dpkg-gensymbols(1) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | MAINTAINING SYMBOLS FILES | OPTIONS | ENVIRONMENT | SEE ALSO | COLOPHON

dpkg-gensymbols(1)             dpkg suite             dpkg-gensymbols(1)

NAME         top

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

SYNOPSIS         top

       dpkg-gensymbols [option...]

DESCRIPTION         top

       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         top

       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) upgrades.

   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 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
       arch-bits=architecture-bits
       arch-endian=architecture-endianness
              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
               (arch=linux-any)linux_specific_symbol@Base 1.0
               (arch=!armel)symbol_armel_does_not_have@Base 1.0

              The architecture-bits is either 32 or 64.

               (arch-bits=32)32bit_specific_symbol@Base 1.0
               (arch-bits=64)64bit_specific_symbol@Base 1.0

              The architecture-endianness is either little or big.

               (arch-endian=little)little_endian_specific_symbol@Base
              1.0
               (arch-endian=big)big_endian_specific_symbol@Base 1.0

              Multiple restrictions can be chained.

               (arch-bits=32|arch-endian=little)32bit_le_symbol@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         top

       -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).

       -ldirectory
              Prepend directory to the list of directories to search for
              private shared libraries (since dpkg 1.19.1). This option
              can be used multiple times.

              Note: Use this option instead of setting LD_LIBRARY_PATH,
              as that environment variable is used to control the run-
              time linker and abusing it to set the shared library paths
              at build-time can be problematic when cross-compiling for
              example.

       -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.

ENVIRONMENT         top

       DPKG_GENSYMBOLS_CHECK_LEVEL
              Overrides the command check level, even if the -c command-
              line argument was given (note that this goes against the
              common convention of command-line arguments having
              precedence over environment variables).

       DPKG_COLORS
              Sets the color mode (since dpkg 1.18.5).  The currently
              accepted values are: auto (default), always and never.

       DPKG_NLS
              If set, it will be used to decide whether to activate
              Native Language Support, also known as
              internationalization (or i18n) support (since dpkg
              1.19.0).  The accepted values are: 0 and 1 (default).

SEE ALSO         top

       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).

COLOPHON         top

       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
       ⟨https://salsa.debian.org/dpkg-team/dpkg.git⟩ on 2021-08-27.  (At
       that time, the date of the most recent commit that was found in
       the repository was 2021-06-17.)  If you discover 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 to
       man-pages@man7.org

1.19.6-2-g6e42d5               2019-03-25             dpkg-gensymbols(1)

Pages that refer to this page: dh_makeshlibs(1)dpkg-shlibdeps(1)deb-symbols(5)debhelper(7)