and
            <file>/usr/lib/X11/</file>, but if they do so, they must
            pre-depend on <tt>x11-common (&gt;=
            1:7.0.0)</tt><footnote>
              <p>
                These libraries used to be all symbolic
                links. However, with <tt>X11R7</tt>,
                <tt>/usr/include/X11</tt> and <tt>/usr/lib/X11</tt>
                are now real directories, and packages
                <strong>should</strong> ship their files here instead
                of in <tt>/usr/X11R6/{include,lib}/X11</tt>.
                <tt>x11-common (&gt;= 1:7.0.0) </tt> is the package
                responsible for converting these symlinks into
                directories.
              </p>
            </footnote>
          </p>
        </sect1>

        <sect1>
          <heading>The OSF/Motif and OpenMotif libraries</heading>

          <p>
            <em>Programs that require the non-DFSG-compliant OSF/Motif or
              OpenMotif libraries</em><footnote>
                OSF/Motif and OpenMotif are collectively referred to as
                "Motif" in this policy document.
            </footnote>
            should be compiled against and tested with LessTif (a free
            re-implementation of Motif) instead.  If the maintainer
            judges that the program or programs do not work
            sufficiently well with LessTif to be distributed and
            supported, but do so when compiled against Motif, then two
            versions of the package should be created; one linked
            statically against Motif and with <tt>-smotif</tt>
            appended to the package name, and one linked dynamically
            against Motif and with <tt>-dmotif</tt> appended to the
            package name.
          </p>

          <p>
            Both Motif-linked versions are dependent
            upon non-DFSG-compliant software and thus cannot be
            uploaded to the <em>main</em> distribution; if the
            software is itself DFSG-compliant it may be uploaded to
            the <em>contrib</em> distribution.  While known existing
            versions of Motif permit unlimited redistribution of
            binaries linked against the library (whether statically or
            dynamically), it is the package maintainer's
            responsibility to determine whether this is permitted by
            the license of the copy of Motif in their possession.
          </p>
        </sect1>
      </sect>

      <sect id="perl">
        <heading>Perl programs and modules</heading>

        <p>
          Perl programs and modules should follow the current Perl policy.
        </p>

        <p>
          The Perl policy can be found in the <tt>perl-policy</tt>
          files in the <tt>debian-policy</tt> package.
          It is also available from the Debian web mirrors at
          <tt><url name="/doc/packaging-manuals/perl-policy/"
                id="http://www.debian.org/doc/packaging-manuals/perl-policy/"></tt>.
        </p>
      </sect>

      <sect id="emacs">
        <heading>Emacs lisp programs</heading>

        <p>
          Please refer to the "Debian Emacs Policy" for details of how to
          package emacs lisp programs.
        </p>

        <p>
          The Emacs policy is available in
          <file>debian-emacs-policy.gz</file> of the
          <package>emacsen-common</package> package.
          It is also available from the Debian web mirrors at
          <tt><url name="/doc/packaging-manuals/debian-emacs-policy"
                id="http://www.debian.org/doc/packaging-manuals/debian-emacs-policy"></tt>.
        </p>
      </sect>

      <sect>
        <heading>Games</heading>

        <p>
          The permissions on <file>/var/games</file> are mode 755, owner
          <tt>root</tt> and group <tt>root</tt>.
        </p>

        <p>
          Each game decides on its own security policy.</p>

        <p>
          Games which require protected, privileged access to
          high-score files, saved games, etc., may be made
          set-<em>group</em>-id (mode 2755) and owned by
          <tt>root.games</tt>, and use files and directories with
          appropriate permissions (770 <tt>root.games</tt>, for
          example).  They must not be made
          set-<em>user</em>-id, as this causes security problems. (If
          an attacker can subvert any set-user-id game they can
          overwrite the executable of any other, causing other players
          of these games to run a Trojan horse program.  With a
          set-group-id game the attacker only gets access to less
          important game data, and if they can get at the other
          players' accounts at all it will take considerably more
          effort.)</p>

        <p>
          Some packages, for example some fortune cookie programs, are
          configured by the upstream authors to install with their
          data files or other static information made unreadable so
          that they can only be accessed through set-id programs
          provided.  You should not do this in a Debian package: anyone can
          download the <file>.deb</file> file and read the data from it,
          so there is no point making the files unreadable.  Not
          making the files unreadable also means that you don't have
          to make so many programs set-id, which reduces the risk of a
          security hole.</p>

        <p>
          As described in the FHS, binaries of games should be
          installed in the directory <file>/usr/games</file>. This also
          applies to games that use the X Window System. Manual pages
          for games (X and non-X games) should be installed in
          <file>/usr/share/man/man6</file>.</p>
      </sect>
    </chapt>


    <chapt id="docs">
      <heading>Documentation</heading>

      <sect>
        <heading>Manual pages</heading>

        <p>
          You should install manual pages in <prgn>nroff</prgn> source
          form, in appropriate places under <file>/usr/share/man</file>.
          You should only use sections 1 to 9 (see the FHS for more
          details).  You must not install a pre-formatted "cat page".
        </p>

        <p>
          Each program, utility, and function should have an
          associated manual page included in the same package. It is
          suggested that all configuration files also have a manual
          page included as well. Manual pages for protocols and other
          auxiliary things are optional.
        </p>

        <p>
          If no manual page is available, this is considered as a bug
          and should be reported to the Debian Bug Tracking System (the
          maintainer of the package is allowed to write this bug report
          themselves, if they so desire).  Do not close the bug report
          until a proper man page is available.<footnote>
              It is not very hard to write a man page. See the
              <url id="http://www.schweikhardt.net/man_page_howto.html"
                name="Man-Page-HOWTO">,
              <manref name="man" section="7">, the examples
              created by <prgn>debmake</prgn> or <prgn>dh_make</prgn>,
              the helper programs <prgn>help2man</prgn>, or the
              directory <file>/usr/share/doc/man-db/examples</file>.
          </footnote>
        </p>

        <p>
          You may forward a complaint about a missing man page to the
          upstream authors, and mark the bug as forwarded in the
          Debian bug tracking system.  Even though the GNU Project do
          not in general consider the lack of a man page to be a bug,
          we do; if they tell you that they don't consider it a bug
          you should leave the bug in our bug tracking system open
          anyway.
        </p>

        <p>
          Manual pages should be installed compressed using <tt>gzip -9</tt>.
        </p>

        <p>
          If one man page needs to be accessible via several names it
          is better to use a symbolic link than the <file>.so</file>
          feature, but there is no need to fiddle with the relevant
          parts of the upstream source to change from <file>.so</file> to
          symlinks: don't do it unless it's easy.  You should not
          create hard links in the manual page directories, nor put
          absolute filenames in <file>.so</file> directives.  The filename
          in a <file>.so</file> in a man page should be relative to the
          base of the man page tree (usually
          <file>/usr/share/man</file>). If you do not create any links
          (whether symlinks, hard links, or <tt>.so</tt> directives)
          in the file system to the alternate names of the man page,
          then you should not rely on <prgn>man</prgn> finding your
          man page under those names based solely on the information in
          the man page's header.<footnote>
              Supporting this in <prgn>man</prgn> often requires
              unreasonable processing time to find a manual page or to
              report that none exists, and moves knowledge into man's
              database that would be better left in the file system.
              This support is therefore deprecated and will cease to
              be present in the future.
          </footnote>
        </p>

        <p>
          Manual pages in locale-specific subdirectories of
          <file>/usr/share/man</file> should use either UTF-8 or the usual
          legacy encoding for that language (normally the one corresponding
          to the shortest relevant locale name in
          <file>/usr/share/i18n/SUPPORTED</file>). For example, pages under
          <file>/usr/share/man/fr</file> should use either UTF-8 or
          ISO-8859-1.<footnote>
            <prgn>man</prgn> will automatically detect whether UTF-8 is in
            use. In future, all manual pages will be required to use
            UTF-8.
          </footnote>
        </p>

        <p>
          A country name (the <tt>DE</tt> in <tt>de_DE</tt>) should not be
          included in the subdirectory name unless it indicates a
          significant difference in the language, as this excludes
          speakers of the language in other countries.<footnote>
            At the time of writing, Chinese and Portuguese are the main
            languages with such differences, so <file>pt_BR</file>,
            <file>zh_CN</file>, and <file>zh_TW</file> are all allowed.
          </footnote>
        </p>

        <p>
          Due to limitations in current implementations, all characters
          in the manual page source should be representable in the usual
          legacy encoding for that language, even if the file is
          actually encoded in UTF-8. Safe alternative ways to write many
          characters outside that range may be found in
          <manref name="groff_char" section="7">.
        </p>
      </sect>

      <sect>
        <heading>Info documents</heading>

        <p>
          Info documents should be installed in <file>/usr/share/info</file>.
          They should be compressed with <tt>gzip -9</tt>.
        </p>

        <p>
          Your package should call <prgn>install-info</prgn> to update
          the Info <file>dir</file> file in its <prgn>postinst</prgn>
          script when called with a <tt>configure</tt> argument, for
          example:
          <example compact="compact">
install-info --quiet --section Development Development \
  /usr/share/info/foobar.info
          </example></p>

        <p>
          It is a good idea to specify a section for the location of
          your program; this is done with the <tt>--section</tt>
          switch.  To determine which section to use, you should look
          at <file>/usr/share/info/dir</file> on your system and choose the most
          relevant (or create a new section if none of the current
          sections are relevant).  Note that the <tt>--section</tt>
          flag takes two arguments; the first is a regular expression
          to match (case-insensitively) against an existing section,
          the second is used when creating a new one.</p>

        <p>
          You should remove the entries in the <prgn>prerm</prgn>
          script when called with a <tt>remove</tt> argument:
          <example compact="compact">
install-info --quiet --remove /usr/share/info/foobar.info
          </example></p>

        <p>
          If <prgn>install-info</prgn> cannot find a description entry
          in the Info file you must supply one.  See <manref
          name="install-info" section="8"> for details.</p>
      </sect>

      <sect>
        <heading>Additional documentation</heading>

        <p>
          Any additional documentation that comes with the package may
          be installed at the discretion of the package maintainer.
          Plain text documentation should be installed in the directory
          <file>/usr/share/doc/<var>package</var></file>, where
          <var>package</var> is the name of the package, and
          compressed with <tt>gzip -9</tt> unless it is small.
        </p>

        <p>
          If a package comes with large amounts of documentation which
          many users of the package will not require you should create
          a separate binary package to contain it, so that it does not
          take up disk space on the machines of users who do not need
          or want it installed.</p>

        <p>
          It is often a good idea to put text information files
          (<file>README</file>s, changelogs, and so forth) that come with
          the source package in <file>/usr/share/doc/<var>package</var></file>
          in the binary package.  However, you don't need to install
          the instructions for building and installing the package, of
          course!</p>

        <p>
          Packages must not require the existence of any files in
          <file>/usr/share/doc/</file> in order to function
          <footnote>
              The system administrator should be able to
              delete files in <file>/usr/share/doc/</file> without causing
              any programs to break.
          </footnote>.
          Any files that are referenced by programs but are also
          useful as stand alone documentation should be installed under
          <file>/usr/share/<var>package</var>/</file> with symbolic links from
          <file>/usr/share/doc/<var>package</var></file>.
        </p>

        <p>
          <file>/usr/share/doc/<var>package</var></file> may be a symbolic
          link to another directory in <file>/usr/share/doc</file> only if
          the two packages both come from the same source and the
          first package Depends on the second.<footnote>
            <p>
              Please note that this does not override the section on
              changelog files below, so the file 
              <file>/usr/share/<var>package</var>/changelog.Debian.gz</file>
              must refer to the changelog for the current version of
              <var>package</var> in question. In practice, this means
              that the sources of the target and the destination of the
              symlink must be the same (same source package and
              version). 
            </p>
          </footnote>
        </p>

        <p>
          Former Debian releases placed all additional documentation
          in <file>/usr/doc/<var>package</var></file>.  This has been
          changed to <file>/usr/share/doc/<var>package</var></file>,
          and packages must not put documentation in the directory
          <file>/usr/doc/<var>package</var></file>. <footnote>
            At this phase of the transition, we no longer require a
            symbolic link in <file>/usr/doc/</file>. At a later point,
            policy shall change to make the symbolic links a bug.
          </footnote>
        </p>
      </sect>

      <sect>
        <heading>Preferred documentation formats</heading>

        <p>
          The unification of Debian documentation is being carried out
          via HTML.</p>

        <p>
          If your package comes with extensive documentation in a
          markup format that can be converted to various other formats
          you should if possible ship HTML versions in a binary
          package, in the directory
          <file>/usr/share/doc/<var>appropriate-package</var></file> or
          its subdirectories.<footnote>
              The rationale: The important thing here is that HTML
              docs should be available in <em>some</em> package, not
              necessarily in the main binary package.
          </footnote>
        </p>

        <p>
          Other formats such as PostScript may be provided at the
          package maintainer's discretion.
        </p>
      </sect>

      <sect id="copyrightfile">
        <heading>Copyright information</heading>

        <p>
          Every package must be accompanied by a verbatim copy of its
          copyright and distribution license in the file
          <file>/usr/share/doc/<var>package</var>/copyright</file>. This
          file must neither be compressed nor be a symbolic link.
        </p>

        <p>
          In addition, the copyright file must say where the upstream
          sources (if any) were obtained.  It should name the original
          authors of the package and the Debian maintainer(s) who were
          involved with its creation.</p>

        <p>
          A copy of the file which will be installed in
          <file>/usr/share/doc/<var>package</var>/copyright</file> should
          be in <file>debian/copyright</file> in the source package.
        </p>

        <p>
          <file>/usr/share/doc/<var>package</var></file> may be a symbolic
          link to another directory in <file>/usr/share/doc</file> only if
          the two packages both come from the same source and the
          first package Depends on the second.  These rules are
          important because copyrights must be extractable by
          mechanical means.
        </p>

        <p>
          Packages distributed under the UCB BSD license, the Artistic
          license, the GNU GPL (version 2 or 3), the GNU LGPL (versions
          2, 2.1, or 3), and the GNU FDL (version 1.2) should refer to
          the corresponding files under
          <file>/usr/share/common-licenses</file>,<footnote>
            <p>
              In particular,
              <file>/usr/share/common-licenses/BSD</file>,
              <file>/usr/share/common-licenses/Artistic</file>,
              <file>/usr/share/common-licenses/GPL-2</file>,
              <file>/usr/share/common-licenses/GPL-3</file>,
              <file>/usr/share/common-licenses/LGPL-2</file>,
              <file>/usr/share/common-licenses/LGPL-2.1</file>,
              <file>/usr/share/common-licenses/LGPL-3</file>, and
              <file>/usr/share/common-licenses/GFDL-1.2</file>,
              respectively.
            </p>
          </footnote> rather than quoting them in the copyright
          file. 
        </p>

        <p>
          You should not use the copyright file as a general <file>README</file>
          file.  If your package has such a file it should be
          installed in <file>/usr/share/doc/<var>package</var>/README</file> or
          <file>README.Debian</file> or some other appropriate place.</p>
      </sect>

      <sect>
        <heading>Examples</heading>

        <p>
          Any examples (configurations, source files, whatever),
          should be installed in a directory
          <file>/usr/share/doc/<var>package</var>/examples</file>.  These
          files should not be referenced by any program: they're there
          for the benefit of the system administrator and users as
          documentation only.  Architecture-specific example files
          should be installed in a directory
          <file>/usr/lib/<var>package</var>/examples</file> with symbolic
          links to them from
          <file>/usr/share/doc/<var>package</var>/examples</file>, or the
          latter directory itself may be a symbolic link to the
          former.
        </p>

        <p>
          If the purpose of a package is to provide examples, then the
          example files may be installed into
          <file>/usr/share/doc/<var>package</var></file>.
        </p>
      </sect>

      <sect id="changelogs">
        <heading>Changelog files</heading>

        <p>
          Packages that are not Debian-native must contain a
          compressed copy of the <file>debian/changelog</file> file from
          the Debian source tree in
          <file>/usr/share/doc/<var>package</var></file> with the name
          <file>changelog.Debian.gz</file>.
        </p>

        <p>
          If an upstream changelog is available, it should be accessible as
          <file>/usr/share/doc/<var>package</var>/changelog.gz</file> in
          plain text.  If the upstream changelog is distributed in
          HTML, it should be made available in that form as
          <file>/usr/share/doc/<var>package</var>/changelog.html.gz</file>
          and a plain text <file>changelog.gz</file> should be generated
          from it using, for example, <tt>lynx -dump -nolist</tt>.  If
          the upstream changelog files do not already conform to this
          naming convention, then this may be achieved either by
          renaming the files, or by adding a symbolic link, at the
          maintainer's discretion.<footnote>
              Rationale: People should not have to look in places for
              upstream changelogs merely because they are given
              different names or are distributed in HTML format.
          </footnote>
        </p>

        <p>
          All of these files should be installed compressed using
          <tt>gzip -9</tt>, as they will become large with time even
          if they start out small.
        </p>

        <p>
          If the package has only one changelog which is used both as
          the Debian changelog and the upstream one because there is
          no separate upstream maintainer then that changelog should
          usually be installed as
          <file>/usr/share/doc/<var>package</var>/changelog.gz</file>; if
          there is a separate upstream maintainer, but no upstream
          changelog, then the Debian changelog should still be called
          <file>changelog.Debian.gz</file>.
        </p>

        <p>
          For details about the format and contents of the Debian
          changelog file, please see <ref id="dpkgchangelog">.
        </p>
      </sect>
    </chapt>

    <appendix id="pkg-scope">
      <heading>Introduction and scope of these appendices</heading>

      <p>
        These appendices are taken essentially verbatim from the
        now-deprecated Packaging Manual, version 3.2.1.0.  They are
        the chapters which are likely to be of use to package
        maintainers and which have not already been included in the
        policy document itself. Most of these sections are very likely
        not relevant to policy; they should be treated as
        documentation for the packaging system. Please note that these
        appendices are included for convenience, and for historical
        reasons: they used to be part of policy package, and they have
        not yet been incorporated into dpkg documentation. However,
        they still have value, and hence they are presented here.
      </p>

      <p>
        They have not yet been checked to ensure that they are
        compatible with the contents of policy, and if there are any
        contradictions, the version in the main policy document takes
        precedence.  The remaining chapters of the old Packaging
        Manual have also not been read in detail to ensure that there
        are not parts which have been left out.  Both of these will be
        done in due course.
      </p>

      <p>
        Certain parts of the Packaging manual were integrated into the
        Policy Manual proper, and removed from the appendices. Links
        have been placed from the old locations to the new ones.
      </p>

      <p>
        <prgn>dpkg</prgn> is a suite of programs for creating binary
        package files and installing and removing them on Unix
        systems.<footnote>
            <prgn>dpkg</prgn> is targeted primarily at Debian
            GNU/Linux, but may work on or be ported to other
            systems.
        </footnote>
      </p>

      <p>
        The binary packages are designed for the management of
        installed executable programs (usually compiled binaries) and
        their associated data, though source code examples and
        documentation are provided as part of some packages.</p>

      <p>
        This manual describes the technical aspects of creating Debian
        binary packages (<file>.deb</file> files).  It documents the
        behavior of the package management programs
        <prgn>dpkg</prgn>, <prgn>dselect</prgn> et al. and the way
        they interact with packages.</p>

      <p>
        It also documents the interaction between
        <prgn>dselect</prgn>'s core and the access method scripts it
        uses to actually install the selected packages, and describes
        how to create a new access method.</p>

      <p>
        This manual does not go into detail about the options and
        usage of the package building and installation tools.  It
        should therefore be read in conjunction with those programs'
        man pages.
      </p>

      <p>
        The utility programs which are provided with <prgn>dpkg</prgn>
        for managing various system configuration and similar issues,
        such as <prgn>update-rc.d</prgn> and
        <prgn>install-info</prgn>, are not described in detail here -
        please see their man pages.
      </p>

      <p>
        It is assumed that the reader is reasonably familiar with the
        <prgn>dpkg</prgn> System Administrators' manual.
        Unfortunately this manual does not yet exist.
      </p>

      <p>
        The Debian version of the FSF's GNU hello program is provided
        as an example for people wishing to create Debian
        packages. The Debian <prgn>debmake</prgn> package is
        recommended as a very helpful tool in creating and maintaining
        Debian packages. However, while the tools and examples are
        helpful, they do not replace the need to read and follow the
        Policy and Programmer's Manual.</p>
    </appendix>

    <appendix id="pkg-binarypkg">
      <heading>Binary packages (from old Packaging Manual)</heading>

      <p>
        The binary package has two main sections.  The first part
        consists of various control information files and scripts used
        by <prgn>dpkg</prgn> when installing and removing.  See <ref
        id="pkg-controlarea">.
      </p>

      <p>
        The second part is an archive containing the files and
        directories to be installed.
      </p>

      <p>
        In the future binary packages may also contain other
        components, such as checksums and digital signatures. The
        format for the archive is described in full in the
        <file>deb(5)</file> man page.
      </p>


      <sect id="pkg-bincreating"><heading>Creating package files -
      <prgn>dpkg-deb</prgn>
        </heading>

        <p>
          All manipulation of binary package files is done by
          <prgn>dpkg-deb</prgn>; it's the only program that has
          knowledge of the format.  (<prgn>dpkg-deb</prgn> may be
          invoked by calling <prgn>dpkg</prgn>, as <prgn>dpkg</prgn>
          will spot that the options requested are appropriate to
          <prgn>dpkg-deb</prgn> and invoke that instead with the same
          arguments.)
        </p>

        <p>
          In order to create a binary package you must make a
          directory tree which contains all the files and directories
          you want to have in the file system data part of the package.
          In Debian-format source packages this directory is usually
          <file>debian/tmp</file>, relative to the top of the package's
          source tree.
        </p>

        <p>
          They should have the locations (relative to the root of the
          directory tree you're constructing) ownerships and
          permissions which you want them to have on the system when
          they are installed.
        </p>

        <p>
          With current versions of <prgn>dpkg</prgn> the uid/username
          and gid/groupname mappings for the users and groups being
          used should be the same on the system where the package is
          built and the one where it is installed.
        </p>

        <p>
          You need to add one special directory to the root of the
          miniature file system tree you're creating:
          <prgn>DEBIAN</prgn>.  It should contain the control
          information files, notably the binary package control file
          (see <ref id="pkg-controlfile">).
        </p>

        <p>
          The <prgn>DEBIAN</prgn> directory will not appear in the
          file system archive of the package, and so won't be installed
          by <prgn>dpkg</prgn> when the package is installed.
        </p>

        <p>
          When you've prepared the package, you should invoke:
          <example>
  dpkg --build <var>directory</var>
          </example>
        </p>

        <p>
          This will build the package in
          <file><var>directory</var>.deb</file>.  (<prgn>dpkg</prgn> knows
          that <tt>--build</tt> is a <prgn>dpkg-deb</prgn> option, so
          it invokes <prgn>dpkg-deb</prgn> with the same arguments to
          build the package.)
        </p>

        <p>
          See the man page <manref name="dpkg-deb" section="8"> for details of how
          to examine the contents of this newly-created file.  You may find the
          output of following commands enlightening:
          <example>
  dpkg-deb --info <var>filename</var>.deb
  dpkg-deb --contents <var>filename</var>.deb
  dpkg --contents <var>filename</var>.deb
          </example>
          To view the copyright file for a package you could use this command:
          <example>
  dpkg --fsys-tarfile <var>filename</var>.deb | tar xOf - \*/copyright | pager
          </example>
        </p>
      </sect>

      <sect id="pkg-controlarea">
        <heading>Package control information files</heading>

        <p>
          The control information portion of a binary package is a
          collection of files with names known to <prgn>dpkg</prgn>.
          It will treat the contents of these files specially - some
          of them contain information used by <prgn>dpkg</prgn> when
          installing or removing the package; others are scripts which
          the package maintainer wants <prgn>dpkg</prgn> to run.
        </p>

        <p>
          It is possible to put other files in the package control
          area, but this is not generally a good idea (though they
          will largely be ignored).
        </p>

        <p>
          Here is a brief list of the control info files supported by
          <prgn>dpkg</prgn> and a summary of what they're used for.
        </p>

        <p>
          <taglist>
            <tag><tt>control</tt>
            <item>
              <p>
                This is the key description file used by
                <prgn>dpkg</prgn>.  It specifies the package's name
                and version, gives its description for the user,
                states its relationships with other packages, and so
                forth.  See <ref id="sourcecontrolfiles"> and
                <ref id="binarycontrolfiles">.
              </p>

              <p>
                It is usually generated automatically from information
                in the source package by the
                <prgn>dpkg-gencontrol</prgn> program, and with
                assistance from <prgn>dpkg-shlibdeps</prgn>.
                See <ref id="pkg-sourcetools">.
              </p>
            </item>

            <tag><tt>postinst</tt>, <tt>preinst</tt>, <tt>postrm</tt>,
                 <tt>prerm</tt>
            </tag>
            <item>
              <p>
                These are executable files (usually scripts) which
                <prgn>dpkg</prgn> runs during installation, upgrade
                and removal of packages.  They allow the package to
                deal with matters which are particular to that package
                or require more complicated processing than that
                provided by <prgn>dpkg</prgn>.  Details of when and
                how they are called are in <ref id="maintainerscripts">.
              </p>

              <p>
                It is very important to make these scripts idempotent.
                See <ref id="idempotency">.
              </p>

              <p>
                The maintainer scripts are guaranteed to run with a
                controlling terminal and can interact with the user.
                See <ref id="controllingterminal">.
              </p>
            </item>

            <tag><tt>conffiles</tt>
            </tag>
            <item>
                This file contains a list of configuration files which
                are to be handled automatically by <prgn>dpkg</prgn>
                (see <ref id="pkg-conffiles">).  Note that not necessarily
                every configuration file should be listed here.
            </item>

            <tag><tt>shlibs</tt>
            </tag>
            <item>
                This file contains a list of the shared libraries
                supplied by the package, with dependency details for
                each.  This is used by <prgn>dpkg-shlibdeps</prgn>
                when it determines what dependencies are required in a
                package control file. The <tt>shlibs</tt> file format
                is described on <ref id="shlibs">.
            </item>
          </taglist>
        </p>

      <sect id="pkg-controlfile">
        <heading>The main control information file: <tt>control</tt></heading>

        <p>
          The most important control information file used by
          <prgn>dpkg</prgn> when it installs a package is
          <tt>control</tt>.  It contains all the package's "vital
          statistics".
        </p>

        <p>
          The binary package control files of packages built from
          Debian sources are made by a special tool,
          <prgn>dpkg-gencontrol</prgn>, which reads
          <file>debian/control</file> and <file>debian/changelog</file> to
          find the information it needs.  See <ref id="pkg-sourcepkg"> for
          more details.
        </p>

        <p>
          The fields in binary package control files are listed in
          <ref id="binarycontrolfiles">.
        </p>

        <p>
          A description of the syntax of control files and the purpose
          of the fields is available in <ref id="controlfields">.
        </p>
      </sect>

      <sect>
        <heading>Time Stamps</heading>

        <p>
          See <ref id="timestamps">.
        </p>
      </sect>
    </appendix>

    <appendix id="pkg-sourcepkg">
      <heading>Source packages (from old Packaging Manual) </heading>

      <p>
        The Debian binary packages in the distribution are generated
        from Debian sources, which are in a special format to assist
        the easy and automatic building of binaries.
      </p>

      <sect id="pkg-sourcetools">
        <heading>Tools for processing source packages</heading>

        <p>
          Various tools are provided for manipulating source packages;
          they pack and unpack sources and help build of binary
          packages and help manage the distribution of new versions.
        </p>

        <p>
          They are introduced and typical uses described here; see
          <manref name="dpkg-source" section="1"> for full
          documentation about their arguments and operation.
        </p>

        <p>
          For examples of how to construct a Debian source package,
          and how to use those utilities that are used by Debian
          source packages, please see the <prgn>hello</prgn> example
          package.
        </p>

        <sect1 id="pkg-dpkg-source">
          <heading>
            <prgn>dpkg-source</prgn> - packs and unpacks Debian source
            packages
          </heading>

          <p>
            This program is frequently used by hand, and is also
            called from package-independent automated building scripts
            such as <prgn>dpkg-buildpackage</prgn>.
          </p>

          <p>
            To unpack a package it is typically invoked with
            <example>
  dpkg-source -x <var>.../path/to/filename</var>.dsc
            </example>
          </p>

           <p>
            with the <file><var>filename</var>.tar.gz</file> and
            <file><var>filename</var>.diff.gz</file> (if applicable) in
            the same directory.  It unpacks into
            <file><var>package</var>-<var>version</var></file>, and if
            applicable
            <file><var>package</var>-<var>version</var>.orig</file>, in
            the current directory.
          </p>

          <p>
            To create a packed source archive it is typically invoked:
            <example>
  dpkg-source -b <var>package</var>-<var>version</var>
          </example>
          </p>

          <p>
            This will create the <file>.dsc</file>, <file>.tar.gz</file> and
            <file>.diff.gz</file> (if appropriate) in the current
            directory.  <prgn>dpkg-source</prgn> does not clean the
            source tree first - this must be done separately if it is
            required.
          </p>

          <p>
            See also <ref id="pkg-sourcearchives">.</p>
        </sect1>


        <sect1 id="pkg-dpkg-buildpackage">
          <heading>
            <prgn>dpkg-buildpackage</prgn> - overall package-building
            control script
          </heading>

          <p>
            <prgn>dpkg-buildpackage</prgn> is a script which invokes
            <prgn>dpkg-source</prgn>, the <file>debian/rules</file>
            targets <tt>clean</tt>, <tt>build</tt> and
            <tt>binary</tt>, <prgn>dpkg-genchanges</prgn> and
            <prgn>gpg</prgn> (or <prgn>pgp</prgn>) to build a signed
            source and binary package upload.
          </p>

          <p>
            It is usually invoked by hand from the top level of the
            built or unbuilt source directory.  It may be invoked with
            no arguments; useful arguments include:
            <taglist compact="compact">
              <tag><tt>-uc</tt>, <tt>-us</tt></tag>
              <item>
                <p>
                  Do not sign the <tt>.changes</tt> file or the
                  source package <tt>.dsc</tt> file, respectively.</p>
              </item>
              <tag><tt>-p<var>sign-command</var></tt></tag>
              <item>
                <p>
                  Invoke <var>sign-command</var> instead of finding
                  <tt>gpg</tt> or <tt>pgp</tt> on the <prgn>PATH</prgn>.
                  <var>sign-command</var> must behave just like
                  <prgn>gpg</prgn> or <tt>pgp</tt>.</p>
              </item>
              <tag><tt>-r<var>root-command</var></tt></tag>
              <item>
                <p>
                  When root privilege is required, invoke the command
                  <var>root-command</var>.  <var>root-command</var>
                  should invoke its first argument as a command, from
                  the <prgn>PATH</prgn> if necessary, and pass its
                  second and subsequent arguments to the command it
                  calls.  If no <var>root-command</var> is supplied
                  then <var>dpkg-buildpackage</var> will take no
                  special action to gain root privilege, so that for
                  most packages it will have to be invoked as root to
                  start with.</p>
              </item>
              <tag><tt>-b</tt>, <tt>-B</tt></tag>
              <item>
                <p>
                  Two types of binary-only build and upload - see
                  <manref name="dpkg-source" section="1">.
                </p>
              </item>
            </taglist>
          </p>
        </sect1>

        <sect1 id="pkg-dpkg-gencontrol">
          <heading>
            <prgn>dpkg-gencontrol</prgn> - generates binary package
            control files
          </heading>

          <p>
            This program is usually called from <file>debian/rules</file>
            (see <ref id="pkg-sourcetree">) in the top level of the source
            tree.
          </p>

          <p>
            This is usually done just before the files and directories in the
            temporary directory tree where the package is being built have their
            permissions and ownerships set and the package is constructed using
            <prgn>dpkg-deb/</prgn>
              <footnote>
                This is so that the control file which is produced has
                the right permissions
            </footnote>.
          </p>

          <p>
            <prgn>dpkg-gencontrol</prgn> must be called after all the
            files which are to go into the package have been placed in
            the temporary build directory, so that its calculation of
            the installed size of a package is correct.
          </p>

          <p>
            It is also necessary for <prgn>dpkg-gencontrol</prgn> to
            be run after <prgn>dpkg-shlibdeps</prgn> so that the
            variable substitutions created by
            <prgn>dpkg-shlibdeps</prgn> in <file>debian/substvars</file>
            are available.
          </p>

          <p>
            For a package which generates only one binary package, and
            which builds it in <file>debian/tmp</file> relative to the top
            of the source package, it is usually sufficient to call
            <prgn>dpkg-gencontrol</prgn>.
          </p>

          <p>
            Sources which build several binaries will typically need
            something like:
            <example>
  dpkg-gencontrol -Pdebian/tmp-<var>pkg</var> -p<var>package</var>
            </example> The <tt>-P</tt> tells
            <prgn>dpkg-gencontrol</prgn> that the package is being
            built in a non-default directory, and the <tt>-p</tt>
            tells it which package's control file should be generated.
          </p>

          <p>
            <prgn>dpkg-gencontrol</prgn> also adds information to the
            list of files in <file>debian/files</file>, for the benefit of
            (for example) a future invocation of
            <prgn>dpkg-genchanges</prgn>.</p>
        </sect1>

        <sect1 id="pkg-dpkg-shlibdeps">
          <heading>
            <prgn>dpkg-shlibdeps</prgn> - calculates shared library
            dependencies
          </heading>

          <p>
            This program is usually called from <file>debian/rules</file>
            just before <prgn>dpkg-gencontrol</prgn> (see <ref
            id="pkg-sourcetree">), in the top level of the source tree.
          </p>

          <p>
            Its arguments are executables.
            <footnote>
              <p>
                In a forthcoming dpkg version,
                <prgn>dpkg-shlibdeps</prgn> would be required to be
                called on shared libraries as well.
              </p>
              <p>
                They may be specified either in the locations in the
                source tree where they are created or in the locations
                in the temporary build tree where they are installed
                prior to binary package creation.
              </p>
            </footnote> for which shared library dependencies should
            be included in the binary package's control file.
          </p>

          <p>
            If some of the found shared libraries should only
            warrant a <tt>Recommends</tt> or <tt>Suggests</tt>, or if
            some warrant a <tt>Pre-Depends</tt>, this can be achieved
            by using the <tt>-d<var>dependency-field</var></tt> option
            before those executable(s).  (Each <tt>-d</tt> option
            takes effect until the next <tt>-d</tt>.)
          </p>

          <p>
            <prgn>dpkg-shlibdeps</prgn> does not directly cause the
            output control file to be modified.  Instead by default it
            adds to the <file>debian/substvars</file> file variable
            settings like <tt>shlibs:Depends</tt>.  These variable
            settings must be referenced in dependency fields in the
            appropriate per-binary-package sections of the source
            control file.
          </p>

          <p>
            For example, a package that generates an essential part
            which requires dependencies, and optional parts that 
            which only require a recommendation, would separate those
            two sets of dependencies into two different fields.<footnote>
                At the time of writing, an example for this was the
                <package/xmms/ package, with Depends used for the xmms
                executable, Recommends for the plug-ins and Suggests for
                even more optional features provided by unzip.
            </footnote>
            It can say in its <file>debian/rules</file>:
            <example>
  dpkg-shlibdeps -dDepends <var>program anotherprogram ...</var> \
                 -dRecommends <var>optionalpart anotheroptionalpart</var>
            </example>
            and then in its main control file <file>debian/control</file>:
            <example>
  <var>...</var>
  Depends: ${shlibs:Pre-Depends}
  Recommends: ${shlibs:Recommends}
  <var>...</var>
            </example>
          </p>

          <p>
            Sources which produce several binary packages with
            different shared library dependency requirements can use
            the <tt>-p<var>varnameprefix</var></tt> option to override
            the default <tt>shlibs:</tt> prefix (one invocation of
            <prgn>dpkg-shlibdeps</prgn> per setting of this option).
            They can thus produce several sets of dependency
            variables, each of the form
            <tt><var>varnameprefix</var>:<var>dependencyfield</var></tt>,
            which can be referred to in the appropriate parts of the
            binary package control files.
          </p>
        </sect1>


        <sect1 id="pkg-dpkg-distaddfile">
          <heading>
            <prgn>dpkg-distaddfile</prgn> - adds a file to
            <file>debian/files</file>
          </heading>

          <p>
            Some packages' uploads need to include files other than
            the source and binary package files.
          </p>

          <p>
            <prgn>dpkg-distaddfile</prgn> adds a file to the
            <file>debian/files</file> file so that it will be included in
            the <file>.changes</file> file when
            <prgn>dpkg-genchanges</prgn> is run.
          </p>

          <p>
            It is usually invoked from the <tt>binary</tt> target of
            <file>debian/rules</file>:
            <example>
  dpkg-distaddfile <var>filename</var> <var>section</var> <var>priority</var>
            </example>
            The <var>filename</var> is relative to the directory where
            <prgn>dpkg-genchanges</prgn> will expect to find it - this
            is usually the directory above the top level of the source
            tree.  The <file>debian/rules</file> target should put the
            file there just before or just after calling
            <prgn>dpkg-distaddfile</prgn>.
          </p>

          <p>
            The <var>section</var> and <var>priority</var> are passed
            unchanged into the resulting <file>.changes</file> file.
          </p>
        </sect1>


        <sect1 id="pkg-dpkg-genchanges">
          <heading>
            <prgn>dpkg-genchanges</prgn> - generates a <file>.changes</file>
            upload control file
          </heading>

          <p>
            This program is usually called by package-independent
            automatic building scripts such as
            <prgn>dpkg-buildpackage</prgn>, but it may also be called
            by hand.
          </p>

          <p>
            It is usually called in the top level of a built source
            tree, and when invoked with no arguments will print out a
            straightforward <file>.changes</file> file based on the
            information in the source package's changelog and control
            file and the binary and source packages which should have
            been built.
          </p>
        </sect1>


        <sect1 id="pkg-dpkg-parsechangelog">
          <heading>
            <prgn>dpkg-parsechangelog</prgn> - produces parsed
            representation of a changelog
          </heading>

          <p>
            This program is used internally by
            <prgn>dpkg-source</prgn> et al.  It may also occasionally
            be useful in <file>debian/rules</file> and elsewhere.  It
            parses a changelog, <file>debian/changelog</file> by default,
            and prints a control-file format representation of the
            information in it to standard output.
          </p>
        </sect1>

        <sect1 id="pkg-dpkg-architecture">
          <heading>
            <prgn>dpkg-architecture</prgn> - information about the build and
            host system
          </heading>

          <p>
            This program can be used manually, but is also invoked by
            <tt>dpkg-buildpackage</tt> or <file>debian/rules</file> to set
            to set environment or make variables which specify the build and
            host architecture for the package building process.
          </p>
        </sect1>
      </sect>

      <sect id="pkg-sourcetree">
        <heading>The Debianised source tree</heading>

        <p>
          The source archive scheme described later is intended to
          allow a Debianised source tree with some associated control
          information to be reproduced and transported easily.  The
          Debianised source tree is a version of the original program
          with certain files added for the benefit of the
          Debianisation process, and with any other changes required
          made to the rest of the source code and installation
          scripts.
        </p>

        <p>
          The extra files created for Debian are in the subdirectory
          <file>debian</file> of the top level of the Debianised source
          tree.  They are described below.
        </p>

        <sect1 id="pkg-debianrules">
          <heading><file>debian/rules</file> - the main building script</heading>

          <p>
            See <ref id="debianrules">.
          </p>
        </sect1>


        <sect1 id="pkg-dpkgchangelog">
          <heading><file>debian/changelog</file></heading>

          <p>
            See <ref id="dpkgchangelog">.
          </p>

          <p>
            It is recommended that the entire changelog be encoded in the
            <url id="http://www.cis.ohio-state.edu/cgi-bin/rfc/rfc2279.html" name="UTF-8">
            encoding of
            <url id="http://www.unicode.org/"
            name="Unicode">.<footnote>
              <p>
                I think it is fairly obvious that we need to
                eventually transition to UTF-8 for our package
                infrastructure; it is really the only sane char-set in
                an international environment.  Now, we can't switch to
                using UTF-8 for package control fields and the like
                until dpkg has better support, but one thing we can
                start doing today is requesting that Debian changelogs
                are UTF-8 encoded. At some point in time, we can start
                requiring them to do so. 
              </p>
              <p>
                Checking for non-UTF8 characters in a changelog is
                trivial.  Dump the file through 
                <example>iconv -f utf-8 -t ucs-4</example>
                  discard the output, and check the return
                value.  If there are any characters in the stream
                which are invalid UTF-8 sequences, iconv will exit
                with an error code; and this will be the case for the
                vast majority of other character sets.
              </p>
            </footnote>
          </p>

          <sect2><heading>Defining alternative changelog formats
            </heading>

            <p>
              It is possible to use a different format to the standard
              one, by providing a parser for the format you wish to
              use.
            </p>

            <p>
              In order to have <tt>dpkg-parsechangelog</tt> run your
              parser, you must include a line within the last 40 lines
              of your file matching the Perl regular expression:
              <tt>\schangelog-format:\s+([0-9a-z]+)\W</tt> The part in
              parentheses should be the name of the format.  For
              example, you might say:
              <example>
  @@@ changelog-format: joebloggs @@@
              </example>
              Changelog format names are non-empty strings of alphanumerics.
            </p>

            <p>
              If such a line exists then <tt>dpkg-parsechangelog</tt>
              will look for the parser as
              <file>/usr/lib/dpkg/parsechangelog/<var>format-name</var></file>
              or
              <file>/usr/local/lib/dpkg/parsechangelog/<var>format-name</var></file>;
              it is an error for it not to find it, or for it not to
              be an executable program.  The default changelog format
              is <tt>dpkg</tt>, and a parser for it is provided with
              the <tt>dpkg</tt> package.
            </p>

            <p>
              The parser will be invoked with the changelog open on
              standard input at the start of the file.  It should read
              the file (it may seek if it wishes) to determine the
              information required and return the parsed information
              to standard output in the form of a series of control
              fields in the standard format.  By default it should
              return information about only the most recent version in
              the changelog; it should accept a
              <tt>-v<var>version</var></tt> option to return changes
              information from all versions present <em>strictly
              after</em> <var>version</var>, and it should then be an
              error for <var>version</var> not to be present in the
              changelog.
            </p>

            <p>
              The fields are:
              <list compact="compact">
                <item><qref id="f-Source"><tt>Source</tt></qref></item>
                <item><qref id="f-Version"><tt>Version</tt></qref> (mandatory)</item>
                <item><qref id="f-Distribution"><tt>Distribution</tt></qref> (mandatory)</item>
                <item><qref id="f-Urgency"><tt>Urgency</tt></qref> (mandatory)</item>
                <item><qref id="f-Maintainer"><tt>Maintainer</tt></qref> (mandatory)</item>
                <item><qref id="f-Date"><tt>Date</tt></qref></item>
                <item><qref id="f-Changes"><tt>Changes</tt></qref> (mandatory)</item>
              </list>
            </p>

            <p>
              If several versions are being returned (due to the use
              of <tt>-v</tt>), the urgency value should be of the
              highest urgency code listed at the start of any of the
              versions requested followed by the concatenated
              (space-separated) comments from all the versions
              requested; the maintainer, version, distribution and
              date should always be from the most recent version.
            </p>

            <p>
              For the format of the <tt>Changes</tt> field see
              <ref id="f-Changes">.
            </p>

            <p>
              If the changelog format which is being parsed always or
              almost always leaves a blank line between individual
              change notes these blank lines should be stripped out,
              so as to make the resulting output compact.
            </p>

            <p>
              If the changelog format does not contain date or package
              name information this information should be omitted from
              the output.  The parser should not attempt to synthesize
              it or find it from other sources.
            </p>

            <p>
              If the changelog does not have the expected format the
              parser should exit with a nonzero exit status, rather
              than trying to muddle through and possibly generating
              incorrect output.
            </p>

            <p>
              A changelog parser may not interact with the user at
              all.
            </p>
          </sect2>
        </sect1>

        <sect1 id="pkg-srcsubstvars">
          <heading><file>debian/substvars</file> and variable substitutions</heading>

          <p>
            See <ref id="substvars">.
          </p>

        </sect1>

        <sect1>
          <heading><file>debian/files</file></heading>

          <p>
            See <ref id="debianfiles">.
          </p>
        </sect1>

        <sect1><heading><file>debian/tmp</file>
          </heading>

          <p>
            This is the canonical temporary location for the
            construction of binary packages by the <tt>binary</tt>
            target.  The directory <file>tmp</file> serves as the root of
            the file system tree as it is being constructed (for
            example, by using the package's upstream makefiles install
            targets and redirecting the output there), and it also
            contains the <tt>DEBIAN</tt> subdirectory.  See <ref
            id="pkg-bincreating">.
          </p>

          <p>
            If several binary packages are generated from the same
            source tree it is usual to use several
            <file>debian/tmp<var>something</var></file> directories, for
            example <file>tmp-a</file> or <file>tmp-doc</file>.
          </p>

          <p>
            Whatever <file>tmp</file> directories are created and used by
            <tt>binary</tt> must of course be removed by the
            <tt>clean</tt> target.</p></sect1>
      </sect>


      <sect id="pkg-sourcearchives"><heading>Source packages as archives
        </heading>

        <p>
          As it exists on the FTP site, a Debian source package
          consists of three related files.  You must have the right
          versions of all three to be able to use them.
        </p>

        <p>
          <taglist>
            <tag>Debian source control file - <tt>.dsc</tt></tag>
            <item>
                This file is a control file used by <prgn>dpkg-source</prgn>
                to extract a source package.
                See <ref id="debiansourcecontrolfiles">.
            </item>

            <tag>
              Original source archive -
              <file>
                <var>package</var>_<var>upstream-version</var>.orig.tar.gz
              </file>
            </tag>

            <item>
              <p>
                This is a compressed (with <tt>gzip -9</tt>)
                <prgn>tar</prgn> file containing the source code from
                the upstream authors of the program.
              </p>
            </item>

            <tag>
              Debianisation diff -
              <file>
                <var>package</var>_<var>upstream_version-revision</var>.diff.gz
              </file>
            </tag>
            <item>

              <p>
                This is a unified context diff (<tt>diff -u</tt>)
                giving the changes which are required to turn the
                original source into the Debian source.  These changes
                may only include editing and creating plain files.
                The permissions of files, the targets of symbolic
                links and the characteristics of special files or
                pipes may not be changed and no files may be removed
                or renamed.
              </p>

              <p>
                All the directories in the diff must exist, except the
                <file>debian</file> subdirectory of the top of the source
                tree, which will be created by
                <prgn>dpkg-source</prgn> if necessary when unpacking.
              </p>

              <p>
                The <prgn>dpkg-source</prgn> program will
                automatically make the <file>debian/rules</file> file
                executable (see below).</p></item>
          </taglist>
        </p>

        <p>
          If there is no original source code - for example, if the
          package is specially prepared for Debian or the Debian
          maintainer is the same as the upstream maintainer - the
          format is slightly different: then there is no diff, and the
          tarfile is named
          <file><var>package</var>_<var>version</var>.tar.gz</file>,
          and preferably contains a directory named
          <file><var>package</var>-<var>version</var></file>.
        </p>
      </sect>

      <sect>
        <heading>Unpacking a Debian source package without <prgn>dpkg-source</prgn></heading>

        <p>
          <tt>dpkg-source -x</tt> is the recommended way to unpack a
          Debian source package.  However, if it is not available it
          is possible to unpack a Debian source archive as follows:
        <enumlist compact="compact">
          <item>
            <p>
              Untar the tarfile, which will create a <file>.orig</file>
              directory.</p>
          </item>
          <item>
            <p>Rename the <file>.orig</file> directory to
              <file><var>package</var>-<var>version</var></file>.</p>
          </item>
            <item>
            <p>
              Create the subdirectory <file>debian</file> at the top of
              the source tree.</p>
          </item>
          <item><p>Apply the diff using <tt>patch -p0</tt>.</p>
          </item>
          <item><p>Untar the tarfile again if you want a copy of the original
              source code alongside the Debianised version.</p>
          </item>
        </enumlist>

        <p>
          It is not possible to generate a valid Debian source archive
          without using <prgn>dpkg-source</prgn>.  In particular,
          attempting to use <prgn>diff</prgn> directly to generate the
          <file>.diff.gz</file> file will not work.
        </p>

        <sect1>
          <heading>Restrictions on objects in source packages</heading>

          <p>
            The source package may not contain any hard links
            <footnote>
                This is not currently detected when building source
                packages, but only when extracting
                them.
            </footnote>
            <footnote>
                Hard links may be permitted at some point in the
                future, but would require a fair amount of
                work.
            </footnote>, device special files, sockets or setuid or
            setgid files.
            <footnote>
                Setgid directories are allowed.
            </footnote>
          </p>

          <p>
            The source packaging tools manage the changes between the
            original and Debianised source using <prgn>diff</prgn> and
            <prgn>patch</prgn>.  Turning the original source tree as
            included in the <file>.orig.tar.gz</file> into the debianised
            source must not involve any changes which cannot be
            handled by these tools.  Problematic changes which cause
            <prgn>dpkg-source</prgn> to halt with an error when
            building the source package are:
            <list compact="compact">
              <item><p>Adding or removing symbolic links, sockets or pipes.</p>
              </item>
              <item><p>Changing the targets of symbolic links.</p>
              </item>
              <item><p>Creating directories, other than <file>debian</file>.</p>
              </item>
              <item><p>Changes to the contents of binary files.</p></item>
            </list> Changes which cause <prgn>dpkg-source</prgn> to
            print a warning but continue anyway are:
            <list compact="compact">
              <item>
                <p>
                  Removing files, directories or symlinks.
                  <footnote>
                      Renaming a file is not treated specially - it is
                      seen as the removal of the old file (which
                      generates a warning, but is otherwise ignored),
                      and the creation of the new one.
                  </footnote>
                </p>
              </item>
              <item>
                <p>
                  Changed text files which are missing the usual final
                  newline (either in the original or the modified
                  source tree).
                </p>
              </item>
            </list>
            Changes which are not represented, but which are not detected by
            <prgn>dpkg-source</prgn>, are:
            <list compact="compact">
              <item><p>Changing the permissions of files (other than
                  <file>debian/rules</file>) and directories.</p></item>
            </list>
          </p>

          <p>
            The <file>debian</file> directory and <file>debian/rules</file>
            are handled specially by <prgn>dpkg-source</prgn> - before
            applying the changes it will create the <file>debian</file>
            directory, and afterwards it will make
            <file>debian/rules</file> world-executable.
          </p>
        </sect1>
      </sect>
    </appendix>

    <appendix id="pkg-controlfields">
      <heading>Control files and their fields (from old Packaging Manual)</heading>

      <p>
        Many of the tools in the <prgn>dpkg</prgn> suite manipulate
        data in a common format, known as control files.  Binary and
        source packages have control data as do the <file>.changes</file>
        files which control the installation of uploaded files, and
        <prgn>dpkg</prgn>'s internal databases are in a similar
        format.
      </p>

      <sect>
        <heading>Syntax of control files</heading>

        <p>
          See <ref id="controlsyntax">.
        </p>

        <p>
          It is important to note that there are several fields which
          are optional as far as <prgn>dpkg</prgn> and the related
          tools are concerned, but which must appear in every Debian
          package, or whose omission may cause problems.
        </p>
      </sect>

      <sect>
        <heading>List of fields</heading>

        <p>
          See <ref id="controlfieldslist">.
        </p>

        <p>
          This section now contains only the fields that didn't belong
          to the Policy manual.
        </p>

        <sect1 id="pkg-f-Filename">
          <heading><tt>Filename</tt> and <tt>MSDOS-Filename</tt></heading>

          <p>
            These fields in <tt>Packages</tt> files give the
            filename(s) of (the parts of) a package in the
            distribution directories, relative to the root of the
            Debian hierarchy.  If the package has been split into
            several parts the parts are all listed in order, separated
            by spaces.
          </p>
        </sect1>

        <sect1 id="pkg-f-Size">
          <heading><tt>Size</tt> and <tt>MD5sum</tt></heading>

          <p>
            These fields in <file>Packages</file> files give the size (in
            bytes, expressed in decimal) and MD5 checksum of the
            file(s) which make(s) up a binary package in the
            distribution.  If the package is split into several parts
            the values for the parts are listed in order, separated by
            spaces.
          </p>
        </sect1>

        <sect1 id="pkg-f-Status">
          <heading><tt>Status</tt></heading>

          <p>
            This field in <prgn>dpkg</prgn>'s status file records
            whether the user wants a package installed, removed or
            left alone, whether it is broken (requiring
            re-installation) or not and what its current state on the
            system is.  Each of these pieces of information is a
            single word.
          </p>
        </sect1>

        <sect1 id="pkg-f-Config-Version">
          <heading><tt>Config-Version</tt></heading>

          <p>
            If a package is not installed or not configured, this
            field in <prgn>dpkg</prgn>'s status file records the last
            version of the package which was successfully
            configured.
          </p>
        </sect1>

        <sect1 id="pkg-f-Conffiles">
          <heading><tt>Conffiles</tt></heading>

          <p>
            This field in <prgn>dpkg</prgn>'s status file contains
            information about the automatically-managed configuration
            files held by a package.  This field should <em>not</em>
            appear anywhere in a package!
          </p>
        </sect1>

        <sect1>
          <heading>Obsolete fields</heading>

          <p>
            These are still recognized by <prgn>dpkg</prgn> but should
            not appear anywhere any more.

            <taglist compact="compact">

              <tag><tt>Revision</tt></tag>
              <tag><tt>Package-Revision</tt></tag>
              <tag><tt>Package_Revision</tt></tag>
              <item>
                  The Debian revision part of the package version was
                  at one point in a separate control file field.  This
                  field went through several names.
              </item>

              <tag><tt>Recommended</tt></tag>
              <item>Old name for <tt>Recommends</tt>.</item>

              <tag><tt>Optional</tt></tag>
              <item>Old name for <tt>Suggests</tt>.</item>

              <tag><tt>Class</tt></tag>
              <item>Old name for <tt>Priority</tt>.</item>

            </taglist>
          </p>
        </sect1>
      </sect>

    </appendix>

    <appendix id="pkg-conffiles">
      <heading>Configuration file handling (from old Packaging Manual)</heading>

      <p>
        <prgn>dpkg</prgn> can do a certain amount of automatic
        handling of package configuration files.
      </p>

      <p>
        Whether this mechanism is appropriate depends on a number of
        factors, but basically there are two approaches to any
        particular configuration file.
      </p>

      <p>
        The easy method is to ship a best-effort configuration in the
        package, and use <prgn>dpkg</prgn>'s conffile mechanism to
        handle updates.  If the user is unlikely to want to edit the
        file, but you need them to be able to without losing their
        changes, and a new package with a changed version of the file
        is only released infrequently, this is a good approach.
      </p>

      <p>
        The hard method is to build the configuration file from
        scratch in the <prgn>postinst</prgn> script, and to take the
        responsibility for fixing any mistakes made in earlier
        versions of the package automatically.  This will be
        appropriate if the file is likely to need to be different on
        each system.
      </p>

      <sect><heading>Automatic handling of configuration files by
      <prgn>dpkg</prgn>
        </heading>

        <p>
          A package may contain a control area file called
          <tt>conffiles</tt>.  This file should be a list of filenames
          of configuration files needing automatic handling, separated
          by newlines.  The filenames should be absolute pathnames,
          and the files referred to should actually exist in the
          package.
        </p>

        <p>
          When a package is upgraded <prgn>dpkg</prgn> will process
          the configuration files during the configuration stage,
          shortly before it runs the package's <prgn>postinst</prgn>
          script,
        </p>

        <p>
          For each file it checks to see whether the version of the
          file included in the package is the same as the one that was
          included in the last version of the package (the one that is
          being upgraded from); it also compares the version currently
          installed on the system with the one shipped with the last
          version.
        </p>

        <p>
          If neither the user nor the package maintainer has changed
          the file, it is left alone.  If one or the other has changed
          their version, then the changed version is preferred - i.e.,
          if the user edits their file, but the package maintainer
          doesn't ship a different version, the user's changes will
          stay, silently, but if the maintainer ships a new version
          and the user hasn't edited it the new version will be
          installed (with an informative message).  If both have
          changed their version the user is prompted about the problem
          and must resolve the differences themselves.
        </p>

        <p>
          The comparisons are done by calculating the MD5 message
          digests of the files, and storing the MD5 of the file as it
          was included in the most recent version of the package.
        </p>

        <p>
          When a package is installed for the first time
          <prgn>dpkg</prgn> will install the file that comes with it,
          unless that would mean overwriting a file already on the
          file system.
        </p>

        <p>
          However, note that <prgn>dpkg</prgn> will <em>not</em>
          replace a conffile that was removed by the user (or by a
          script).  This is necessary because with some programs a
          missing file produces an effect hard or impossible to
          achieve in another way, so that a missing file needs to be
          kept that way if the user did it.
        </p>

        <p>
          Note that a package should <em>not</em> modify a
          <prgn>dpkg</prgn>-handled conffile in its maintainer
          scripts.  Doing this will lead to <prgn>dpkg</prgn> giving
          the user confusing and possibly dangerous options for
          conffile update when the package is upgraded.</p>
      </sect>

      <sect><heading>Fully-featured maintainer script configuration
      handling
        </heading>

        <p>
          For files which contain site-specific information such as
          the hostname and networking details and so forth, it is
          better to create the file in the package's
          <prgn>postinst</prgn> script.
        </p>

        <p>
          This will typically involve examining the state of the rest
          of the system to determine values and other information, and
          may involve prompting the user for some information which
          can't be obtained some other way.
        </p>

        <p>
          When using this method there are a couple of important
          issues which should be considered:
        </p>

        <p>
          If you discover a bug in the program which generates the
          configuration file, or if the format of the file changes
          from one version to the next, you will have to arrange for
          the postinst script to do something sensible - usually this
          will mean editing the installed configuration file to remove
          the problem or change the syntax.  You will have to do this
          very carefully, since the user may have changed the file,
          perhaps to fix the very problem that your script is trying
          to deal with - you will have to detect these situations and
          deal with them correctly.
        </p>

        <p>
          If you do go down this route it's probably a good idea to
          make the program that generates the configuration file(s) a
          separate program in <file>/usr/sbin</file>, by convention called
          <file><var>package</var>config</file> and then run that if
          appropriate from the post-installation script.  The
          <tt><var>package</var>config</tt> program should not
          unquestioningly overwrite an existing configuration - if its
          mode of operation is geared towards setting up a package for
          the first time (rather than any arbitrary reconfiguration
          later) you should have it check whether the configuration
          already exists, and require a <tt>--force</tt> flag to
          overwrite it.</p></sect>
    </appendix>

    <appendix id="pkg-alternatives"><heading>Alternative versions of
        an interface - <prgn>update-alternatives</prgn> (from old
    Packaging Manual)
      </heading>

      <p>
        When several packages all provide different versions of the
        same program or file it is useful to have the system select a
        default, but to allow the system administrator to change it
        and have their decisions respected.
      </p>

      <p>
        For example, there are several versions of the <prgn>vi</prgn>
        editor, and there is no reason to prevent all of them from
        being installed at once, each under their own name
        (<prgn>nvi</prgn>, <prgn>vim</prgn> or whatever).
        Nevertheless it is desirable to have the name <tt>vi</tt>
        refer to something, at least by default.
      </p>

      <p>
        If all the packages involved cooperate, this can be done with
        <prgn>update-alternatives</prgn>.
      </p>

      <p>
        Each package provides its own version under its own name, and
        calls <prgn>update-alternatives</prgn> in its postinst to
        register its version (and again in its prerm to deregister
        it).
      </p>

      <p>
        See the man page <manref name="update-alternatives"
        section="8"> for details.
      </p>

      <p>
        If <prgn>update-alternatives</prgn> does not seem appropriate
        you may wish to consider using diversions instead.</p>
    </appendix>

    <appendix id="pkg-diversions"><heading>Diversions - overriding a
    package's version of a file (from old Packaging Manual)
      </heading>

      <p>
        It is possible to have <prgn>dpkg</prgn> not overwrite a file
        when it reinstalls the package it belongs to, and to have it
        put the file from the package somewhere else instead.
      </p>

      <p>
        This can be used locally to override a package's version of a
        file, or by one package to override another's version (or
        provide a wrapper for it).
      </p>

      <p>
        Before deciding to use a diversion, read <ref
        id="pkg-alternatives"> to see if you really want a diversion
        rather than several alternative versions of a program.
      </p>

      <p>
        There is a diversion list, which is read by <prgn>dpkg</prgn>,
        and updated by a special program <prgn>dpkg-divert</prgn>.
        Please see <manref name="dpkg-divert" section="8"> for full
        details of its operation.
      </p>

      <p>
        When a package wishes to divert a file from another, it should
        call <prgn>dpkg-divert</prgn> in its preinst to add the
        diversion and rename the existing file.  For example,
        supposing that a <prgn>smailwrapper</prgn> package wishes to
        install a wrapper around <file>/usr/sbin/smail</file>:
        <example>
  if [ install = "$1"  ]; then
     dpkg-divert --package smailwrapper --add --rename \
        --divert /usr/sbin/smail.real /usr/sbin/smail
  fi
        </example> Testing <tt>$1</tt> is necessary so that the script
        doesn't try to add the diversion again when
        <prgn>smailwrapper</prgn> is upgraded.  The <tt>--package
        smailwrapper</tt> ensures that <prgn>smailwrapper</prgn>'s
        copy of <file>/usr/sbin/smail</file> can bypass the diversion and
        get installed as the true version.
      </p>

      <p>
        The postrm has to do the reverse:
        <example>
  if [ remove = "$1" ]; then
     dpkg-divert --package smailwrapper --remove --rename \
        --divert /usr/sbin/smail.real /usr/sbin/smail
  fi
        </example>
      </p>

      <p>
        Do not attempt to divert a file which is vitally important for
        the system's operation - when using <prgn>dpkg-divert</prgn>
        there is a time, after it has been diverted but before
        <prgn>dpkg</prgn> has installed the new version, when the file
        does not exist.</p>
    </appendix>

  </book>
</debiandoc>
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