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

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.  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
       -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
       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
       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).

1.19.7                                      2019-06-03                         dpkg-gensymbols(1)

Usage: dpkg-gensymbols [<option>...]

Options:
  -l<library-path>         add directory to private shared library search list.
  -p<package>              generate symbols file for package.
  -P<package-build-dir>    temporary build directory instead of debian/tmp.
  -e<library>              explicitly list libraries to scan.
  -v<version>              version of the packages (defaults to
                           version extracted from debian/changelog).
  -c<level>                compare generated symbols file with the reference
                           template in the debian directory and fail if
                           difference is too important; level goes from 0 for
                           no check, to 4 for all checks (default level is 1).
  -q                       keep quiet and never emit any warnings or
                           generate a diff between generated symbols
                           file and the reference template.
  -I<file>                 force usage of <file> as reference symbols
                           file instead of the default file.
  -O[<file>]               write to stdout (or <file>), not .../DEBIAN/symbols.
  -t                       write in template mode (tags are not
                           processed and included in output).
  -V                       verbose output; write deprecated symbols and pattern
                           matching symbols as comments (in template mode only).
  -a<arch>                 assume <arch> as host architecture when processing
                           symbol files.
  -d                       display debug information during work.
  -?, --help               show this help message.
      --version            show the version.