Ubuntu Manpage: dpkg-gensymbols - generate symbols files (shared library dependency information)

Provided by: dpkg-dev_1.22.11ubuntu3_all

NAME

       dpkg-gensymbols - generate symbols files (shared library dependency information)

SYNOPSIS

       dpkg-gensymbols [option...]

DESCRIPTION

dpkg-gensymbols scans a temporary build tree (debian/tmp by default) looking for libraries
and generates a symbols file describing them. This file, if non-empty, is then installed
in the DEBIAN subdirectory of the build tree so that it ends up included in the control
information of the package.

When generating those files, it uses as input some symbols files provided by the
maintainer. It looks for the following files (and uses the first that is found):

• debian/package.symbols.arch

• debian/symbols.arch

• debian/package.symbols

• debian/symbols

The main interest of those files is to provide the minimal version associated to each
symbol provided by the libraries. Usually it corresponds to the first version of that
package that provided the symbol, but it can be manually incremented by the maintainer if
the ABI of the symbol is extended without breaking backwards compatibility. It's the
responsibility of the maintainer to keep those files up-to-date and accurate, but dpkg-
gensymbols helps with that.

When the generated symbols files differ from the maintainer supplied one, dpkg-gensymbols
will print a diff between the two versions. Furthermore if the difference is too
significant, it will even fail (you can customize how much difference you can tolerate,
see the -c option).

This program was introduced in dpkg 1.14.8.

MAINTAINING SYMBOLS FILES

The base interchange format of the symbols file is described in deb-symbols(5), which is
used by the symbols files included in binary packages. These are generated from template
symbols files with a format based on the former, described in deb-src-symbols(5) and
included in source packages.

The symbols files are really useful only if they reflect the evolution of the package
through several releases. Thus the maintainer has to update them every time that a new
symbol is added so that its associated minimal version matches reality.

The diffs contained in the build logs can be used as a starting point, but the maintainer,
additionally, has to make sure that the behaviour of those symbols has not changed in a
way that would make anything using those symbols and linking against the new version, stop
working with the old version.

In most cases, the diff applies directly to the debian/package.symbols file. That said,
further tweaks are usually needed: it's recommended for example to drop the Debian
revision from the minimal version so that backports with a lower version number but the
same upstream version still satisfy the generated dependencies. If the Debian revision
can't be dropped because the symbol really got added by the Debian specific change, then
one should suffix the version with ‘~’.

Before applying any patch to the symbols file, the maintainer should double-check that
it's sane. Public symbols are not supposed to disappear, so the patch should ideally only
add new lines.

Note that you can put comments in symbols files.

Do not forget to check if old symbol versions need to be increased. There is no way dpkg-
gensymbols can warn about this. Blindly applying the diff or assuming there is nothing to
change if there is no diff, without checking for such changes, can lead to packages with
loose dependencies that claim they can work with older packages they cannot work with.
This will introduce hard to find bugs with (partial) upgrades.

Good library management
A well-maintained library has the following features:

• its API is stable (public symbols are never dropped, only new public symbols are
added) and changes in incompatible ways only when the SONAME changes;

• ideally, it uses symbol versioning to achieve ABI stability despite internal changes
and API extension;

• it doesn't export private symbols (such symbols can be tagged optional as workaround).

While maintaining the symbols file, it's easy to notice appearance and disappearance of
symbols. But it's more difficult to catch incompatible API and ABI change. Thus the
maintainer should read thoroughly the upstream changelog looking for cases where the rules
of good library management have been broken. If potential problems are discovered, the
upstream author should be notified as an upstream fix is always better than a Debian
specific work-around.

OPTIONS

       -Ppackage-build-dir
           Scan package-build-dir instead of debian/tmp.

       -ppackage
           Define the package name.  Required if more than one binary package is listed in
           debian/control (or if there's no debian/control file).

       -vversion
           Define the package version.  Defaults to the version extracted from debian/changelog.
           Required if called outside of a source package tree.

       -elibrary-file
           Only analyze libraries explicitly listed instead of finding all public libraries.  You
           can use shell patterns used for pathname expansions (see the File::Glob manual page
           for details) in library-file to match multiple libraries with a single argument
           (otherwise you need multiple -e).

       -ldirectory
           Prepend directory to the list of directories to search for private shared libraries
           (since dpkg 1.19.1).  This option can be used multiple times.

           Note: Use this option instead of setting LD_LIBRARY_PATH, as that environment variable
           is used to control the run-time linker and abusing it to set the shared library paths
           at build-time can be problematic when cross-compiling for example.

       -Ifilename
           Use filename as reference file to generate the symbols file that is integrated in the
           package itself.

       -O[filename]
           Print the generated symbols file to standard output or to filename if specified,
           rather than to debian/tmp/DEBIAN/symbols (or package-build-dir/DEBIAN/symbols if -P
           was used).  If filename is pre-existing, its contents are used as basis for the
           generated symbols file.  You can use this feature to update a symbols file so that it
           matches a newer upstream version of your library.

       -t  Write the symbol file in template mode rather than the format compatible with
           deb-symbols(5).  The main difference is that in the template mode symbol names and
           tags are written in their original form contrary to the post-processed symbol names
           with tags stripped in the compatibility mode.  Moreover, some symbols might be omitted
           when writing a standard deb-symbols(5) file (according to the tag processing rules)
           while all symbols are always written to the symbol file template.

       -c[0-4]
           Define the checks to do when comparing the generated symbols file with the template
           file used as starting point.  By default the level is 1.  Increasing levels do more
           checks and include all checks of lower levels.

           Level 0
               Never fails.

           Level 1
               Fails if some symbols have disappeared.

           Level 2
               Fails if some new symbols have been introduced.

           Level 3
               Fails if some libraries have disappeared.

           Level 4
               Fails if some libraries have been introduced.

           This value can be overridden by the environment variable DPKG_GENSYMBOLS_CHECK_LEVEL.

       -q  Keep quiet and never generate a diff between generated symbols file and the template
           file used as starting point or show any warnings about new/lost libraries or new/lost
           symbols.  This option only disables informational output but not the checks themselves
           (see -c option).

       -aarch
           Assume arch as host architecture when processing symbol files.  Use this option to
           generate a symbol file or diff for any architecture provided its binaries are already
           available.

       -d  Enable debug mode.  Numerous messages are displayed to explain what dpkg-gensymbols
           does.

       -V  Enable verbose mode.  The generated symbols file contains deprecated symbols as
           comments.  Furthermore in template mode, pattern symbols are followed by comments
           listing real symbols that have matched the pattern.

       -?, --help
           Show the usage message and exit.

       --version
           Show the version and exit.

ENVIRONMENT