+
+ 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: +
+ â + library API + compiler ABI = library ABI + â +
+ 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. +
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 +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 link 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: +
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.[0-3].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.
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
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
gcc-4.2.0: GCC_4.2.0
gcc-4.3.0: GCC_4.3.0
+ Release versioning on the libstdc++.so binary, implemented in
+ the same was 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: +
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 (See Note 1)
gcc-3.0.4: libstdc++.so.3.0.4
gcc-3.1.0: libstdc++.so.4.0.0 (Incompatible with previous)
gcc-3.1.1: libstdc++.so.4.0.1
gcc-3.2.0: libstdc++.so.5.0.0 (Incompatible with previous)
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 (See Note 2)
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 (Incompatible with previous)
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
gcc-4.1.2: libstdc++.so.6.0.8
gcc-4.2.0: libstdc++.so.6.0.9
gcc-4.2.1: libstdc++.so.6.0.9 (See Note 3)
gcc-4.2.2: libstdc++.so.6.0.9
gcc-4.2.3: libstdc++.so.6.0.9
gcc-4.2.4: libstdc++.so.6.0.9
gcc-4.3.0: libstdc++.so.6.0.10
gcc-4.3.1: libstdc++.so.6.0.10
gcc-4.3.2: libstdc++.so.6.0.10
+ 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++/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 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
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.[0-3].x: 102 (when n=1)
gcc-3.4.x, gcc-4.[0-3].x: 1000 + n (when n>1)
gcc-3.4.x, gcc-4.[0-3].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.[0-3].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++/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
+ 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++/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.[0-3].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++/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-4.3.2: include/c++/4.3.2
+ 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++) 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++/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++: +
+
+ 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". +
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. +
+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. +
+ 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. +
+ 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.
+
+ 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_internalDefaults 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 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++ 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. +
+ 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 +
+ ABIcheck, a vague idea of checking ABI compatibility + . + + + .
+ C++ ABI Reference + . + + + .
+ Intel® Compilers for Linux* -Compatibility with the GNU Compilers + . + + + .
+ Intel® Compilers for Linux* -Compatibility with the GNU Compilers + . + + + .
+ Sun Solaris 2.9 : Linker and Libraries Guide (document 816-1386) + . + + + .
+ Sun Solaris 2.9 : C++ Migration Guide (document 816-2459) + . + + + .
+ ELF Symbol Versioning + . + + + .
+ C++ ABI for the ARM Architecture + . + + + .
+ Dynamic Shared Objects: Survey and Issues + . + ISO C++ J16/06-0046 + . + + + .
+ Versioning With Namespaces + . + ISO C++ J16/06-0083 + . + + + .