ABI Policy and Guidelines

ABI Policy and Guidelines
Prev	Appendix B. + Porting and Maintenance + +	Next

The C++ Interface

+ C++ applications often depend on specific language support + routines, say for throwing exceptions, or catching exceptions, and + perhaps also depend 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 (i.e., g++ and libstdc++) that + implements the C++ ABI in question. +

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: +
For m68k-linux the versions differ as follows:
- gcc-3.4.x, gcc-4.[0-5].x: libgcc_s.so.1 + when configuring --with-sjlj-exceptions, or + libgcc_s.so.2
For hppa-linux the versions differ as follows:
- gcc-3.4.x, gcc-4.[0-1].x: either libgcc_s.so.1 + when configuring --with-sjlj-exceptions, or + libgcc_s.so.2
- gcc-4.[2-5].x: either libgcc_s.so.3 when configuring + --with-sjlj-exceptions) or libgcc_s.so.4 +
Symbol versioning on the libgcc_s.so binary.
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 preceding + release.
This corresponds to the mapfile: gcc/libgcc-std.ver
+ Release versioning on the libstdc++.so binary, implemented in + the same way as the libgcc_s.so binary above. Listed is the + filename: DT_SONAME can be deduced from + the filename by removing the last two period-delimited numbers. For + example, filename libstdc++.so.5.0.4 + corresponds to a DT_SONAME of + libstdc++.so.5. Binaries with equivalent + DT_SONAMEs are forward-compatibile: in + the table below, releases incompatible with the previous + one are explicitly noted. +
It is versioned as follows: +
+ Note 1: Error should be libstdc++.so.3.0.3. +
+ Note 2: Not strictly required. +
+ Note 3: This release (but not previous or subsequent) has one + known incompatibility, see 33678 + in the GCC bug database. +
Symbol versioning on the libstdc++.so binary.
mapfile: libstdc++-v3/config/abi/pre/gnu.ver
It is versioned with the following labels and version + definitions, where the version definition is the maximum for a + particular release. Note, only symbols 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 preceding 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
- gcc-4.2.0: GLIBCXX_3.4.9
- gcc-4.3.0: GLIBCXX_3.4.10, CXXABI_1.3.2
- gcc-4.4.0: GLIBCXX_3.4.11, CXXABI_1.3.3
- gcc-4.4.1: GLIBCXX_3.4.12, CXXABI_1.3.3
- gcc-4.4.2: GLIBCXX_3.4.13, CXXABI_1.3.3
- gcc-4.5.0: GLIBCXX_3.4.14, CXXABI_1.3.4
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.[0-5].x: 102 (when n=1)
- gcc-3.4.x, gcc-4.[0-5].x: 1000 + n (when n>1)
- gcc-3.4.x, gcc-4.[0-5].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.[0-5].x: -fabi-version=2 (Incompatible with previous)
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
- gcc-4.1.2: 20070214
- gcc-4.2.0: 20070514
- gcc-4.2.1: 20070719
- gcc-4.2.2: 20071007
- gcc-4.2.3: 20080201
- gcc-4.2.4: 20080519
- gcc-4.3.0: 20080306
- gcc-4.3.1: 20080606
- gcc-4.3.2: 20080827
- gcc-4.3.3: 20090124
- gcc-4.4.0: 20090421
- gcc-4.4.1: 20090722
- gcc-4.4.2: 20091015
+ 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.[0-5].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
- gcc-4.1.2: include/c++/4.1.2
- gcc-4.2.0: include/c++/4.2.0
- gcc-4.2.1: include/c++/4.2.1
- gcc-4.2.2: include/c++/4.2.2
- gcc-4.2.3: include/c++/4.2.3
- gcc-4.2.4: include/c++/4.2.4
- gcc-4.3.0: include/c++/4.3.0
- gcc-4.3.1: include/c++/4.3.1
- gcc-4.3.3: include/c++/4.3.3
- gcc-4.3.4: include/c++/4.3.4
- gcc-4.4.0: include/c++/4.4.0
- gcc-4.4.1: include/c++/4.4.1
- gcc-4.4.2: include/c++/4.4.2
- gcc-4.5.0: include/c++/4.5.0

+ 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. +

Prerequisites

+ 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 Linux and BSD versions, particularly ones using + gcc-3.1.x tools and more recent vintages, will meet the + requirements above, as does Solaris 2.5 and up. +

Configuring

+ 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. +

Checking Active

+ When the GNU C++ library is being built with symbol versioning + on, you should see the following at configure time for + libstdc++: +

+
+  checking versioning on shared library symbols... gnu
+
+

+ or another of the supported styles. + 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. +

Allowed 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. +

Prohibited 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 passing instances of this +class in registers, the compiler will be forced to use memory. See the +section on Function +Calling Conventions and APIs + of the C++ ABI documentation for further details. +

Implementation

+ 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, i.e., +__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

Single ABI Testing

+ 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! +

Multiple ABI Testing

+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.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++ ABIs 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

+ . + ABIcheck, a vague idea of checking ABI compatibility + .

+ . + C++ ABI Reference + .

+ . + Intel Compilers for Linux Compatibility with the GNU Compilers + .

+ . + Linker and Libraries Guide (document 819-0690) + .

+ . + Sun Studio 11: C++ Migration Guide (document 819-3689) + .

+ . + How to Write Shared Libraries + . Ulrich Drepper.

+ . + C++ ABI for the ARM Architecture + .

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

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

+ . + Binary Compatibility of Shared Libraries Implemented in C++ + on GNU/Linux Systems + . + SYRCoSE 2009 + . Pavel Shved. Denis Silakov.

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