dpkg\-gensymbols

Section: dpkg utilities (1)
Updated: 2009-08-07
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NAME

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

SYNOPSIS

dpkg-gensymbols [options]

DESCRIPTION

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 the package. 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):

*: 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 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 option).

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.

Using #PACKAGE# substitution

In some rare cases, the name of the library varies between architectures. To avoid hardcoding the name of the package in the symbols file, you can use the marker #PACKAGE#. It will be replaced by the real package name during installation of the symbols files. Contrary to the #MINVER# marker, #PACKAGE# will never appear in a symbols file inside a binary package.

Using symbol tags

Symbol tagging is useful for marking symbols that are special in some way. Any symbol can have an arbitrary number of tags associated with it. While all tags are parsed and stored, only a some of them are understood by dpkg-gensymbols and trigger special handling of the symbols. See subsection Standard symbol tags for reference of these tags. Tag specification comes right before the symbol name (no whitespace is allowed in between). It always starts with an opening bracket (, ends with a closing bracket ) and must contain at least one tag. Multiple tags are separated by the | character. Each tag can optionally have a value which is separated form the tag name by the = character. Tag names and values can be arbitrary strings except they cannot contain any of the special ) | = characters. Symbol names following a tag specification can optionally be quoted with either ' or " characters to allow whitespaces in them. However, if there are no tags specified for the symbol, quotes are treated as part of the symbol name which continues up until the first space.
(tag1=i am marked|tag name with space)"tagged quoted symbol"@Base 1.0
(optional)tagged_unquoted_symbol@Base 1.0 1
untagged_symbol@Base 1.0 The first symbol in the example is named tagged quoted symbol and has two tags: tag1 with value i am marked and tag name with space that has no value. The second symbol named tagged_unquoted_symbol is only tagged with the tag named optional. The last symbol is an example of the normal untagged symbol. Since symbol tags are an extension of the deb-symbols(5) format, they can only be part of the symbols files used in source packages (those files should then be seen as templates used to build the symbols files that are embedded in binary packages). When dpkg-gensymbols is called without the -t option, it will output symbols files compatible to the deb-symbols(5) format: it fully processes symbols according to the requirements of their standard tags and strips all tags from the output. On the contrary, in template mode (-t) all symbols and their tags (both standard and unknown ones) are kept in the output and are written in their orignal form as they were loaded.

Standard symbol tags

optional

A symbol marked as optional can disappear from the library at any time and that will never cause dpkg-gensymbols to fail. However, disappeared optional symbols will continuously appear as MISSING in the diff in each new package revision. This behaviour serves as a reminder for the maintainer that such a symbol needs to be removed from the symbol file or readded to the library. When the optional symbol, which was previously declared as MISSING, suddenly reappears in the next revision, it will be upgraded back to the "existing" status with its minimum version unchanged.

This tag is useful for symbols which are private where their disappearance do not cause ABI breakage. For example, most of C++ template instantiations fall into this category. Like any other tag, this one may also have an arbitrary value: it could be used to indicate why the symbol is considered optional.

arch=architecture list

This tag allows to restrict the set of architectures where the symbol is supposed to exist. When the symbols list is updated with the symbols discovered in the library, all arch-specific symbols which do not concern the current host architecture are treated as if they did not exist. If an arch-specific symbol matching the current host architecture does not exist in the library, normal procedures for missing symbols apply and it may cause dpkg-gensymbols to fail. On the other hand, if the arch-specific symbol is found when it was not supposed to exist (because the current host architecture is not listed in the tag), it is made arch neutral (i.e. the arch tag is dropped and the symbol will appear in the diff due to this change), but it is not considered as new.

When operating in the default non-template mode, among arch-specific symbols only those that match the current host architecture are written to the symbols file. On the contrary, all arch-specific symbols (including those from foreign arches) are always written to the symbol file when operating in template mode.

The format of architecture list is the same as the one used in the Build-Depends field of debian/control (except the enclosing square brackets []). For example, the first symbol from the list below will be considered only on alpha, amd64, kfreebsd-amd64 and ia64 architectures while the second one anywhere except on armel.

(arch=alpha amd64 kfreebsd-amd64 ia64)a_64bit_specific_symbol@Base 1.0
(arch=!armel)symbol_armel_does_not_have@Base 1.0

ignore-blacklist

dpkg-gensymbols has an internal blacklist of symbols that should not appear in symbols files as they are usually only side-effects of implementation details of the toolchain. If for some reason, you really want one of those symbols to be included in the symbols file, you should tag the symbol with ignore-blacklist. It can be necessary for some low level toolchain libraries like libgcc.

Using includes

When the set of exported symbols differ between architectures, it may become inefficient to use a single symbol file. In those cases, an include directive may prove to be useful in a couple of ways:

*

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:

#include "packages.symbols.common"

*

The include directive may also be tagged like any symbol:

(tag|..|tagN)#include "file_to_include"

As a result, all symbols included from file_to_include will be considered to be tagged with tag .. tagN by default. You can use this feature to create a common package.symbols file which includes architecture specific symbol files:

common_symbol1@Base 1.0
(arch=amd64 ia64 alpha)#include "package.symbols.64bit"
(arch=!amd64 !ia64 !alpha)#include "package.symbols.32bit"
common_symbol2@Base 1.0 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. Any symbol (or even another #include directive) in the included file can specify additional tags or override values of the inherited tags in its tag specification. However, there is no way for the symbol to remove any of the inherited tags. 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:

#include "libsomething1.symbols.common"
arch_specific_symbol@Base 1.0

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. An example:

libc.so.6 libc6 #MINVER#
*@GLIBC_2.0 2.0
[...]
*@GLIBC_2.7 2.7
access@GLIBC_2.0 2.2 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 package.

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 a regular expression in library-file to match multiple libraries with a single argument (otherwise you need multiple -e).
-Ifilename: 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.
-Ofilename: 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.
-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 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 introduced.
This value can be overridden by the environment variable DPKG_GENSYMBOLS_CHECK_LEVEL.
-d: Enable debug mode. Numerous messages are displayed to explain what dpkg-gensymbols does.
-h, --help: Show the usage message and exit.
--version: Show the version and exit.

AUTHORS

This is free software; see the GNU General Public Licence version 2 or later for copying conditions. There is NO WARRANTY.