dpkg-gensymbols(1)              dpkg utilities              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 informa-
       tion of the package.

       When generating those files, it uses as input some symbols  files  pro-
       vided by the maintainer. It looks for the following files (and uses the
       first that is found):

       o   debian/package.symbols.arch

       o   debian/symbols.arch

       o   debian/package.symbols

       o   debian/symbols

       The main interest of those files is  to  provide  the  minimal  version
       associated  to each symbol provided by the libraries. Usually it corre-
       sponds 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.  Fur-
       thermore 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 associ-
       ated 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  disap-
       pear, 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 spe-
       cial 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  architec-
       tures.   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 under-
       stood 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 spec-
       ification 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 contin-
       ues 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  nor-
       mal 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 out-
       put. 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. How-
              ever, disappeared optional symbols will continuously  appear  as
              MISSING  in  the diff in each new package revision.  This behav-
              iour 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 architec-
              ture  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-spe-
              cific symbols only those that match the current  host  architec-
              ture 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-black-
              list. 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 subsec-
              tion 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 spe-
       cial 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 destruc-
          tor which under diamond inheritance needs a non-virtual  thunk  sym-
          bol.  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 def-
          inition,  this is not necessarily true for demangled names. A couple
          of distinct real symbols may have the same demangled name. For exam-
          ple,  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  spe-
          cific 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  excep-
          tion of the symbol access@GLIBC_2.0. The latter will lead to a mini-
          mal dependency on libc6 version 2.2 despite being in  the  scope  of
          the "(symver)GLIBC_2.0" pattern because specific symbols take prece-
          dence 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",    "mys-
          tack_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 sym-
          bols  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 speci-
       fied. 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 multi-
       ple  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 pat-
       terns.

       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 gener-
       ates 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:

       o   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"

       o   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 con-
           sidered 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.  How-
       ever,  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:

       o   its  API is stable (public symbols are never dropped, only new pub-
           lic symbols are added) and changes in incompatible ways  only  when
           the SONAME changes;

       o   ideally, it uses symbol versioning to achieve ABI stability despite
           internal changes and API extension;

       o   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 pack-
              age 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  argu-
              ment (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  file-
              name  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 dis-
              appeared. 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 archi-
              tecture 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  depre-
              cated 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).



Debian Project                    2013-09-06                dpkg-gensymbols(1)
