dpkg-gensymbols(1) dpkg utilities dpkg-gensymbols(1)
dpkg-gensymbols - generate symbols files (shared library dependency information)
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 main-
tainer. 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 sym-
bol 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 responsibil-
ity of the maintainer to keep those files up-to-date and accurate, but dpkg-gensymbols
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 signifi-
cant, it will even fail (you can customize how much difference you can tolerate, see the
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 prop-
erly 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 gen-
erated 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 hard-
coding 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. Con-
trary to the #MINVER# marker, #PACKAGE# will never appear in a symbols file inside a
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 spec-
ified 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
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-gensym-
bols 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 original form as they were loaded.
Standard symbol tags
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
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.
This tag allows one to restrict the set of architectures where the symbol is sup-
posed 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 architec-
ture 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
dpkg-gensymbols has an internal blacklist of symbols that should not appear in sym-
bols 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.
c++ Denotes c++ symbol pattern. See Using symbol patterns subsection below.
symver Denotes symver (symbol version) symbol pattern. See Using symbol patterns subsec-
regex Denotes regex symbol pattern. See Using symbol patterns subsection below.
Using symbol patterns
Unlike a standard symbol specification, a pattern may cover multiple real symbols from the
library. dpkg-gensymbols will attempt to match each pattern against each real symbol that
does not have a specific symbol counterpart defined in the symbol file. Whenever the first
matching pattern is found, all its tags and properties will be used as a basis specifica-
tion of the symbol. If none of the patterns matches, the symbol will be considered as new.
A pattern is considered lost if it does not match any symbol in the library. By default
this will trigger a dpkg-gensymbols failure under -c1 or higher level. However, if the
failure is undesired, the pattern may be marked with the optional tag. Then if the pattern
does not match anything, it will only appear in the diff as MISSING. Moreover, like any
symbol, the pattern may be limited to the specific architectures with the arch tag. Please
refer to Standard symbol tags subsection above for more information.
Patterns are an extension of the deb-symbols(5) format hence they are only valid in symbol
file templates. Pattern specification syntax is not any different from the one of a spe-
cific symbol. However, symbol name part of the specification serves as an expression to be
matched against name@version of the real symbol. In order to distinguish among different
pattern types, a pattern will typically be tagged with a special tag.
At the moment, dpkg-gensymbols supports three basic pattern types:
This pattern is denoted by the c++ tag. It matches only C++ symbols by their demangled
symbol name (as emitted by c++filt(1) utility). This pattern is very handy for matching
symbols which mangled names might vary across different architectures while their
demangled names remain the same. One group of such symbols is non-virtual thunks which
have architecture specific offsets embedded in their mangled names. A common instance
of this case is a virtual destructor which under diamond inheritance needs a non-vir-
tual thunk symbol. For example, even if _ZThn8_N3NSB6ClassDD1Ev@Base on 32bit architec-
tures will probably be _ZThn16_N3NSB6ClassDD1Ev@Base on 64bit ones, it can be matched
with a single c++ pattern:
libdummy.so.1 libdummy1 #MINVER#
(c++)"non-virtual thunk to NSB::ClassD::~ClassD()@Base" 1.0
The demangled name above can be obtained by executing the following command:
$ echo '_ZThn8_N3NSB6ClassDD1Ev@Base' | c++filt
Please note that while mangled name is unique in the library by definition, this is not
necessarily true for demangled names. A couple of distinct real symbols may have the
same demangled name. For example, that's the case with non-virtual thunk symbols in
complex inheritance configurations or with most constructors and destructors (since g++
typically generates two real symbols for them). However, as these collisions happen on
the ABI level, they should not degrade quality of the symbol file.
This pattern is denoted by the symver tag. Well maintained libraries have versioned
symbols where each version corresponds to the upstream version where the symbol got
added. If that's the case, you can use a symver pattern to match any symbol associated
to the specific version. For example:
libc.so.6 libc6 #MINVER#
All symbols associated with versions GLIBC_2.0 and GLIBC_2.7 will lead to minimal ver-
sion of 2.0 and 2.7 respectively with the exception of the symbol access@GLIBC_2.0. The
latter will lead to a minimal dependency on libc6 version 2.2 despite being in the
scope of the "(symver)GLIBC_2.0" pattern because specific symbols take precedence over
Please note that while old style wildcard patterns (denoted by "*@version" in the sym-
bol name field) are still supported, they have been deprecated by new style syntax
"(symver|optional)version". For example, "*@GLIBC_2.0 2.0" should be written as
"(symver|optional)GLIBC_2.0 2.0" if the same behaviour is needed.
Regular expression patterns are denoted by the regex tag. They match by the perl regu-
lar expression specified in the symbol name field. A regular expression is matched as
it is, therefore do not forget to start it with the ^ character or it may match any
part of the real symbol name@version string. For example:
libdummy.so.1 libdummy1 #MINVER#
Symbols like "mystack_new@Base", "mystack_push@Base", "mystack_pop@Base" etc. will be
matched by the first pattern while e.g. "ng_mystack_new@Base" won't. The second pat-
tern will match all symbols having the string "private" in their names and matches will
inherit optional tag from the pattern.
Basic patterns listed above can be combined where it makes sense. In that case, they are
processed in the order in which the tags are specified. For example, both
will match symbols "_ZN3NSA6ClassA7Private11privmethod1Ei@Base" and "_ZN3NSA6ClassA7Pri-
vate11privmethod2Ei@Base". When matching the first pattern, the raw symbol is first deman-
gled as C++ symbol, then the demangled name is matched against the regular expression. On
the other hand, when matching the second pattern, regular expression is matched against
the raw symbol name, then the symbol is tested if it is C++ one by attempting to demangle
it. A failure of any basic pattern will result in the failure of the whole pattern.
Therefore, for example, "__N3NSA6ClassA7Private11privmethod\dEi@Base" will not match
either of the patterns because it is not a valid C++ symbol.
In general, all patterns are divided into two groups: aliases (basic c++ and symver) and
generic patterns (regex, all combinations of multiple basic patterns). Matching of basic
alias-based patterns is fast (O(1)) while generic patterns are O(N) (N - generic pattern
count) for each symbol. Therefore, it is recommended not to overuse generic patterns.
When multiple patterns match the same real symbol, aliases (first c++, then symver) are
preferred over generic patterns. Generic patterns are matched in the order they are found
in the symbol file template until the first success. Please note, however, that manual
reordering of template file entries is not recommended because dpkg-gensymbols generates
diffs based on the alphanumerical order of their names.
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:
o 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:
o The include directive may also be tagged like any symbol:
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.sym-
bols file which includes architecture specific symbol files:
(arch=amd64 ia64 alpha)#include "package.symbols.64bit"
(arch=!amd64 !ia64 !alpha)#include "package.symbols.32bit"
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 direc-
tive 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:
Good library management
A well-maintained library has the following features:
o 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;
o ideally, it uses symbol versioning to achieve ABI stability despite internal changes
and API extension;
o 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 main-
tainer 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 spe-
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 debian/control file).
Define the package version. Defaults to the version extracted from
debian/changelog. Required if called outside of a source package tree.
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 con-
tent 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 process-
ing rules) while all symbols are always written to the symbol file template.
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_GENSYM-
-q Keep quiet and never generate a diff between generated symbols file and the tem-
plate 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
-d Enable debug mode. Numerous messages are displayed to explain what dpkg-gensymbols
-V Enable verbose mode. The generated symbols file contains deprecated symbols as com-
ments. Furthermore in template mode, pattern symbols are followed by comments list-
ing real symbols that have matched the pattern.
Show the usage message and exit.
Show the version and exit.
Copyright (C) 2007-2009 Raphael Hertzog
This is free software; see the GNU General Public Licence version 2 or later for copying
conditions. There is NO WARRANTY.
Debian Project 2009-08-07 dpkg-gensymbols(1)