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.
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 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:
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.
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.
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:
The include directive may also be tagged like any symbol:
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).