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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):

       ·   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 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
       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 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
	      or does not match the endianness and bits), it is made arch neu‐
	      tral  (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-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

	      The architecture-bits is either 32 or 64.

	       (arch-bits=32)a_32bit_specific_symbol@Base 1.0
	       (arch-bits=64)a_64bit_specific_symbol@Base 1.0

	      The architecture-endianness is either little or big.

	       (arch-endian=little)a_little_endian_specific_symbol@Base 1.0
	       (arch-endian=big)a_big_endian_specific_symbol@Base 1.0

	      Multiple restrictions can be chained.

	       (arch-bits=32|arch-endian=little)a_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-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:

       ·   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 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:

       ·   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;

       ·   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 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			  2014-12-29		    dpkg-gensymbols(1)
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