deb-src-symbols(5) dpkg suite deb-src-symbols(5)
NAME
deb-src-symbols - Debian's extended shared library template file
SYNOPSIS
debian/package.symbols.arch, debian/symbols.arch,
debian/package.symbols, debian/symbols
DESCRIPTION
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”.
ignore-blacklist
A deprecated alias for allow-internal (since dpkg 1.20.1, supported
since dpkg 1.15.3).
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
SEE ALSO
deb-symbols(5), dpkg-shlibdeps(1), dpkg-gensymbols(1).
1.22.21 2025-06-30 deb-src-symbols(5)
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