Extending existing, stable ABIs. Versioning gives subsequent releases of library binaries the ability to add new symbols and add functionality, all the while retaining compatibility with the previous releases in the series. Thus, program binaries linked with the initial release of a library binary will still run correctly if the library binary is replaced by carefully-managed subsequent library binaries. This is called forward compatibility.

The reverse (backwards compatibility) is not true. It is not possible to take program binaries linked with the latest version of a library binary in a release series (with additional symbols added), substitute in the initial release of the library binary, and 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:

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) or the Sun linker, a shared executable compiled with g++, and shared libraries (libgcc_s, libstdc++) 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 GNU/Linux and BSD versions, particularly ones using GCC 3.1 and later, will meet the requirements above, as does Solaris 2.5 and up.

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: here

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++ (showing either 'gnu' or another of the supported styles):

  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

On Solaris 2, you can use pvs -r instead:

%g++ hello.cc -o hello.out

%pvs -r hello.out
        libstdc++.so.6 (GLIBCXX_3.4, GLIBCXX_3.4.12);
        libgcc_s.so.1 (GCC_3.0);
        libc.so.1 (SUNWprivate_1.1, SYSVABI_1.3);

ldd -v works too, but is very verbose.

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.

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++ Makefile.

This is a proactive check of 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 PR 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 a C++ shared library compiled with GCC 3.3, and uses io, exceptions, locale, etc. The dependent library libb is a C++ shared library compiled with GCC 3.4, 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.