NAME | SYNOPSIS | DESCRIPTION | MAINTAINING SYMBOLS FILES | OPTIONS | 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).

       -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         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 
       ⟨git://git.debian.org/git/dpkg/dpkg.git⟩ on 2017-04-25.  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
       to man-pages@man7.org

1.18.15-3-ga2ef                  1970-01-01               dpkg-gensymbols(1)

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