Provided by: dpkg-dev_126.96.36.199ubuntu3_all
dpkg-gensymbols - generate symbols files (shared library dependency
dpkg-gensymbols scans a temporary build tree (debian/tmp by default)
looking for libraries and generate 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
When generating those files, it uses as input some symbols files
provided by the maintainer. It looks for the following files (and use
the first that is found):
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 him.
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 are too significant, it will even fail
(you can customize how much difference you can tolerate, see the -c
MAINTAINING SYMBOLS FILES
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. To do this properly he can
use the diffs contained in the build logs. In most cases, the diff
applies directly to his 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.
When the set of exported symbols differ between architectures, it’s no
more possible to use a common symbols file. Using one file per
architecture works, but it can also lead to duplication of information.
In those cases, you can factorize the common part in some external file
and include that file in your package.symbols.arch file by using an
include directive like this:
The symbols files are read line by line, and include directives are
processed as soon as they are encountered. This means that the content
of the included file can override any content that appeared before the
include directive and that any content after the directive can override
anything contained in the included file.
An included file can repeat the header line containing the SONAME of
the library. In that case, it overrides any header line previously
read. However, in general it’s best to avoid duplicating header lines.
One way to do it is the following:
Using wildcards with versioned symbols
Well maintained libraries have versioned symbols where each version
corresponds to the upstream version where the symbol got added. If
that’s the case, it’s possible to write a symbols file with wildcard
entries like "*@GLIBC_2.0" that would match any symbol associated to
the version GLIBC_2.0. It’s still possible to include specific symbols
in the file, they’ll take precedence over any matching wildcard entry.
libc.so.6 libc6 #MINVER#
The symbol access@GLIBC_2.0 will lead to a minimal dependency on libc6
version 2.2 despite the wildcard entry *@GLIBC_2.0 which associates
symbols versioned as GLIBC_2.0 with the minimal version 2.0. Note that
using wildcards means that dpkg-gensymbols can’t check for symbols that
might have disappeared and can’t generate a diff between the
maintainer-supplied symbols file and the generated one in the binary
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.
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.
Scan package-build-dir instead of debian/tmp.
Define the package name. Required if more than one binary
package is listed in debian/control (or if there’s no
Define the package version. Defaults to the version extracted
from debian/changelog. Required if called outside of a source
Only analyze libraries explicitly listed instead of finding all
public libraries. You can use a regular expression in library-
file to match multiple libraries with a single argument
(otherwise you need multiple -e).
Use filename as reference file to generate the symbols file that
is integrated in the package itself.
-O Print the generated symbols file to standard output, rather than
being stored in the package build tree.
Store the generated symbols file as filename. If filename is
pre-existing, its content is 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.
Define the checks to do when comparing the generated symbols
file with the 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 disables all checks. 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
This value can be overridden by the environment variable
-d Enable debug mode. Numerous messages are displayed to explain
what dpkg-gensymbols does.
Show the usage message and exit.
Show the version and exit.
Copyright (C) 2007 Raphaël Hertzog
This is free software; see the GNU General Public Licence version 2 or
later for copying conditions. There is NO WARRANTY.