dpkg-gensymbols(1) dpkg utilities dpkg-gensymbols(1)
NAME
dpkg-gensymbols - generate symbols files (shared library dependency
information)
SYNOPSIS
dpkg-gensymbols [options]
DESCRIPTION
dpkg-gensymbols scans a temporary build tree (debian/tmp by default)
looking for libraries and generate 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 pro‐
vided by the maintainer. It looks for the following files (and use 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 him.
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 are 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 he can use
the diffs contained in the build logs. In most cases, the diff applies
directly to his 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 ver‐
sion number but the same upstream version still satisfy the generated
dependencies. If the Debian revision can't be dropped because the sym‐
bol 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.
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 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 orignal 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 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 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), 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, amd64, kfreebsd-amd64
and ia64 architectures while the second one anywhere except on
armel.
(arch=alpha amd64 kfreebsd-amd64 ia64)a_64bit_specific_sym‐
bol@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.
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. 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
Using wildcards with versioned symbols
Well maintained libraries have versioned symbols where each version
corresponds to the upstream version where the symbol got added. If
that's the case, it's possible to write a symbols file with wildcard
entries like "*@GLIBC_2.0" that would match any symbol associated to
the version GLIBC_2.0. It's still possible to include specific symbols
in the file, they'll take precedence over any matching wildcard entry.
An example:
libc.so.6 libc6 #MINVER#
*@GLIBC_2.0 2.0
[...]
*@GLIBC_2.7 2.7
access@GLIBC_2.0 2.2
The symbol access@GLIBC_2.0 will lead to a minimal dependency on libc6
version 2.2 despite the wildcard entry *@GLIBC_2.0 which associates
symbols versioned as GLIBC_2.0 with the minimal version 2.0.
Note that using wildcards means that dpkg-gensymbols can't check for
symbols that might have disappeared and can't generate a diff between
the maintainer-supplied symbols file and the generated one in the
binary package.
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 a regular expression in library-
file to match multiple libraries with a single argument (other‐
wise you need multiple -e).
-Ifilename
Use filename as reference file to generate the symbols file that
is integrated in the package itself.
-O Print the generated symbols file to standard output, rather than
being stored in the package build tree.
-Ofilename
Store the generated symbols file as filename. If filename is
pre-existing, its content is 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 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 disables all checks. 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.
-d Enable debug mode. Numerous messages are displayed to explain
what dpkg-gensymbols does.
-h, --help
Show the usage message and exit.
--version
Show the version and exit.
SEE ALSO
http://people.redhat.com/drepper/symbol-versioning
http://people.redhat.com/drepper/goodpractice.pdf
http://people.redhat.com/drepper/dsohowto.pdf
deb-symbols(5), dpkg-shlibdeps(1).
AUTHORS
Copyright © 2007-2009 Raphaël Hertzog
This is free software; see the GNU General Public Licence version 2 or
later for copying conditions. There is NO WARRANTY.
Debian Project 2009-08-07 dpkg-gensymbols(1)
Thursday, December 30, 2010
dpkg-gensymbols
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