dpkg-gensymbols

       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. To do  this  properly  the  diffs
       contained  in  the  build  logs  can  be  used. In most cases, the diff
       applies directly to the debian/package.symbols file. That said, further
       tweaks  are  usually  needed:  it's recommended for example to drop the
       Debian revision from the minimal version so that backports with a lower
       version  number  but the same upstream version still satisfy the gener-
       ated 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.
       binary package.

   Using symbol tags
       Symbol tagging is useful for marking symbols that are special  in  some
       way.   Any  symbol can have an arbitrary number of tags associated with
       it. While all tags are parsed and stored,  only  a  some  of  them  are
       understood  by dpkg-gensymbols and trigger special handling of the sym-
       bols. 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.
              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.
       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

       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:

           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
              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

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