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):
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
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
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: any line with ‘#’ as
the first character is a comment except if it starts with ‘#include’
(see section Using includes). Lines starting with ‘#MISSING:’ are
special comments documenting symbols that have disappeared.
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)
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 some of them are
understood by dpkg-gensymbols and trigger special handling of the
symbols. See subsection Standard symbol tags for reference of these
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
(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-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 original form as they were loaded.
Standard symbol tagsoptional
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.
These tags allow one to restrict the set of architectures
where the symbol is supposed to exist. The arch-bits and
arch-endian tags are supported since dpkg 1.18.0. 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 or does not match the
endianness and bits), it is made arch neutral (i.e. the arch,
arch-bits and arch-endian tags are 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,
any-amd64 and ia64 architectures, the second only on linux
architectures, while the third one anywhere except on armel.
(arch=alpha any-amd64 ia64)64bit_specific_symbol@Base 1.0
The architecture-bits is either 32 or 64.
The architecture-endianness is either little or big.
Multiple restrictions can be chained.
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.
c++ Denotes c++ symbol pattern. See Using symbol patterns
symver Denotes symver (symbol version) symbol pattern. See Usingsymbol patterns subsection below.
regex Denotes regex symbol pattern. See Using symbol patterns
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 specification 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 specific 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-
virtual thunk symbol. For example, even if
_ZThn8_N3NSB6ClassDD1Ev@Base on 32bit architectures 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
$ 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 version 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 patterns.
Please note that while old style wildcard patterns (denoted by
"*@version" in the symbol 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 regular 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 pattern 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
"_ZN3NSA6ClassA7Private11privmethod2Ei@Base". When matching the first
pattern, the raw symbol is first demangled 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
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
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:
· 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.symbols 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 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:
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.
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 shell patterns used for
pathname expansions (see the File::Glob(3perl) manual page for
details) 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.
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
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
This value can be overridden by the environment variable
-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.
Show the usage message and exit.
Show the version and exit.
This page is part of the dpkg (Debian Package Manager) project.
Information about the project can be found at
⟨https://wiki.debian.org/Teams/Dpkg/⟩. If you have a bug report for
this manual page, see
⟨http://bugs.debian.org/cgi-bin/pkgreport.cgi?src=dpkg⟩. This page
was obtained from the project's upstream Git repository
⟨git://git.debian.org/git/dpkg/dpkg.git⟩ on 2017-03-13. If you dis‐
cover any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
1.18.15-3-ga2ef 1970-01-01 dpkg-gensymbols(1)