C++ Standard Library ABI

The latest version of this document is always available at http://gcc.gnu.org/onlinedocs/libstdc++/abi.html.

The C++ interface

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.

Versioning

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.

Goals of versioning

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.

Version History

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:
- gcc-3.0.0: libgcc_s.so.1
- gcc-3.0.1: libgcc_s.so.1
- gcc-3.0.2: libgcc_s.so.1
- gcc-3.0.3: libgcc_s.so.1
- gcc-3.0.4: libgcc_s.so.1
- gcc-3.1.0: libgcc_s.so.1
- gcc-3.1.1: libgcc_s.so.1
- gcc-3.2.0: libgcc_s.so.1
- gcc-3.2.1: libgcc_s.so.1
- gcc-3.2.2: libgcc_s.so.1
- gcc-3.2.3: libgcc_s.so.1
- gcc-3.3.0: libgcc_s.so.1
- gcc-3.3.1: libgcc_s.so.1
- gcc-3.3.2: libgcc_s.so.1
- gcc-3.3.3: libgcc_s.so.1
- gcc-3.4.x, gcc-4.0.x, gcc-4.1.x: on m68k-linux and hppa-linux this is either libgcc_s.so.1 (when configuring --with-sjlj-exceptions) or libgcc_s.so.2. For all others, this is libgcc_s.so.1.
Symbol versioning on the libgcc_s.so binary.
mapfile: gcc/libgcc-std.ver

It is versioned with the following labels and version definitions, where the version definition is the maximum for a particular release. Labels are cumulative. If a particular release is not listed, it has the same version labels as the preceeding release.
- gcc-3.0.0: GCC_3.0
- gcc-3.3.0: GCC_3.3
- gcc-3.3.1: GCC_3.3.1
- gcc-3.3.2: GCC_3.3.2
- gcc-3.3.4: GCC_3.3.4
- gcc-3.4.0: GCC_3.4
- gcc-3.4.2: GCC_3.4.2
- gcc-3.4.4: GCC_3.4.4
- gcc-4.0.0: GCC_4.0.0
- gcc-4.1.0: GCC_4.1.0
Release versioning on the libstdc++.so binary, implemented in the same was as the libgcc_s.so binary, above.
It is versioned as follows:
- gcc-3.0.0: libstdc++.so.3.0.0
- gcc-3.0.1: libstdc++.so.3.0.1
- gcc-3.0.2: libstdc++.so.3.0.2
- gcc-3.0.3: libstdc++.so.3.0.2 (Error should be libstdc++.so.3.0.3)
- gcc-3.0.4: libstdc++.so.3.0.4
- gcc-3.1.0: libstdc++.so.4.0.0
- gcc-3.1.1: libstdc++.so.4.0.1
- gcc-3.2.0: libstdc++.so.5.0.0
- gcc-3.2.1: libstdc++.so.5.0.1
- gcc-3.2.2: libstdc++.so.5.0.2
- gcc-3.2.3: libstdc++.so.5.0.3 (Not strictly required)
- gcc-3.3.0: libstdc++.so.5.0.4
- gcc-3.3.1: libstdc++.so.5.0.5
- gcc-3.3.2: libstdc++.so.5.0.5
- gcc-3.3.3: libstdc++.so.5.0.5
- gcc-3.4.0: libstdc++.so.6.0.0
- gcc-3.4.1: libstdc++.so.6.0.1
- gcc-3.4.2: libstdc++.so.6.0.2
- gcc-3.4.3: libstdc++.so.6.0.3
- gcc-3.4.4: libstdc++.so.6.0.3
- gcc-3.4.5: libstdc++.so.6.0.3
- gcc-3.4.6: libstdc++.so.6.0.3
- gcc-4.0.0: libstdc++.so.6.0.4
- gcc-4.0.1: libstdc++.so.6.0.5
- gcc-4.0.2: libstdc++.so.6.0.6
- gcc-4.0.3: libstdc++.so.6.0.7
- gcc-4.1.0: libstdc++.so.6.0.7
- gcc-4.1.1: libstdc++.so.6.0.8
Symbol versioning on the libstdc++.so binary.
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.) If a particular release is not listed, it has the same version labels as the preceeding release.
- gcc-3.0.0: (Error, not versioned)
- gcc-3.0.1: (Error, not versioned)
- gcc-3.0.2: (Error, not versioned)
- gcc-3.0.3: (Error, not versioned)
- gcc-3.0.4: (Error, not versioned)
- gcc-3.1.0: GLIBCPP_3.1, CXXABI_1
- gcc-3.1.1: GLIBCPP_3.1, CXXABI_1
- gcc-3.2.0: GLIBCPP_3.2, CXXABI_1.2
- gcc-3.2.1: GLIBCPP_3.2.1, CXXABI_1.2
- gcc-3.2.2: GLIBCPP_3.2.2, CXXABI_1.2
- gcc-3.2.3: GLIBCPP_3.2.2, CXXABI_1.2
- gcc-3.3.0: GLIBCPP_3.2.2, CXXABI_1.2.1
- gcc-3.3.1: GLIBCPP_3.2.3, CXXABI_1.2.1
- gcc-3.3.2: GLIBCPP_3.2.3, CXXABI_1.2.1
- gcc-3.3.3: GLIBCPP_3.2.3, CXXABI_1.2.1
- gcc-3.4.0: GLIBCXX_3.4, CXXABI_1.3
- gcc-3.4.1: GLIBCXX_3.4.1, CXXABI_1.3
- gcc-3.4.2: GLIBCXX_3.4.2
- gcc-3.4.3: GLIBCXX_3.4.3
- gcc-4.0.0: GLIBCXX_3.4.4, CXXABI_1.3.1
- gcc-4.0.1: GLIBCXX_3.4.5
- gcc-4.0.2: GLIBCXX_3.4.6
- gcc-4.0.3: GLIBCXX_3.4.7
- gcc-4.1.1: GLIBCXX_3.4.8
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':
- gcc-3.0.x: 100
- gcc-3.1.x: 100 (Error, should be 101)
- gcc-3.2.x: 102
- gcc-3.3.x: 102
- gcc-3.4.x, gcc-4.0.x, gcc-4.1.x: 102 (when n=1)
- gcc-3.4.x, gcc-4.0.x, gcc-4.1.x: 1000 + n (when n>1)
- gcc-3.4.x, gcc-4.0.x, gcc-4.1.x: 999999 (when n=0)
Changes to the default compiler option for -fabi-version.

It is versioned as follows:
- gcc-3.0.x: (Error, not versioned)
- gcc-3.1.x: (Error, not versioned)
- gcc-3.2.x: -fabi-version=1
- gcc-3.3.x: -fabi-version=1
- gcc-3.4.x, gcc-4.0.x, gcc-4.1.x: -fabi-version=2
Incremental 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.

This macro is defined in the file "c++config" in the "libstdc++-v3/include/bits" directory. (Up to gcc-4.1.0, it was changed every night by an automated script. Since gcc-4.1.0, it is the same value as gcc/DATESTAMP.)

It is versioned as follows:
- gcc-3.0.0: 20010615
- gcc-3.0.1: 20010819
- gcc-3.0.2: 20011023
- gcc-3.0.3: 20011220
- gcc-3.0.4: 20020220
- gcc-3.1.0: 20020514
- gcc-3.1.1: 20020725
- gcc-3.2.0: 20020814
- gcc-3.2.1: 20021119
- gcc-3.2.2: 20030205
- gcc-3.2.3: 20030422
- gcc-3.3.0: 20030513
- gcc-3.3.1: 20030804
- gcc-3.3.2: 20031016
- gcc-3.3.3: 20040214
- gcc-3.4.0: 20040419
- gcc-3.4.1: 20040701
- gcc-3.4.2: 20040906
- gcc-3.4.3: 20041105
- gcc-3.4.4: 20050519
- gcc-3.4.5: 20051201
- gcc-3.4.6: 20060306
- gcc-4.0.0: 20050421
- gcc-4.0.1: 20050707
- gcc-4.0.2: 20050921
- gcc-4.0.3: 20060309
- gcc-4.1.0: 20060228
- gcc-4.1.1: 20060524
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:
- gcc-3.0.0: "3.0.0"
- gcc-3.0.1: "3.0.0" (Error, should be "3.0.1")
- gcc-3.0.2: "3.0.0" (Error, should be "3.0.2")
- gcc-3.0.3: "3.0.0" (Error, should be "3.0.3")
- gcc-3.0.4: "3.0.0" (Error, should be "3.0.4")
- gcc-3.1.0: "3.1.0"
- gcc-3.1.1: "3.1.1"
- gcc-3.2.0: "3.2"
- gcc-3.2.1: "3.2.1"
- gcc-3.2.2: "3.2.2"
- gcc-3.2.3: "3.2.3"
- gcc-3.3.0: "3.3"
- gcc-3.3.1: "3.3.1"
- gcc-3.3.2: "3.3.2"
- gcc-3.3.3: "3.3.3"
- gcc-3.4.x: "version-unused"
- gcc-4.0.x: "version-unused"
- gcc-4.1.x: "version-unused"
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:
- gcc-3.0.0: include/g++-v3
- gcc-3.0.1: include/g++-v3
- gcc-3.0.2: include/g++-v3
- gcc-3.0.3: include/g++-v3
- gcc-3.0.4: include/g++-v3
- gcc-3.1.0: include/g++-v3
- gcc-3.1.1: include/c++/3.1.1
- gcc-3.2.0: include/c++/3.2
- gcc-3.2.1: include/c++/3.2.1
- gcc-3.2.2: include/c++/3.2.2
- gcc-3.2.3: include/c++/3.2.3
- gcc-3.3.0: include/c++/3.3
- gcc-3.3.1: include/c++/3.3.1
- gcc-3.3.2: include/c++/3.3.2
- gcc-3.3.3: include/c++/3.3.3
- gcc-3.4.0: include/c++/3.4.0
- gcc-3.4.1: include/c++/3.4.1
- gcc-3.4.2: include/c++/3.4.2
- gcc-3.4.3: include/c++/3.4.3
- gcc-3.4.4: include/c++/3.4.4
- gcc-3.4.5: include/c++/3.4.5
- gcc-3.4.6: include/c++/3.4.6
- gcc-4.0.0: include/c++/4.0.0
- gcc-4.0.1: include/c++/4.0.1
- gcc-4.0.2: include/c++/4.0.2
- gcc-4.0.3: include/c++/4.0.3
- gcc-4.1.0: include/c++/4.1.0
- gcc-4.1.1: include/c++/4.1.1

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 requirements for a versioned ABI

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.

What configure options impact symbol versioning?

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.

How to tell if symbol versioning is, indeed, active?

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

Library allowed ABI changes

The following will cause the library minor version number to increase, say from "libstdc++.so.3.0.4" to "libstdc++.so.3.0.5".

adding an exported global or static data member
adding an exported function, static or non-virtual member function
adding an exported symbol or symbols by additional instantiations

Other allowed changes are possible.

Library disallowed ABI changes

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".

changes in the gcc/g++ compiler ABI
changing size of an exported symbol
changing alignment of an exported symbol
changing the layout of an exported symbol
changing mangling on an exported symbol
deleting an exported symbol
changing the inheritance properties of a type by adding or removing base classes
changing the size, alignment, or layout of types specified in the C++ standard. These may not necessarily be instantiated or otherwise exported in the library binary, and include all the required locale facets, as well as things like std::basic_streambuf, et al.
adding an explicit copy constructor or destructor to a class that would otherwise have implicit versions. This will change the way the compiler deals with this class in by-value return statements or parameters: instead of being passing instances of this class in registers, the compiler will be forced to use memory. See this part of the C++ ABI documentation for further details.

Library implementation strategy

Separation of interface and implementation
This is accomplished by two techniques that separate the API from the ABI: forcing undefined references to link against a library binary for definitions.

Include files have declarations, source files have defines

For non-templatized types, such as much of 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 on required types

For parts of the standard that have an explicit list of required instantiations, the GNU extension syntax 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.
Namespaces linking symbol definitions to export mapfiles
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 std

Defaults to exporting all symbols in label GLIBCXX 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_cxx

Defaults to not exporting any symbols in label GLIBCXX, select items are allowed to be visible.

namespace __gnu_internal

Defaults to not exported, no items are allowed to be visible.

namespace __cxxabiv1, aliased to namespace abi

Defaults to not exporting any symbols in label CXXABI, select items are allowed to be visible.
Freezing the API
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 ABI changes

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!

Testing Multi-ABI binaries

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.

Outstanding Issues

Some features in the C++ language make versioning especially difficult. In particular, compiler generated constructs such as implicit instantiations for templates, typeinfo information, and virtual tables all may cause ABI leakage across shared library boundaries. Because of this, mixing C++ ABI's is not recommended at this time.

For more background on this issue, see these bugzilla entries:

24660: versioning weak symbols in libstdc++

19664: libstdc++ headers should have pop/push of the visibility around the declarations

Bibliography / Further Reading

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

C++ ABI for the ARM Architecture
http://www.arm.com/miscPDFs/8033.pdf

Benjamin Kosnik, ISO C++ J16/06-0046
Dynamic Shared Objects: Survey and Issues

Benjamin Kosnik, ISO C++ J16/06-0083
Versioning With Namespaces