Ubuntu Manpage: dpkg-gensymbols - generate symbols files (shared library dependency information)

Provided by: dpkg-dev_1.19.7ubuntu3.2_all

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