Bug#571776: document symbols
Hello folks,
I took some time today and wrote a first draft of a new section of Policy
documenting symbols files, and the revisions to shlibs for their
interaction. Please review. There's quite a lot of material here,
including details from dpkg-shlibdeps, dpkg-gensymbols, and deb-symbols
documentation as well as additional requirements and recommendations
around how to maintain the minimal-version field.
I tried sending a unified diff, but the new sections are largely
unreadable since they're intermixed with the old sections being removed.
Hence, for review purposes, here are the symbols and shlibs sections in
their entirety, followed by a diff for the changes elsewhere in Policy.
You can also refer to branch bug571776-rra in the Policy repository.
<sect id="sharedlibs-symbols">
<heading>Dependencies between the library and other packages -
the <tt>symbols</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. These dependencies must be added to the binary
package when it is built, since they may change based on which
version of a shared library the binary or library was linnked
with. To allow these dependencies to be constructed, shared
libraries must provide either a <file>symbols</file> file or
a <file>shlibs</file> file, which provide information on the
package dependencies required to ensure the presence of this
library. Any package which uses a shared library must use these
files to determine the required dependencies when it is built.
</p>
<p>
<file>shlibs</file> files were the original mechanism for
handling library dependencies. They are documented
in <ref id="sharedlibs-shlibdeps">. <file>symbols</file> files,
documented in this section, are recommended for most packages,
since they provide dependency information for each exported
symbol and therefore generate more accurate dependencies for
binaries that do not use symbols from newer versions of the
shared library. However, <file>shlibs</file> files must be used
for udebs. Packages which provide a <file>symbols</file> file
are not required to provide a <file>shlibs</file> file.
</p>
<p>
When a package that contains any shared libraries or compiled
binaries is built, it must run <prgn>dpkg-shlibdeps</prgn> on
each shared library and compiled binary to determine the
libraries used and hence the dependencies needed by the
package.<footnote>
<prgn>dpkg-shlibdeps</prgn> will use a program
like <prgn>objdump</prgn> or <prgn>readelf</prgn> to find the
libraries and the symbols in those libraries directly needed
by the binaries or shared libraries in the package.
</footnote>
</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, the library is listed in the
ELF <tt>NEEDED</tt> attribute, caused by adding <tt>-lbar</tt>
to the link line when the binary is created). 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, but not the libraries it indirectly uses. The
dependencies for those libraries will automatically pull in the
other libraries. <prgn>dpkg-shlibdeps</prgn> will handle this
logic automatically, but package maintainers need to be aware of
this distinction between directly and indirectly using a library
if they have to override its results for some reason.
<footnote>
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) and depends on <tt>libdgf</tt>. If we
used <prgn>ldd</prgn> to add dependencies for every library
directly or indirectly linked with a binary, 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. Since dependencies are only added based on
ELF <tt>NEEDED</tt> attribute, 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.
</footnote>
</p>
<p>
In the following sections, we will first describe where the
various <file>symbols</file> files are to be found, then how to
use <prgn>dpkg-shlibdeps</prgn>, and finally
the <file>symbols</file> file format and how to create them if
your package contains a shared library.
</p>
<sect1 id="symbols-paths">
<heading>The <file>symbols</file> files present on the
system</heading>
<p>
<file>symbols</file> files for a shared library are normally
provided by the shared library package, but there are several
override paths that are checked first in case that information
is wrong or missing. The following list gives them in the
order in which they are read by <prgn>dpkg-shlibdeps</prgn>
The first one that contains the required information is used.
<list>
<item>
<p><file>debian/*/DEBIAN/symbols</file></p>
<p>
During the package build, if the package itself contains
shared libraries with <file>symbols</file> files, they
will be generated in these staging directories
by <prgn>dpkg-gensymbols</prgn>. <file>symbols</file>
files found in the build tree take precedence
over <file>symbols</file> files from other binary
packages.
</p>
<p>
These files must exist
before <prgn>dpkg-shlibdeps</prgn> is run or the
dependencies of binaries and libraries from a source
package on other libraries from that same source package
will not be correct. In practice, this means
that <prgn>dpkg-gensymbols</prgn> must be run
before <prgn>dpkg-shlibdeps</prgn> during the package
build.<footnote>
An example may clarify. Suppose the source
package <tt>foo</tt> generates two binary
packages, <tt>libfoo2</tt> and <tt>foo-runtime</tt>.
When building the binary packages, the contents of the
packages are staged 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 contain
a <tt>symbols</tt> file, which will be installed
in <file>debian/libfoo2/DEBIAN/symbols</file>,
eventually to be included as a control file in that
package. 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/symbols</file> file to
determine whether <tt>foo-prog</tt>'s library
dependencies are satisfied by any of the libraries
provided by <tt>libfoo2</tt>. Since those binaries
were linked against the just-built shared library as
part of the build process, the <file>symbols</file>
file for the newly-built <tt>libfoo2</tt> must take
precedence over a <file>symbols</file> file for any
other <tt>libfoo2</tt> package already installed on
the system.
</footnote>
</p>
</item>
<item>
<p>
<file>/etc/dpkg/symbols/<var>package</var>.symbols.<var>arch</var></file>
and <file>/etc/dpkg/symbols/<var>package</var>.symbols</file>
</p>
<p>
Per-system overrides of shared library dependencies.
These files normally do not exist. They are maintained
by the local system administrator and must not be
created by any Debian package.
</p>
</item>
<item>
<p><file>symbols</file> control files for packages
installed on the system</p>
<p>
The <file>symbols</file> control files for all the
packages currently installed on the system are searched
last. This will be the most common source of shared
library dependency information. These are normally
found in <file>/var/lib/dpkg/info/*.symbols</file>, but
packages should not rely on this and instead should
use <tt>dpkg-query --control-path <var>package</var>
symbols</tt> if for some reason these files need to be
examined.
</p>
</item>
</list>
</p>
<p>
Be aware that if a <file>debian/shlibs.local</file> exists in
the source package, it will override any <file>symbols</file>
files. This is the only case where a <file>shlibs</file> is
used despite <file>symbols</file> files being present. See
<ref id="shlibs-paths"> and <ref id="sharedlibs-shlibdeps">
for more information.
</p>
</sect1>
<sect1 id="dpkg-shlibdeps">
<heading>How to use <prgn>dpkg-shlibdeps</prgn> and the
<tt>symbols</tt> files</heading>
<p>
If your package contains any compiled binaries or shared
libraries, put a call to <prgn>dpkg-shlibdeps</prgn> into
your <file>debian/rules</file> file in the source package.
List all of the compiled binaries, libraries, or loadable
modules in your package. If your source package builds only a
single binary package that contains only compiled binaries and
libraries (but no scripts) and is not multiarch, 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>
but normally finding all of the binaries is more
complex.<footnote>
The easiest way to do this is to use a package helper
framework such as <tt>debhelper</tt>. 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 of every binary package built by
this source package that contains compiled binaries,
libraries, or loadable modules. If you have multiple binary
packages, you will need to call <prgn>dpkg-shlibdeps</prgn> on
each one which contains compiled libraries or binaries, using
the <tt>-T</tt> option to the <tt>dpkg</tt> utilities to
specify a different <file>substvars</file> file for each
binary package.<footnote>
Again, <prgn>dh_shlibdeps</prgn>
and <prgn>dh_gencontrol</prgn> will handle all of this for
you if you're using <tt>debhelper</tt>.
</footnote>
</p>
<p>
For more details on <prgn>dpkg-shlibdeps</prgn>,
see <manref name="dpkg-shlibdeps" section="1">.
</p>
</sect1>
<sect1 id="symbols">
<heading>The <file>symbols</file> File Format</heading>
<p>
The following documents the format of the <file>symbols</file>
control file as included in binary packages. These files are
built from template <file>symbols</file> files in the source
package by <prgn>dpkg-gensymbols</prgn>. The template files
support a richer syntax that
allows <prgn>dpkg-gensymbols</prgn> to do some of the tedious
work involved in maintaining <file>symbols</file> files, such
as handling C++ symbols or optional symbols that may not exist
on particular architectures. When
writing <file>symbols</file> files for a shared library
package, refer to <manref name="dpkg-gensymbols" section="1">
for the richer syntax.
</p>
<p>
A <file>symbols</file> may contain one or more entries, one
for each shared library contained in the package corresponding
to that <file>symbols</file>. Each entry has the following
format:
</p>
<p>
<example>
<var>library-soname</var> <var>main-dependency-template</var>
[ | <var>alternative-dependency-template</var> ]
[ ... ]
[ * <var>field-name</var>: <var>field-value</var> ]
[ ... ]
<var>symbol</var> <var>minimal-version</var>[ <var>id-of-dependency-template</var> ]
</example>
</p>
<p>
To explain this format, we'll use the the <tt>zlib1g</tt>
package as an example, which (at the time of writing) installs
the shared library <file>/usr/lib/libz.so.1.2.3.4</file>.
Mandatory lines will be described first, followed by optional
lines.
</p>
<p>
<var>library-soname</var> must contain exactly the value of
the ELF <tt>SONAME</tt> attribute of the shared library. In
our example, this is <tt>libz.so.1</tt>.<footnote>
This can be determined by using the command
<example compact="compact">
readelf -d /usr/lib/libz.so.1.2.3.4 | grep SONAME
</example>
</footnote>
</p>
<p>
<var>main-dependency-template</var> has the same syntax as a
dependency field in a binary package control file, except that
the string <tt>#MINVER#</tt> is replaced by a version
restriction like <tt>(>= <var>version</var>)</tt> or by
nothing if an unversioned dependency is deemed sufficient.
The version restriction will be based on which symbols from
the shared library are referenced and the version at which
they were introduced (see below). In nearly all
cases, <var>main-dependency-template</var> will
be <tt><var>package</var> #MINVER#</tt>,
where <var>package</var> is the name of the binary package
containing the shared library. This adds a simple,
possibly-versioned dependency on the shared library package.
In some rare cases, such as when multiple packages provide the
same shared library ABI, the dependency template may need to
be more complex.
</p>
<p>
In our example, the first line of
the <tt>zlib1g</tt> <file>symbols</file> file would be:
<example compact="compact">
libz.so.1 zlib1g #MINVER#
</example>
</p>
<p>
Each public symbol exported by the shared library must have a
corresponding symbol line, indented by one
space. <var>symbol</var> is the exported symbol (which, for
C++, means the mangled symbol) followed by <tt>@</tt> and the
symbol version, or the string <tt>Base</tt> if there is no
symbol version. <var>minimal-version</var> is the most recent
version of the shared library that changed the behavior of
that symbol, whether by adding it, changing its function
signature (the parameters, their types, or the return type),
or its behavior in a way that is visible to a
caller. <var>id-of-dependency-template</var> is an optional
field that references
an <var>alternative-dependency-template</var>; see below for a
full description.
</p>
<p>
For example, <tt>libz.so.1</tt> contains the
symbols <tt>compress</tt>
and <tt>compressBound</tt>. <tt>compress</tt> has no symbol
version and last changed its behavior in upstream
version <tt>1:1.1.4</tt>. <tt>compressBound</tt> has the
symbol version <tt>ZLIB_1.2.0</tt>, was introduced in upstream
version <tt>1:1.2.0</tt>, and has not changed its behavior.
Its <file>symbols</file> file therefore contains the lines:
<example compact="compact">
compress@Base 1:1.1.4
compressBound@ZLIB_1.2.0 1:1.2.0
</example>
Packages using only <tt>compress</tt> would then get a
dependency of <tt>zlib1g (>= 1:1.1.4)</tt>, but packages
using <tt>compressBound</tt> would get a dependency
of <tt>zlib1g (>= 1:1.2.0)</tt>.
</p>
<p>
One or more <var>alternative-dependency-template</var> lines
may be provided. These are used in cases where some symbols
in the shared library should use one dependency template while
others should use a different template. The alternative
dependency templates are used only if a symbol line contains
the <var>id-of-dependency-template</var> field. The first
alternative dependency template is numbered 1, the second 2,
and so forth.<footnote>
An example of where this may be needed is with a library
that implements the libGL interface. All GL implementations
provide the same set of base interfaces, and then may
provide some additional interfaces only used by programs
that require that specific GL implementation. So, for
example, libgl1-mesa-glx may use the
following <file>symbols</file> file:
<example>
libGL.so.1 libgl1
| libgl1-mesa-glx #MINVER#
publicGlSymbol@Base 6.3-1
[...]
implementationSpecificSymbol@Base 6.5.2-7 1
[...]
</example>
Binaries or shared libraries using
only <tt>publicGlSymbol</tt> would depend only
on <tt>libgl1</tt> (which may be provided by multiple
packages), but ones
using <tt>implementationSpecificSymbol</tt> would get a
dependency on <tt>libgl1-mesa-glx (>= 6.5.2-7)</tt>
</footnote>
</p>
<p>
Finally, the entry for the library may contain one or more
metadata fields. Currently, the only
supported <var>field-name</var>
is <tt>Build-Depends-Package</tt>, whose value lists
the <qref id="sharedlibs-dev">library development
package</qref> on which packages using this shared library
declare a build dependency. If this field is
present, <prgn>dpkg-shlibdeps</prgn> uses it to ensure that
the resulting binary package dependency on the shared library
is at least as strict as the source package dependency on the
shared library development package.<footnote>
This field should normally not be necessary, since if the
behavior of any symbol has changed, the corresponding
symbol <var>minimal-version</var> should have been
increased. But including it makes the <tt>symbols</tt>
system more robust by tightening the dependency in cases
where the package using the shared library specifically
requires at least a particular version of the shared library
development package for some reason.
</footnote>
For our example, the <tt>zlib1g</tt> <file>symbols</file> file
would contain:
<example compact="compact">
* Build-Depends-Package: zlib1g-dev
</example>
(Don't forget the leading space.)
</p>
<p>
Also see <manref name="deb-symbols" section="5">.
</p>
</sect1>
<sect1 id="providing-symbols">
<heading>Providing a <file>symbols</file> file</heading>
<p>
If your package provides a shared library, you should arrange
to include a <file>symbols</file> control file following the
format described above in that package. You must include
either a <file>symbols</file> control file or
a <file>shlibs</file> control file.
</p>
<p>
Normally, this is done by creating a <file>symbols</file> in
the source package
named <file>debian/<var>package</var>.symbols</file>
or <file>debian/symbols</file>, possibly
with <file>.<var>arch</var></file> appended if the symbols
information varies by architecture. This file may use the
extended syntax documented
in <manref name="dpkg-gensymbols" section="1">. Then,
call <prgn>dpkg-gensymbols</prgn> as part of the package build
process. It will create <file>symbols</file> files in the
package staging area based on the binaries and libraries in
the package staging area and the <file>symbols</file> files in
the source package.<footnote>
If you are
using <tt>debhelper</tt>, <prgn>dh_makeshlibs</prgn> will
take care of calling either <prgn>dpkg-gensymbols</prgn>
or generating a <file>shlibs</file> file as appropriate.
</footnote>
</p>
<p>
Packages that provide <file>symbols</file> files must keep
them up-to-date to ensure correct dependencies in packages
that use the shared libraries. This means updating
the <file>symbols</file> file whenever a new public symbol is
added, changing the <var>minimal-version</var> field whenever
a symbol changes behavior or signature, and changing
the <var>library-soname</var>
and <var>main-dependency-template</var>, and probably all of
the <var>minimal-version</var> fields, when the library
changes <tt>SONAME</tt>. Removing a public symbol from
the <file>symbols</file> file because it's no longer provided
by the library normally requires changing the <tt>SONAME</tt>
of the library. See <ref id="sharedlibs-runtime">.
</p>
<p>
Special care should be taken in updating
the <var>minimal-version</var> field when the behavior of a
public symbol changes. This is easy to neglect, since there
is no automated method of determining such changes, but
failing to update <var>minimal-version</var> in this case may
result in binary packages with too-weak dependencies that will
fail at runtime, possibly in ways that can cause security
vulnerabilities. If the package maintainer believes that a
symbol behavior change may have occurred but isn't sure, it's
safer to update the <var>minimal-version</var> of all possibly
affected symbols to the current upstream version rather than
leave them unmodified. This may result in unnecessarily
strict dependencies, but it ensures that packages whose
dependencies are satisfied will work properly.
</p>
<p>
A common example of when a change
to <var>minimal-version</var> is required is a function that
takes an enum or struct argument that controls what the
function does. For example:
<example>
enum library_op { OP_FOO, OP_BAR };
int library_do_operation(enum library_op);
</example>
If a new operation, <tt>OP_BAZ</tt>, is added,
the <var>minimal-version</var>
of <tt>library_do_operation</tt> must be increased to the
version at which <tt>OP_BAZ</tt> was introduced. Otherwise, a
binary built against the new version of the library (having
detected at compile-time that the library
supports <tt>OP_BAZ</tt>) may be installed with a shared
library that doesn't support <tt>OP_BAZ</tt> and will fail at
runtime when it tries to pass <tt>OP_BAZ</tt> into this
function.
</p>
<p>
The <var>minimal-version</var> field normally should not
contain the Debian revision of the package, since the library
behavior is normally fixed for a particular upstream version
and any Debian packaging of that upstream version will have
the same behavior. In the rare case that the library behavior
was changed in a particular Debian revision,
appending <tt>~</tt> to the end of
the <var>minimal-version</var> that includes the Debian
revision is recommended, since this allows backports of the
shared library package using the normal backport versioning
convention to satisfy the dependency.
</p>
</sect1>
</sect>
<sect id="sharedlibs-shlibdeps">
<heading>Dependencies between the library and other packages -
the <tt>shlibs</tt> system</heading>
<p>
The <tt>shlibs</tt> system is an alternative to
the <tt>symbols</tt> system for declaring dependencies for
shared libraries. It predated the <tt>symbols</tt> system and
is therefore frequently seen in older packages. It is also
required for udebs, which do not support <tt>symbols</tt>.
</p>
<p>
<file>shlibs</file> files do not provide as detailed of
information as <file>symbols</file> files. They only provide
information about the library as a whole, not individual
symbols, and therefore have to force tighter dependencies since
they have no way of relaxing dependencies for binaries and
libraries that only use symbols whose behavior has not changed.
Because of this, and because of some problems with
how <file>shlibs</file> files represent the
library <tt>SONAME</tt>, <file>symbols</file> files are
recommended instead for any shared library package that isn't a
udeb.
</p>
<p>
In the following sections, we will first describe where the
various <file>shlibs</file> files are to be found, then how to
use <prgn>dpkg-shlibdeps</prgn>, and finally
the <file>shlibs</file> file format and how to create them if
your package contains a shared library. Much of the information
about <file>shlibs</file> files is the same as
for <file>symbols</file> files, so only the differences will be
mentioned.
</p>
<sect1 id="shlibs-paths">
<heading>The <file>shlibs</file> 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 <prgn>dpkg-shlibdeps</prgn>. (The first one
which gives the required information is used.)
<list>
<item>
<p><file>debian/shlibs.local</file></p>
<p>
This lists overrides for this package. This file should
normally not be used, but may be needed temporarily in
unusual situations to work around bugs in other
packages, or in unusual cases where the normally
declared dependency information in the
installed <file>shlibs</file> file for a library cannot
be used. This file overrides information obtained from
any other source.
</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 information 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 same package.
</p>
</item>
<item>
<p><file>shlibs</file> control files for packages
installed on the system</p>
<p>
The <file>shlibs</file> control files for all the
packages currently installed on the system. These are
normally found
in <file>/var/lib/dpkg/info/*.symbols</file>, but
packages should not rely on this and instead should
use <tt>dpkg-query --control-path <var>package</var>
shlibs</tt> if for some reason these files need to be
examined.
</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>
<p>
If a <file>symbols</file> file for a shared library package
is available, <prgn>dpkg-shlibdeps</prgn> will always use it
in preference to a <file>shlibs</file>, with the exception
of <file>debian/shlibs.local</file>. The latter overrides any
other <file>shlibs</file> or <file>symbols</file> files.
</p>
</sect1>
<sect1>
<heading>How to use <prgn>dpkg-shlibdeps</prgn> and the
<file>shlibs</file> files</heading>
<p>
Use of <prgn>dpkg-shlibdeps</prgn> with <file>shlibs</file>
files is generally the same as with <file>symbols</file>
files. See <ref id="dpkg-shlibdeps">.
</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 the <tt>-tudeb</tt> 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>
</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.1.2.3.4</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
ELF <tt>SONAME</tt> attribute of the library.
The <tt>SONAME</tt> 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>.
The version part is the part which comes after
<tt>.so.</tt>, so in our case, it is <tt>1</tt>. The soname
may instead be of the
form <tt><var>name</var>-<var>major-version</var>.so</tt>,
such as <tt>libdb-5.1.so</tt>, in which case the name would
be <tt>libdb</tt> and the version would be <tt>5.1</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 last change to the <tt>zlib1g</tt>
package that could change behavior for a client of that
library was in version <tt>1:1.2.3.3.dfsg-1</tt>, then
the <tt>shlibs</tt> entry for this library could say:
<example compact="compact">
libz 1 zlib1g (>= 1:1.2.3.3.dfsg-1)
</example>
This version restriction must be new enough that any binary
built against the current version of the library will work
with any version of the shared library that satisfies that
dependency.
</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 (>= 1:1.2.3.3.dfsg-1)
</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/<var>package</var>.shlibs</file> instead).
Then let <file>debian/rules</file> install it in the control
information file 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/<var>package</var>.shlibs debian/<var>package</var>/DEBIAN/shlibs
</example>
An alternative way of doing this is to create
the <file>shlibs</file> file in the control information file
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 <package>debhelper</package> 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>
Since <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>
</sect>
</chapt>
diff --git a/policy.sgml b/policy.sgml
index 79281e9..cca4b6d 100644
--- a/policy.sgml
+++ b/policy.sgml
@@ -840,10 +840,11 @@
Among those files are the package maintainer scripts
and <file>control</file>, the <qref id="binarycontrolfiles">binary
package control file</qref> that contains the control fields for
- the package. Other control information files
- include <qref id="sharedlibs-shlibdeps">the <file>shlibs</file>
- file</qref> used to store shared library dependency information
- and the <file>conffiles</file> file that lists the package's
+ the package. Other control information files include
+ the <qref id="sharedlibs-symbols"><file>symbols</file> file</qref>
+ or <qref id="sharedlibs-shlibdeps"><file>shlibs</file> file</qref>
+ used to store shared library dependency information and
+ the <file>conffiles</file> file that lists the package's
configuration files (described in <ref id="config-files">).
</p>
@@ -5521,9 +5522,9 @@ Replaces: mail-transport-agent
linked against the old shared library. Correct versioning of
dependencies on the newer shared library by binaries that use
the new interfaces is handled via
- the <qref id="sharedlibs-shlibdeps"><tt>shlibs</tt>
- system</qref> or via symbols files (see
- <manref name="deb-symbols" section="5">).
+ the <qref id="sharedlibs-symbols"><tt>symbols</tt> system</qref>
+ or the <qref id="sharedlibs-shlibdeps"><tt>shlibs</tt>
+ system</qref>.
</p>
<p>
@@ -7702,8 +8153,9 @@ INSTALL = install -s # (or use strip on the files in debian/tmp)
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
+ functioning of the <qref id="sharedlibs-symbols">symbols</qref>
+ and <qref id="sharedlibs-shlibdeps">shlibs</qref>
+ systems 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,
@@ -10481,89 +10933,6 @@ END-INFO-DIR-ENTRY
<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 and shared libraries
- <footnote>
- <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: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
--
Russ Allbery (rra@debian.org) <http://www.eyrie.org/~eagle/>
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