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NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | COLOPHON |
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deb-src-symbols(5) dpkg suite deb-src-symbols(5)
deb-src-symbols - Debian's extended shared library template file
debian/package.symbols.arch, debian/symbols.arch,
debian/package.symbols, debian/symbols
The symbol file templates are shipped in Debian source packages,
and its format is a superset of the symbols files shipped in
binary packages, see deb-symbols(5).
Comments
Comments are supported in template symbol 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.
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
behavior 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 arm64,
any-amd64 and riscv64 architectures, the second only on linux
architectures, while the third one anywhere except on armel.
(arch=arm64 any-amd64 riscv64)arch_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
allow-internal
dpkg-gensymbols has a list of internal symbols that should not
appear in symbols files as they are usually only side-effects
of implementation details of the toolchain (since dpkg
1.20.1). If for some reason, you really want one of those
symbols to be included in the symbols file, you should tag the
symbol with allow-internal. 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 32-bit
architectures will probably be _ZThn16_N3NSB6ClassDD1Ev@Base
on 64-bit 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 behavior 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 "ng_mystack_new@Base" would not. 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 "I<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-bits=64)#include "package.symbols.64-bit"
(arch-bits=32)#include "package.symbols.32-bit"
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
deb-symbols(5), dpkg-shlibdeps(1), dpkg-gensymbols(1).
This page is part of the dpkg (Debian Package Manager) project.
Information about the project can be found at
⟨https://wiki.debian.org/Teams/Dpkg/⟩. If you have a bug report
for this manual page, see
⟨http://bugs.debian.org/cgi-bin/pkgreport.cgi?src=dpkg⟩. This
page was obtained from the project's upstream Git repository ⟨git
clone https://git.dpkg.org/git/dpkg/dpkg.git⟩ on 2025-08-11. (At
that time, the date of the most recent commit that was found in
the repository was 2025-08-06.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
1.22.19-74-gf1ca0 2025-05-18 deb-src-symbols(5)
Pages that refer to this page: dpkg-gensymbols(1), deb-symbols(5)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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