The latest version of this document is always available at http://gcc.gnu.org/onlinedocs/libstdc++/abi.html.
To the libstdc++-v3 homepage.
C++ applications often dependent on specific language support routines, say for throwing exceptions, or catching exceptions, and perhaps also dependent on features in the C++ Standard Library.
The C++ Standard Library has many include files, types defined in those include files, specific named functions, and other behavior. The text of these behaviors, as written in source include files, is called the Application Programing Interface, or API.
Furthermore, C++ source that is compiled into object files is transformed by the compiler: it arranges objects with specific alignment and in a particular layout, mangling names according to a well-defined algorithm, has specific arrangements for the support of virtual functions, etc. These details are defined as the compiler Application Binary Interface, or ABI. The GNU C++ compiler uses an industry-standard C++ ABI starting with version 3. Details can be found in the ABI specification.
The GNU C++ compiler, g++, has a compiler command line option to
switch between various different C++ ABIs. This explicit version
switch is the flag -fabi-version. In addition, some
g++ command line options may change the ABI as a side-effect of
use. Such flags include -fpack-struct and
-fno-exceptions, but include others: see the complete
list in the GCC manual under the heading Options
for Code Generation Conventions.
The configure options used when building a specific libstdc++ version may also impact the resulting library ABI. The available configure options, and their impact on the library ABI, are documented here.
Putting all of these ideas together results in the C++ Standard library ABI, which is the compilation of a given library API by a given compiler ABI. In a nutshell:
library API + compiler ABI = library ABI
The library ABI is mostly of interest for end-users who have unresolved symbols and are linking dynamically to the C++ Standard library, and who thus must be careful to compile their application with a compiler that is compatible with the available C++ Standard library binary. In this case, compatible is defined with the equation above: given an application compiled with a given compiler ABI and library API, it will work correctly with a Standard C++ Library created with the same constraints.
To use a specific version of the C++ ABI, one must use a corresponding GNU C++ toolchain (Ie, g++ and libstdc++) that implements the C++ ABI in question.
The C++ interface has evolved throughout the history of the GNU C++ toolchain. With each release, various details have been changed so as to give distinct versions to the C++ interface.
Extending existing, stable ABIs. Versioning gives subsequent stable releases series libraries the ability to add new symbols and add functionality, all the while retaining backwards compatibility with the previous releases in the series. Note: the reverse is not true. It is not possible to take binaries linked with the latest version of a release series (if symbols have been added) and expect the initial release of the series to remain link compatible.
Allows multiple, incompatible ABIs to coexist at the same time.
How can this complexity be managed? What does C++ versioning mean? Because library and compiler changes often make binaries compiled with one version of the GNU tools incompatible with binaries compiled with other (either newer or older) versions of the same GNU tools, specific techniques are used to make managing this complexity easier.
The following techniques are used:
Release versioning on the libgcc_s.so binary. This is implemented via file names and the ELF DT_SONAME mechanism (at least on ELF systems).
It is versioned as follows:
--with-sjlj-exceptions) or
libgcc_s.so.2. For all others, this is libgcc_s.so.1.
It is versioned as follows:
mapfile: gcc/libgcc-std.ver
It is versioned with the following labels and version definitions:
mapfile: libstdc++-v3/config/linker-map.gnu
It is versioned with the following labels and version definitions, where the version definition is the maximum for a particular release. Note, only symbol which are newly introduced will use the maximum version definition. Thus, for release series with the same label, but incremented version definitions, the later release has both versions. (An example of this would be the gcc-3.2.1 release, which has GLIBCPP_3.2.1 for new symbols and GLIBCPP_3.2 for symbols that were introduced in the gcc-3.2.0 release.)
Incremental bumping of a compiler pre-defined macro, __GXX_ABI_VERSION. This macro is defined as the version of the compiler v3 ABI, with g++ 3.0.x being version 100. This macro will be automatically defined whenever g++ is used (the curious can test this by invoking g++ with the '-v' flag.)
This macro was defined in the file "lang-specs.h" in the gcc/cp directory. Later versions defined it in "c-common.c" in the gcc directory, and from G++ 3.4 it is defined in c-cppbuiltin.c and its value determined by the '-fabi-version' command line option.
It is versioned as follows, where 'n' is given by '-fabi-version=n':
Changes to the default compiler option for
-fabi-version.
It is versioned as follows:
-fabi-version=1-fabi-version=1-fabi-version=2Incremental bumping of a library pre-defined macro. For releases before 3.4.0, the macro is __GLIBCPP__. For later releases, it's __GLIBCXX__. (The libstdc++ project generously changed from CPP to CXX throughout its source to allow the "C" pre-processor the CPP macro namespace.) These macros are defined as the date the library was released, in compressed ISO date format, as an unsigned long.
In addition, the pre-defined macro is defined in the file "c++config" in the "libstdc++-v3/include/bits" directory and is changed every night by an automated script.
It is versioned as follows:
Incremental bumping of a library pre-defined macro, _GLIBCPP_VERSION. This macro is defined as the released version of the library, as a string literal. This is only implemented in gcc-3.1.0 releases and higher, and is deprecated in 3.4 (where it is called _GLIBCXX_VERSION).
This macro is defined in the file "c++config" in the "libstdc++-v3/include/bits" directory and is generated automatically by autoconf as part of the configure-time generation of config.h.
It is versioned as follows:
Matching each specific C++ compiler release to a specific set of C++ include files. This is only implemented in gcc-3.1.1 releases and higher.
All C++ includes are installed in include/c++, then nest in a directory hierarchy corresponding to the C++ compiler's released version. This version corresponds to the variable "gcc_version" in "libstdc++-v3/acinclude.m4," and more details can be found in that file's macro GLIBCXX_CONFIGURE (GLIBCPP_CONFIGURE before gcc-3.4.0).
C++ includes are versioned as follows:
Taken together, these techniques can accurately specify interface and implementation changes in the GNU C++ tools themselves. Used properly, they allow both the GNU C++ tools implementation, and programs using them, an evolving yet controlled development that maintains backward compatibility.
Minimum environment that supports a versioned ABI: A supported dynamic linker, a GNU linker of sufficient vintage to understand demangled C++ name globbing (ld), a shared executable compiled with g++, and shared libraries (libgcc_s, libstdc++-v3) compiled by a compiler (g++) with a compatible ABI. Phew.
On top of all that, an additional constraint: libstdc++ did not attempt to version symbols (or age gracefully, really) until version 3.1.0.
Most modern Linux and BSD versions, particularly ones using gcc-3.1.x tools and more recent vintages, will meet the requirements above.
It turns out that most of the configure options that change default behavior will impact the mangled names of exported symbols, and thus impact versioning and compatibility.
For more information on configure options, including ABI impacts, see: http://gcc.gnu.org/onlinedocs/libstdc++/configopts.html
There is one flag that explicitly deals with symbol versioning: --enable-symvers.
In particular, libstdc++-v3/acinclude.m4 has a macro called GLIBCXX_ENABLE_SYMVERS that defaults to yes (or the argument passed in via --enable-symvers=foo). At that point, the macro attempts to make sure that all the requirement for symbol versioning are in place. For more information, please consult acinclude.m4.
When the GNU C++ library is being built with symbol versioning on, you should see the following at configure time for libstdc++-v3:
checking versioning on shared library symbols... gnu
If you don't see this line in the configure output, or if this line appears but the last word is 'no', then you are out of luck.
If the compiler is pre-installed, a quick way to test is to compile the following (or any) simple C++ file and link it to the shared libstdc++ library:
#include <iostream>
int main()
{ std::cout << "hello" << std::endl; return 0; }
%g++ hello.cc -o hello.out
%ldd hello.out
libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0x00764000)
libm.so.6 => /lib/tls/libm.so.6 (0x004a8000)
libgcc_s.so.1 => /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x40016000)
libc.so.6 => /lib/tls/libc.so.6 (0x0036d000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x00355000)
%nm hello.out
If you see symbols in the resulting output with "GLIBCXX_3" as part of the name, then the executable is versioned. Here's an example:
U _ZNSt8ios_base4InitC1Ev@@GLIBCXX_3.4
The following will cause the library minor version number to increase, say from "libstdc++.so.3.0.4" to "libstdc++.so.3.0.5".
Other allowed changes are possible.
The following non-exhaustive list will cause the library major version number to increase, say from "libstdc++.so.3.0.4" to "libstdc++.so.4.0.0".
This is accomplished by two techniques that separate the API from the ABI: forcing undefined references to link against a library binary for definitions.
class
locale, the appropriate standard C++ include, say
locale, can contain full declarations, while various
source files (say locale.cc, locale_init.cc,
localename.cc) contain definitions. extern template
can be used to control where template definitions
reside. By marking required instantiations as extern
template in include files, and providing explicit
instantiations in the appropriate instantiation files, non-inlined
template functions can be versioned. This technique is mostly used
on parts of the standard that require char and
wchar_t instantiations, and includes
basic_string, the locale facets, and the types in
iostreams.In addition, these techniques have the additional benefit that they reduce binary size, which can increase runtime performance.
All symbols in the shared library binary are processed by a linker script at build time that either allows or disallows external linkage. Because of this, some symbols, regardless of normal C/C++ linkage, are not visible. Symbols that are internal have several appealing characteristics: by not exporting the symbols, there are no relocations when the shared library is started and thus this makes for faster runtime loading performance by the underlying dynamic loading mechanism. In addition, they have the possibility of changing without impacting ABI compatibility.
The following namespaces are transformed by the mapfile:
namespace stdGLIBCXX that do not begin with an underscore, ie
__test_func would not be exported by default. Select
exceptional symbols are allowed to be visible.namespace __gnu_cxxGLIBCXX, select items are allowed to be visible.namespace __gnu_internalnamespace __cxxabiv1, aliased to namespace abiCXXABI, select items are allowed to be visible.
Disallowed changes, as above, are not made on a stable release branch. Enforcement tends to be less strict with GNU extensions that standard includes.
Testing for GNU C++ ABI changes is composed of two distinct areas: testing the C++ compiler (g++) for compiler changes, and testing the C++ library (libstdc++) for library changes.
Testing the C++ compiler ABI can be done various ways.
One. Intel ABI checker. More information can be obtained here.
Two. The second is yet unreleased, but has been announced on the gcc mailing list. It is yet unspecified if these tools will be freely available, and able to be included in a GNU project. Please contact Mark Mitchell (mark@codesourcery.com) for more details, and current status.
Three. Involves using the vlad.consistency test framework. This has also been discussed on the gcc mailing lists.
Testing the C++ library ABI can also be done various ways.
One. (Brendan Kehoe, Jeff Law suggestion to run 'make check-c++' two ways, one with a new compiler and an old library, and the other with an old compiler and a new library, and look for testsuite regressions)
Details on how to set this kind of test up can be found here: http://gcc.gnu.org/ml/gcc/2002-08/msg00142.html
Two. Use the 'make check-abi' rule in the libstdc++-v3 Makefile.
This is a proactive check the library ABI. Currently, exported symbol names that are either weak or defined are checked against a last known good baseline. Currently, this baseline is keyed off of 3.4.0 binaries, as this was the last time the .so number was incremented. In addition, all exported names are demangled, and the exported objects are checked to make sure they are the same size as the same object in the baseline. Notice that each baseline is relative to a default configured library and compiler: in particular, if options such as --enable-clocale, or --with-cpu, in case of multilibs, are used at configure time, the check may fail, either because of substantive differences or because of limitations of the current checking machinery.
This dataset is insufficient, yet a start. Also needed is a comprehensive check for all user-visible types part of the standard library for sizeof() and alignof() changes.
Verifying compatible layouts of objects is not even attempted. It should be possible to use sizeof, alignof, and offsetof to compute offsets for each structure and type in the standard library, saving to another datafile. Then, compute this in a similar way for new binaries, and look for differences.
Another approach might be to use the -fdump-class-hierarchy flag to get information. However, currently this approach gives insufficient data for use in library testing, as class data members, their offsets, and other detailed data is not displayed with this flag. (See g++/7470 on how this was used to find bugs.)
Perhaps there are other C++ ABI checkers. If so, please notify us. We'd like to know about them!
A "C" application, dynamically linked to two shared libraries, liba, libb. The dependent library liba is C++ shared library compiled with gcc-3.3.x, and uses io, exceptions, locale, etc. The dependent library libb is a C++ shared library compiled with gcc-3.4.x, and also uses io, exceptions, locale, etc.
As above, libone is constructed as follows:
%$bld/H-x86-gcc-3.4.0/bin/g++ -fPIC -DPIC -c a.cc %$bld/H-x86-gcc-3.4.0/bin/g++ -shared -Wl,-soname -Wl,libone.so.1 -Wl,-O1 -Wl,-z,defs a.o -o libone.so.1.0.0 %ln -s libone.so.1.0.0 libone.so %$bld/H-x86-gcc-3.4.0/bin/g++ -c a.cc %ar cru libone.a a.o
And, libtwo is constructed as follows:
%$bld/H-x86-gcc-3.3.3/bin/g++ -fPIC -DPIC -c b.cc %$bld/H-x86-gcc-3.3.3/bin/g++ -shared -Wl,-soname -Wl,libtwo.so.1 -Wl,-O1 -Wl,-z,defs b.o -o libtwo.so.1.0.0 %ln -s libtwo.so.1.0.0 libtwo.so %$bld/H-x86-gcc-3.3.3/bin/g++ -c b.cc %ar cru libtwo.a b.o
...with the resulting libraries looking like
%ldd libone.so.1.0.0
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x40016000)
libm.so.6 => /lib/tls/libm.so.6 (0x400fa000)
libgcc_s.so.1 => /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x4011c000)
libc.so.6 => /lib/tls/libc.so.6 (0x40125000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x00355000)
%ldd libtwo.so.1.0.0
libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0x40027000)
libm.so.6 => /lib/tls/libm.so.6 (0x400e1000)
libgcc_s.so.1 => /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x40103000)
libc.so.6 => /lib/tls/libc.so.6 (0x4010c000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x00355000)
Then, the "C" compiler is used to compile a source file that uses functions from each library.
gcc test.c -g -O2 -L. -lone -ltwo /usr/lib/libstdc++.so.5 /usr/lib/libstdc++.so.6
Which gives the expected:
%ldd a.out
libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0x00764000)
libstdc++.so.6 => /usr/lib/libstdc++.so.6 (0x40015000)
libc.so.6 => /lib/tls/libc.so.6 (0x0036d000)
libm.so.6 => /lib/tls/libm.so.6 (0x004a8000)
libgcc_s.so.1 => /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x400e5000)
/lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x00355000)
This resulting binary, when executed, will be able to safely use code from both liba, and the dependent libstdc++.so.6, and libb, with the dependent libstdc++.so.5.
ABIcheck, a vague idea of checking ABI compatibility
http://abicheck.sourceforge.net/
C++ ABI reference
http://www.codesourcery.com/cxx-abi/
Intel ABI documentation, "Intel® Compilers for Linux* -Compatibility with the GNU Compilers"
http://developer.intel.com/software/products/compilers/techtopics/LinuxCompilersCompatibility.htm
Sun Solaris 2.9 docs
Linker and Libraries Guide (document 816-1386)
C++ Migration Guide (document 816-2459)
http://docs.sun.com/db/prod/solaris.9
http://docs.sun.com/?p=/doc/816-1386&a=load
Ulrich Drepper, "ELF Symbol Versioning"
http://people.redhat.com/drepper/symbol-versioning