apt/doc/dpkg-tech.dbk

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<book lang="en">

<title>dpkg technical manual</title>

<bookinfo>

<authorgroup>
  <author>
    <personname>Tom Lees</personname><email>tom@lpsg.demon.co.uk</email>
  </author>
</authorgroup>

<releaseinfo>Version &apt-product-version;</releaseinfo>

<abstract>
<para>
This document describes the minimum necessary workings for the APT dselect
replacement. It gives an overall specification of what its external interface
must look like for compatibility, and also gives details of some internal
quirks.
</para>
</abstract>

<copyright><year>1997</year><holder>Tom Lees</holder></copyright>

<legalnotice>
<title>License Notice</title>
<para>
APT and this document are free software; you can redistribute them and/or
modify them under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at your
option) any later version.
</para>
<para>
For more details, on Debian systems, see the file
/usr/share/common-licenses/GPL for the full license.
</para>
</legalnotice>

</bookinfo>

<chapter id="ch1"><title>Quick summary of dpkg's external interface</title>

<section id="control"><title>Control files</title>
<para>
The basic dpkg package control file supports the following major features:-
</para>
<itemizedlist>
<listitem>
<para>
5 types of dependencies:-
</para>
<itemizedlist>
<listitem>
<para>
Pre-Depends, which must be satisfied before a package may be unpacked
</para>
</listitem>
<listitem>
<para>
Depends, which must be satisfied before a package may be configured
</para>
</listitem>
<listitem>
<para>
Recommends, to specify a package which if not installed may severely limit the
usefulness of the package
</para>
</listitem>
<listitem>
<para>
Suggests, to specify a package which may increase the productivity of the
package
</para>
</listitem>
<listitem>
<para>
Conflicts, to specify a package which must NOT be installed in order for the
package to be configured
</para>
</listitem>
<listitem>
<para>
Breaks, to specify a package which is broken by the package and which should
therefore not be configured while broken
</para>
</listitem>
</itemizedlist>
<para>
Each of these dependencies can specify a version and a dependency on that
version, for example "&lt;= 0.5-1", "== 2.7.2-1", etc. The comparators
available are:-
</para>
<itemizedlist>
<listitem>
<para>
"&lt;&lt;" - less than
</para>
</listitem>
<listitem>
<para>
"&lt;=" - less than or equal to
</para>
</listitem>
<listitem>
<para>
"&gt;&gt;" - greater than
</para>
</listitem>
<listitem>
<para>
"&gt;=" - greater than or equal to
</para>
</listitem>
<listitem>
<para>
"==" - equal to
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
The concept of "virtual packages", which many other packages may provide,
using the Provides mechanism. An example of this is the "httpd" virtual
package, which all web servers should provide. Virtual package names may be
used in dependency headers. However, current policy is that virtual packages
do not support version numbers, so dependencies on virtual packages with
versions will always fail.
</para>
</listitem>
<listitem>
<para>
Several other control fields, such as Package, Version, Description, Section,
Priority, etc., which are mainly for classification purposes. The package
name must consist entirely of lowercase characters, plus the characters '+',
'-', and '.'. Fields can extend across multiple lines - on the second and
subsequent lines, there is a space at the beginning instead of a field name
and a ':'. Empty lines must consist of the text " .", which will be ignored,
as will the initial space for other continuation lines. This feature is
usually only used in the Description field.
</para>
</listitem>
</itemizedlist>
</section>

<section id="s1.2"><title>The dpkg status area</title>
<para>
The "dpkg status area" is the term used to refer to the directory where dpkg
keeps its various status files (GNU would have you call it the dpkg shared
state directory). This is always, on Debian systems, /var/lib/dpkg. However,
the default directory name should not be hard-coded, but #define'd, so that
alteration is possible (it is available via configure in dpkg 1.4.0.9 and
above). Of course, in a library, code should be allowed to override the
default directory, but the default should be part of the library (so that
the user may change the dpkg admin dir simply by replacing the library).
</para>
<para>
Dpkg keeps a variety of files in its status area. These are discussed later
on in this document, but a quick summary of the files is here:-
</para>
<itemizedlist>
<listitem>
<para>
available - this file contains a concatenation of control information from all
the packages which dpkg knows about. This is updated using the dpkg commands
"--update-avail &lt;file&gt;", "--merge-avail &lt;file&gt;", and
"--clear-avail".
</para>
</listitem>
<listitem>
<para>
status - this file contains information on the following things for every
package:-
</para>
<itemizedlist>
<listitem>
<para>
Whether it is installed, not installed, unpacked, removed, failed
configuration, or half-installed (deconfigured in favour of another package).
</para>
</listitem>
<listitem>
<para>
Whether it is selected as install, hold, remove, or purge.
</para>
</listitem>
<listitem>
<para>
If it is "ok" (no installation problems), or "not-ok".
</para>
</listitem>
<listitem>
<para>
It usually also contains the section and priority (so that dselect may classify
packages not in available)
</para>
</listitem>
<listitem>
<para>
For packages which did not initially appear in the "available" file when they
were installed, the other control information for them.
</para>
</listitem>
</itemizedlist>
<para>
The exact format for the "Status:" field is:
</para>
<screen>
      Status: Want Flag Status
</screen>
<para>
Where <replaceable>Want</replaceable> may be one of
<emphasis>unknown</emphasis>, <emphasis>install</emphasis>,
<emphasis>hold</emphasis>, <emphasis>deinstall</emphasis>,
<emphasis>purge</emphasis>. <replaceable>Flag</replaceable> may
be one of <emphasis>ok</emphasis>, <emphasis>reinstreq</emphasis>.
<replaceable>Status</replaceable> may
be one of <emphasis>not-installed</emphasis>, <emphasis>config-files</emphasis>,
<emphasis>half-installed</emphasis>, <emphasis>unpacked</emphasis>,
<emphasis>half-configured</emphasis> and <emphasis>installed</emphasis>.
The states are as follows:-
</para>
<variablelist>
<varlistentry>
<term>not-installed</term>
<listitem>
<para>
No files are installed from the package, it has no config files left, it
uninstalled cleanly if it ever was installed.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>unpacked</term>
<listitem>
<para>
The basic files have been unpacked (and are listed in
/var/lib/dpkg/info/[package].list. There are config files present, but the
postinst script has _NOT_ been run.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>half-configured</term>
<listitem>
<para>
The package was installed and unpacked, but the postinst script failed in some
way.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>installed</term>
<listitem>
<para>
All files for the package are installed, and the configuration was also
successful.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>half-installed</term>
<listitem>
<para>
An attempt was made to remove the package but there was a failure in the
prerm script.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>config-files</term>
<listitem>
<para>
The package was "removed", not "purged". The config files are left, but
nothing else.
</para>
</listitem>
</varlistentry>
</variablelist>
<para>
The two last items are only left in dpkg for compatibility - they are
understood by it, but never written out in this form.
</para>
<para>
Please see the dpkg source code, <literal>lib/parshelp.c</literal>,
<emphasis>statusinfos</emphasis>, <emphasis>eflaginfos</emphasis> and
<emphasis>wantinfos</emphasis> for more details.
</para>
</listitem>
<listitem>
<para>
info - this directory contains files from the control archive of every
package currently installed. They are installed with a prefix of
"&lt;packagename&gt;.". In addition to this, it also contains a file
called &lt;package&gt;.list for every package, which contains a list
of files. Note also that the control file is not copied into here; it
is instead found as part of status or available.
</para>
</listitem>
<listitem>
<para>
methods - this directory is reserved for "method"-specific files - each
"method" has a subdirectory underneath this directory (or at least,
it can have). In addition, there is another subdirectory "mnt", where
misc. filesystems (floppies, CD-ROMs, etc.) are mounted.
</para>
</listitem>
<listitem>
<para>
alternatives - directory used by the "update-alternatives" program. It
contains one file for each "alternatives" interface, which contains
information about all the needed symlinked files for each alternative.
</para>
</listitem>
<listitem>
<para>
diversions - file used by the "dpkg-divert" program. Each diversion takes
three lines. The first is the package name (or ":" for user diversion), the
second the original filename, and the third the diverted filename.
</para>
</listitem>
<listitem>
<para>
updates - directory used internally by dpkg. This is discussed later, in the
section <xref linkend="updates"/>.
</para>
</listitem>
<listitem>
<para>
parts - temporary directory used by dpkg-split
</para>
</listitem>
</itemizedlist>
</section>

<section id="s1.3"><title>The dpkg library files</title>
<para>
These files are installed under /usr/lib/dpkg (usually), but
/usr/local/lib/dpkg is also a possibility (as Debian policy dictates). Under
this directory, there is a "methods" subdirectory. The methods subdirectory in
turn contains any number of subdirectories for each general method processor
(note that one set of method scripts can, and is, used for more than one of
the methods listed under dselect).
</para>
<para>
The following files may be found in each of these subdirectories:-
</para>
<itemizedlist>
<listitem>
<para>
names - One line per method, two-digit priority to appear on menu at
beginning, followed by a space, the name, and then another space and
the short description.
</para>
</listitem>
<listitem>
<para>
desc.&lt;name&gt; - Contains the long description displayed by dselect
when the cursor is put over the &lt;name&gt; method.
</para>
</listitem>
<listitem>
<para>
setup - Script or program which sets up the initial values to be used
by this method. Called with first argument as the status area directory
(/var/lib/dpkg), second argument as the name of the method (as in the
directory name), and the third argument as the option (as in the names file).
</para>
</listitem>
<listitem>
<para>
install - Script/program called when the "install" option of dselect is run
with this method. Same arguments as for setup.
</para>
</listitem>
<listitem>
<para>
update - Script/program called when the "update" option of dselect is
run. Same arguments as for setup/install.
</para>
</listitem>
</itemizedlist>
</section>

<section id="s1.4"><title>The "dpkg" command-line utility</title>

<section id="s1.4.1"><title>"Documented" command-line interfaces</title>
<para>
As yet unwritten. You can refer to the other manuals for now. See
<citerefentry><refentrytitle>dpkg</refentrytitle><manvolnum>8</manvolnum></citerefentry>.
</para>
</section>

<section id="s1.4.2"><title>Environment variables which dpkg responds to</title>
<itemizedlist>
<listitem>
<para>
SHELL - used to determine which shell to run.
</para>
</listitem>
<listitem>
<para>
CC - used as the C compiler to call to determine the target architecture. The
default is "gcc".
</para>
</listitem>
<listitem>
<para>
PATH - dpkg checks that it can find at least the following files in the path
when it wants to run package installation scripts, and gives an error if it
cannot find all of them:-
</para>
<itemizedlist>
<listitem>
<para>
ldconfig
</para>
</listitem>
<listitem>
<para>
start-stop-daemon
</para>
</listitem>
<listitem>
<para>
install-info
</para>
</listitem>
<listitem>
<para>
update-rc.d
</para>
</listitem>
</itemizedlist>
</listitem>
</itemizedlist>
</section>

<section id="s1.4.3"><title>Assertions</title>
<para>
The dpkg utility itself is required for quite a number of packages, even if
they have been installed with a tool totally separate from dpkg. The reason
for this is that some packages, in their pre-installation scripts, check that
your version of dpkg supports certain features. This was broken from the
start, and it should have actually been a control file header "Dpkg-requires",
or similar. What happens is that the configuration scripts will abort or
continue according to the exit code of a call to dpkg, which will stop them
from being wrongly configured.
</para>
<para>
These special command-line options, which simply return as true or false are
all prefixed with "--assert-". Here is a list of them (without the prefix):-
</para>
<itemizedlist>
<listitem>
<para>
support-predepends - Returns success or failure according to whether a version
of dpkg which supports predepends properly (1.1.0 or above) is installed,
according to the database.
</para>
</listitem>
<listitem>
<para>
working-epoch - Return success or failure according to whether a version of
dpkg which supports epochs in version properly (1.4.0.7 or above) is installed,
according to the database.
</para>
</listitem>
</itemizedlist>
<para>
Both these options check the status database to see what version of the
"dpkg" package is installed, and check it against a known working version.
</para>
</section>

<section id="s1.4.4"><title>--predep-package</title>
<para>
This strange option is described as follows in the source code:
</para>
<screen>
/* Print a single package which:
 *  (a) is the target of one or more relevant predependencies.
 *  (b) has itself no unsatisfied pre-dependencies.
 * If such a package is present output is the Packages file entry,
 *  which can be massaged as appropriate.
 * Exit status:
 *  0 = a package printed, OK
 *  1 = no suitable package available
 *  2 = error
 */
</screen>
<para>
On further inspection of the source code, it appears that what is does is
this:-
</para>
<itemizedlist>
<listitem>
<para>
Looks at the packages in the database which are selected as "install",
and are installed.
</para>
</listitem>
<listitem>
<para>
It then looks at the Pre-Depends information for each of these packages
from the available file. When it find a package for which any of the
pre-dependencies are not satisfied, it breaks from the loop through the
packages.
</para>
</listitem>
<listitem>
<para>
It then looks through the unsatisfied pre-dependencies, and looks for
packages which would satisfy this pre-dependency, stopping on the first
it finds. If it finds none, it bombs out with an error.
</para>
</listitem>
<listitem>
<para>
It then continues this for every dependency of the initial package.
</para>
</listitem>
</itemizedlist>
<para>
Eventually, it writes out the record of all the packages to satisfy the
pre-dependencies. This is used by the disk method to make sure that its
dependency ordering is correct. What happens is that all pre-depending
packages are first installed, then it runs dpkg -iGROEB on the directory,
which installs in the order package files are found. Since pre-dependencies
mean that a package may not even be unpacked unless they are satisfied, it
is necessary to do this (usually, since all the package files are unpacked
in one phase, the configured in another, this is not needed).
</para>
</section>

</section>

</chapter>

<chapter id="ch2"><title>dpkg-deb and .deb file internals</title>
<para>
This chapter describes the internals to the "dpkg-deb" tool, which is used by
"dpkg" as a back-end. dpkg-deb has its own tar extraction functions, which is
the source of many problems, as it does not support long filenames, using
extension blocks.
</para>

<section id="s2.1"><title>The .deb archive format</title>
<para>
The main principal of the new-format Debian archive (I won't describe the old
format - for that have a look at deb-old.5), is that the archive really is an
archive - as used by "ar" and friends. However, dpkg-deb uses this format
internally, rather than calling "ar". Inside this archive, there are usually
the following members:-
</para>
<itemizedlist>
<listitem>
<para>
debian-binary
</para>
</listitem>
<listitem>
<para>
control.tar.gz
</para>
</listitem>
<listitem>
<para>
data.tar.gz
</para>
</listitem>
</itemizedlist>
<para>
The debian-binary member consists simply of the string "2.0", indicating
the format version. control.tar.gz contains the control files (and scripts),
and the data.tar.gz contains the actual files to populate the filesystem
with. Both tarfiles extract straight into the current directory. Information
on the tar formats can be found in the GNU tar info page. Since dpkg-deb
calls "tar -cf" to build packages, the Debian packages use the GNU extensions.
</para>
</section>

<section id="s2.2"><title>The dpkg-deb command-line</title>
<para>
dpkg-deb documents itself thoroughly with its '--help' command-line
option. However, I am including a reference to these for
completeness. dpkg-deb supports the following options:-
</para>
<itemizedlist>
<listitem>
<para>
--build (-b) &lt;dir&gt; - builds a .deb archive, takes a directory which
contains all the files as an argument. Note that the directory
&lt;dir&gt;/DEBIAN will be packed separately into the control archive.
</para>
</listitem>
<listitem>
<para>
--contents (-c) &lt;debfile&gt; - Lists the contents of the "data.tar.gz"
member.
</para>
</listitem>
<listitem>
<para>
--control (-e) &lt;debfile&gt; - Extracts the control archive into a directory
called DEBIAN. Alternatively, with another argument, it will extract it into a
different directory.
</para>
</listitem>
<listitem>
<para>
--info (-I) &lt;debfile&gt; - Prints the contents of the "control" file in the
control archive to stdout. Alternatively, giving it other arguments will cause
it to print the contents of those files instead.
</para>
</listitem>
<listitem>
<para>
--field (-f) &lt;debfile&gt; &lt;field&gt; ... - Prints any number of fields
from the "control" file. Giving it extra arguments limits the fields it prints
to only those specified. With no command-line arguments other than a filename,
it is equivalent to -I and just the .deb filename.
</para>
</listitem>
<listitem>
<para>
--extract (-x) &lt;debfile&gt; &lt;dir&gt; - Extracts the data archive of a
debian package under the directory &lt;dir&gt;.
</para>
</listitem>
<listitem>
<para>
--vextract (-X) &lt;debfile&gt; &lt;dir&gt; - Same as --extract, except it
is equivalent of giving tar the '-v' option - it prints the filenames as it
extracts them.
</para>
</listitem>
<listitem>
<para>
--fsys-tarfile &lt;debfile&gt; - This option outputs a gunzip'd version of
data.tar.gz to stdout.
</para>
</listitem>
<listitem>
<para>
--new - sets the archive format to be used to the new Debian format
</para>
</listitem>
<listitem>
<para>
--old - sets the archive format to be used to the old Debian format
</para>
</listitem>
<listitem>
<para>
--debug - Tells dpkg-deb to produce debugging output
</para>
</listitem>
<listitem>
<para>
--nocheck - Tells dpkg-deb not to check the sanity of the control file
</para>
</listitem>
<listitem>
<para>
--help (-h) - Gives a help message
</para>
</listitem>
<listitem>
<para>
--version - Shows the version number
</para>
</listitem>
<listitem>
<para>
--licence/--license (UK/US spellings) - Shows a brief outline of the GPL
</para>
</listitem>
</itemizedlist>

<section id="s2.2.1"><title>Internal checks used by dpkg-deb when building packages</title>
<para>
Here is a list of the internal checks used by dpkg-deb when building
packages. It is in the order they are done.
</para>
<itemizedlist>
<listitem>
<para>
First, the output Debian archive argument, if it is given, is checked using
stat. If it is a directory, an internal flag is set. This check is only made
if the archive name is specified explicitly on the command-line. If the
argument was not given, the default is the directory name, with ".deb"
appended.
</para>
</listitem>
<listitem>
<para>
Next, the control file is checked, unless the --nocheck flag was specified on
the command-line. dpkg-deb will bomb out if the second argument to --build was
a directory, and --nocheck was specified. Note that dpkg-deb will not be able
to determine the name of the package in this case. In the control file, the
following things are checked:-
</para>
<itemizedlist>
<listitem>
<para>
The package name is checked to see if it contains any invalid characters (see
<xref linkend="control"/> for this).
</para>
</listitem>
<listitem>
<para>
The priority field is checked to see if it uses standard values, and
user-defined values are warned against. However, note that this check is now
redundant, since the control file no longer contains the priority - the
changes file now does this.
</para>
</listitem>
<listitem>
<para>
The control file fields are then checked against the standard list of fields
which appear in control files, and any "user-defined" fields are reported as
warnings.
</para>
</listitem>
<listitem>
<para>
dpkg-deb then checks that the control file contains a valid version number.
</para>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<para>
After this, in the case where a directory was specified to build the .deb file
in, the filename is created as "directory/pkg_ver.deb" or
"directory/pkg_ver_arch.deb", depending on whether the control file contains
an architecture field.
</para>
</listitem>
<listitem>
<para>
Next, dpkg-deb checks for the &lt;dir&gt;/DEBIAN directory. It complains if it
doesn't exist, or if it has permissions &lt; 0755, or &gt; 0775.
</para>
</listitem>
<listitem>
<para>
It then checks that all the files in this subdir are either symlinks or plain
files, and have permissions between 0555 and 0775.
</para>
</listitem>
<listitem>
<para>
The conffiles file is then checked to see if the filenames are too
long. Warnings are produced for each that is. After this, it checks
that the package provides initial copies of each of these conffiles,
and that they are all plain files.
</para>
</listitem>
</itemizedlist>
</section>

</section>

</chapter>

<chapter id="ch3"><title>dpkg internals</title>
<para>
This chapter describes the internals of dpkg itself. Although the low-level
formats are quite simple, what dpkg does in certain cases often does not make
sense.
</para>

<section id="updates"><title>Updates</title>
<para>
This describes the /var/lib/dpkg/updates directory. The function of this
directory is somewhat strange, and seems only to be used internally. A
function called cleanupdates is called whenever the database is scanned. This
function in turn uses
<citerefentry><refentrytitle>scandir</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
to sort the files in this directory. Files who names do not consist entirely
of digits are discarded. dpkg also causes a fatal error if any of the
filenames are different lengths.
</para>
<para>
After having scanned the directory, dpkg in turn parses each file the same way
it parses the status file (they are sorted by the scandir to be in numerical
order). After having done this, it then writes the status information back to
the "status" file, and removes all the "updates" files.
</para>
<para>
These files are created internally by dpkg's "checkpoint" function, and are
cleaned up when dpkg exits cleanly.
</para>
<para>
Judging by the use of the updates directory I would call it a Journal. Inorder
to efficiently ensure the complete integrity of the status file dpkg will
"checkpoint" or journal all of it's activities in the updates directory. By
merging the contents of the updates directory (in order!!) against the original
status file it can get the precise current state of the system, even in the
event of a system failure while dpkg is running.
</para>
<para>
The other option would be to sync-rewrite the status file after each operation,
which would kill performance.
</para>
<para>
It is very important that any program that uses the status file abort if the
updates directory is not empty!  The user should be informed to run dpkg
manually (what options though??) to correct the situation.
</para>
</section>

<section id="s3.2"><title>What happens when dpkg reads the database</title>
<para>
First, the status file is read. This gives dpkg an initial idea of the
packages that are there. Next, the updates files are read in, overriding the
status file, and if necessary, the status file is re-written, and updates files
are removed. Finally, the available file is read. The available file is read
with flags which preclude dpkg from updating any status information from it,
though - installed version, etc., and is also told to record that the packages
it reads this time are available, not installed.
</para>
<para>
More information on updates is given above.
</para>
</section>

<section id="s3.3"><title>How dpkg compares version numbers</title>
<para>
Version numbers consist of three parts: the epoch, the upstream version, and
the Debian revision. Dpkg compares these parts in that order. If the epochs
are different, it returns immediately, and so on.
</para>
<para>
However, the important part is how it compares the versions which are
essentially stored as just strings. These are compared in two distinct
parts: those consisting of numerical characters (which are evaluated, and
then compared), and those consisting of other characters. When comparing
non-numerical parts, they are compared as the character values (ASCII),
but non-alphabetical characters are considered "greater than" alphabetical
ones. Also note that longer strings (after excluding differences where
numerical values are equal) are considered "greater than" shorter ones.
</para>
<para>
Here are a few examples of how these rules apply:-
</para>
<screen>
15 &gt; 10
0010 == 10

d.r &gt; dsr
32.d.r == 0032.d.r
d.rnr &lt; d.rnrn
</screen>
</section>

</chapter>

</book>