Provided by: dpkg-dev_1.17.5ubuntu5.8_all 
NAME
dpkg-gensymbols - generate symbols files (shared library dependency information)
SYNOPSIS
dpkg-gensymbols [option...]
DESCRIPTION
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):
• 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 with that.
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 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 the diffs contained in the build logs can
be used. 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.
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 original 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 one 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, any-amd64 and ia64 architectures, the second only on
linux architectures, while the third one anywhere except on armel.
(arch=alpha any-amd64 ia64)a_64bit_specific_symbol@Base 1.0
(arch=linux-any)linux_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.
c++ Denotes c++ symbol pattern. See Using symbol patterns subsection below.
symver Denotes symver (symbol version) symbol pattern. See Using symbol patterns subsection below.
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 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:
c++
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 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.
symver
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#
(symver)GLIBC_2.0 2.0
[...]
(symver)GLIBC_2.7 2.7
access@GLIBC_2.0 2.2
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.
regex
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#
(regex)"^mystack_.*@Base$" 1.0
(regex|optional)"private" 1.0
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
(c++|regex)"^NSA::ClassA::Private::privmethod\d\(int\)@Base" 1.0
(regex|c++)N3NSA6ClassA7Private11privmethod\dEi@Base 1.0
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 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.
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
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
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).
-Ifilename
Use filename as reference file to generate the symbols file that is integrated in the package
itself.
-O[filename]
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 template.
-c[0-4]
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_GENSYMBOLS_CHECK_LEVEL.
-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.
-?, --help
Show the usage message and exit.
--version
Show the version and exit.
SEE ALSO
https://people.redhat.com/drepper/symbol-versioning
https://people.redhat.com/drepper/goodpractice.pdf
https://people.redhat.com/drepper/dsohowto.pdf
deb-symbols(5), dpkg-shlibdeps(1).