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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
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
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 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 AT 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 AT 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:
o 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"
o 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
SEE ALSO
deb-symbols(5), dpkg-shlibdeps(1), dpkg-gensymbols(1).
1.21.1 2024-02-23 deb-src-symbols(5)
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