Ubuntu Manpage: deb-src-symbols - Debian's extended shared library template file

Provided by: dpkg-dev_1.22.18ubuntu2_all

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

NAME

SYNOPSIS

DESCRIPTION

SEE ALSO