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 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.
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.
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.
- 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.
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@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 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 "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.
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=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
deb-symbols(5),
dpkg-shlibdeps(1),
dpkg-gensymbols(1).