The symbols files are really useful only if they reflect the
evolution of the package through several releases. Thus the
maintainer has to update them every time that a new symbol is
added so that its associated minimal version matches reality.
The diffs contained in the build logs can be used as a starting
point, but the maintainer, additionally, has to make sure that
the behaviour of those symbols has not changed in a way that
would make anything using those symbols and linking against the
new version, stop working with the old version. In most cases,
the diff applies directly to the debian/package.symbols file.
That said, further tweaks are usually needed: it's recommended
for example to drop the Debian revision from the minimal version
so that backports with a lower version number but the same
upstream version still satisfy the generated dependencies. If
the Debian revision can't be dropped because the symbol really
got added by the Debian specific change, then one should suffix
the version with '~'.
Before applying any patch to the symbols file, the maintainer
should double-check that it's sane. Public symbols are not
supposed to disappear, so the patch should ideally only add new
lines.
Note that you can put comments in symbols 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.
Do not forget to check if old symbol versions need to be
increased. There is no way dpkg-gensymbols can warn about this.
Blindly applying the diff or assuming there is nothing to change
if there is no diff, without checking for such changes, can lead
to packages with loose dependencies that claim they can work with
older packages they cannot work with. This will introduce hard to
find bugs with (partial) upgrades.
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
ignore-blacklist
dpkg-gensymbols has an internal blacklist of symbols that
should not appear in symbols files as they are usually
only side-effects of implementation details of the
toolchain. If for some reason, you really want one of
those symbols to be included in the symbols file, you
should tag the symbol with ignore-blacklist. It can be
necessary for some low level toolchain libraries like
libgcc.
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@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 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:
• 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 "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
Good library management
A well-maintained library has the following features:
• its API is stable (public symbols are never dropped, only new
public symbols are added) and changes in incompatible ways
only when the SONAME changes;
• ideally, it uses symbol versioning to achieve ABI stability
despite internal changes and API extension;
• it doesn't export private symbols (such symbols can be tagged
optional as workaround).
While maintaining the symbols file, it's easy to notice
appearance and disappearance of symbols. But it's more difficult
to catch incompatible API and ABI change. Thus the maintainer
should read thoroughly the upstream changelog looking for cases
where the rules of good library management have been broken. If
potential problems are discovered, the upstream author should be
notified as an upstream fix is always better than a Debian
specific work-around.
