How to use symbols files

This page is meant to collect some generic recommendations for handling symbols files. It's related to the improved dpkg-shlibdeps project.

Maintainers should not drop symbols file directly into Debian without understanding the mechanism for packaging these symbols files. Please read this page carefully (and the manual pages of dpkg-shlibdeps / dpkg-gensymbols) to understand the various issues.

How to manage the content of symbols files

The initial list of symbols for a previously built library package, e.g., libfoo can be obtained as follows.

$ dpkg-deb -x libfoo_<version>-<rev>.deb /tmp/libfoo
$ dpkg-gensymbols -v<version> -plibfoo -P/tmp/libfoo -Olibfoo.symbols

If a library package contains multiple versions of library files, you may need to specify the path to each of them with -e for each version instead of using -P.

You can create a symbols file which contains the minimal version by scanning successively oldstable + stable + testing + unstable versions (or all versions from Debian snapshots) of the library as above.

One needs to also check the following;

Check the list of symbols exported by the library

If you find (on some arches) symbols that are exported which are not supposed to be public, you must not use symbols versioning at all. You might want to update the libtool scripts of your package from the latest version in Debian to fix the problem. There are old libtool versions which are not doing their job properly on some arches.

If your library exports private symbols, you might want to discuss with upstream to implement a version script to fix this. For libtool using packages, it can be easily generated using libtool's -export-symbols and -export-symbols-regex options.

Be responsive to porters

In some cases, the usage of symbols file might create problems to unofficial architectures. Be kind to them and incorporate quickly their patches, or offer them to NMU your package if you're busy.

C++ libraries

For C++ libraries it is often better not to ship symbols files.

Essays from Russ Allbery about symbols files in C++ libraries:

In essence from above one can apply C++ filter to the generated symbols to ease maintenance and understanding:

$ cat libfoo.symbols 
libqtubuntu_sensors.so.1 qtubuntu-sensors #MINVER#
 _ZN17OrientationSensor11qt_metacallEN11QMetaObject4CallEiPPv@Base 0.5.1daily13.04.16ubuntu.unity.next
 _ZN17OrientationSensor11qt_metacastEPKc@Base 0.5.1daily13.04.16ubuntu.unity.next
 _ZN17OrientationSensor16staticMetaObjectE@Base 0.5.1daily13.04.16ubuntu.unity.next
 _ZN17OrientationSensor4typeE@Base 0.5.1daily13.04.16ubuntu.unity.next
 _ZN17OrientationSensorD0Ev@Base 0.5.1daily13.04.16ubuntu.unity.next
 _ZN17OrientationSensorD1Ev@Base 0.5.1daily13.04.16ubuntu.unity.next
 _ZN17OrientationSensorD2Ev@Base 0.5.1daily13.04.16ubuntu.unity.next

$ sed 's/ \(_.*\) \(.*\)/ (c++)"\1" \2/' libfoo.symbols | c++filt
libqtubuntu_sensors.so.1 qtubuntu-sensors #MINVER#
 (c++)"OrientationSensor::qt_metacall(QMetaObject::Call, int, void**)@Base" 0.5.1daily13.04.16ubuntu.unity.next
 (c++)"OrientationSensor::qt_metacast(char const*)@Base" 0.5.1daily13.04.16ubuntu.unity.next
 (c++)"OrientationSensor::staticMetaObject@Base" 0.5.1daily13.04.16ubuntu.unity.next
 (c++)"OrientationSensor::type@Base" 0.5.1daily13.04.16ubuntu.unity.next
 (c++)"OrientationSensor::~OrientationSensor()@Base" 0.5.1daily13.04.16ubuntu.unity.next
 (c++)"OrientationSensor::~OrientationSensor()@Base" 0.5.1daily13.04.16ubuntu.unity.next
 (c++)"OrientationSensor::~OrientationSensor()@Base" 0.5.1daily13.04.16ubuntu.unity.next

Other tags and symbol patterns are: optional, arch, ignore-blacklist, symver and regex. See man dpkg-gensymbols(1) for further reference.

Proper visibility for C++ symbols

If your upstream is on board with this, they should be building with -fvisibility=hidden, and decorate everything intentionally public with a macro that (when building with gcc or clang) expands to __attribute__((__visibility__("default"))).

Some upstreams will be doing something similar already, because they are portable to Windows and need to decorate public symbols with __declspec(dllexport) on Windows.

For further information please read: