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  Porting and Maintenance
  
</th><td width="20%" align="right"> <a accesskey="n" href="api.html">Next</a></td></tr></table><hr /></div><div class="sect1" lang="en" xml:lang="en"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="appendix.porting.abi"></a>ABI Policy and Guidelines</h2></div></div></div><p>
</p><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="abi.cxx_interface"></a>The C++ Interface</h3></div></div></div><p>
  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.
</p><p> 
  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.
</p><p> 
  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 <a class="ulink" href="http://www.codesourcery.com/cxx-abi/abi.html" target="_top"> ABI
  specification</a>.
</p><p>
 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 <code class="code">-fabi-version</code>. In addition, some
  g++ command line options may change the ABI as a side-effect of
  use. Such flags include <code class="code">-fpack-struct</code> and
  <code class="code">-fno-exceptions</code>, but include others: see the complete
  list in the GCC manual under the heading <a class="ulink" href="http://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code%20Gen%20Options" target="_top">Options
  for Code Generation Conventions</a>.
</p><p> 
  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
<a class="link" href="configure.html" title="Configure">here</a>.
</p><p> 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:
</p><p>
  “<span class="quote">
    library API + compiler ABI = library ABI
  </span>”
</p><p>
 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.
</p><p>
  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.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="abi.versioning"></a>Versioning</h3></div></div></div><p> 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.
</p><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="abi.versioning.goals"></a>Goals</h4></div></div></div><p>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.
</p><p>
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.
</p><p>Allows multiple, incompatible ABIs to coexist at the same time.
</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="abi.versioning.history"></a>History</h4></div></div></div><p>
 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.
</p><p>
  The following techniques are used:
</p><div class="orderedlist"><ol type="1"><li><p>Release versioning on the libgcc_s.so binary. </p><p>This is implemented via file names and the ELF
    <code class="constant">DT_SONAME</code> mechanism (at least on ELF
    systems). It is versioned as follows:
    </p><div class="itemizedlist"><ul type="disc"><li><p>gcc-3.0.0: libgcc_s.so.1</p></li><li><p>gcc-3.0.1: libgcc_s.so.1</p></li><li><p>gcc-3.0.2: libgcc_s.so.1</p></li><li><p>gcc-3.0.3: libgcc_s.so.1</p></li><li><p>gcc-3.0.4: libgcc_s.so.1</p></li><li><p>gcc-3.1.0: libgcc_s.so.1</p></li><li><p>gcc-3.1.1: libgcc_s.so.1</p></li><li><p>gcc-3.2.0: libgcc_s.so.1</p></li><li><p>gcc-3.2.1: libgcc_s.so.1</p></li><li><p>gcc-3.2.2: libgcc_s.so.1</p></li><li><p>gcc-3.2.3: libgcc_s.so.1</p></li><li><p>gcc-3.3.0: libgcc_s.so.1</p></li><li><p>gcc-3.3.1: libgcc_s.so.1</p></li><li><p>gcc-3.3.2: libgcc_s.so.1</p></li><li><p>gcc-3.3.3: libgcc_s.so.1</p></li><li><p>gcc-3.4.x, gcc-4.[0-3].x: on m68k-linux and
    hppa-linux this is either libgcc_s.so.1 (when configuring
    <code class="code">--with-sjlj-exceptions</code>) or libgcc_s.so.2. For all
    others, this is libgcc_s.so.1.  </p></li></ul></div></li><li><p>Symbol versioning on the libgcc_s.so binary.</p><p>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.</p><p>This corresponds to the mapfile: gcc/libgcc-std.ver</p><div class="itemizedlist"><ul type="disc"><li><p>gcc-3.0.0: GCC_3.0</p></li><li><p>gcc-3.3.0: GCC_3.3</p></li><li><p>gcc-3.3.1: GCC_3.3.1</p></li><li><p>gcc-3.3.2: GCC_3.3.2</p></li><li><p>gcc-3.3.4: GCC_3.3.4</p></li><li><p>gcc-3.4.0: GCC_3.4</p></li><li><p>gcc-3.4.2: GCC_3.4.2</p></li><li><p>gcc-3.4.4: GCC_3.4.4</p></li><li><p>gcc-4.0.0: GCC_4.0.0</p></li><li><p>gcc-4.1.0: GCC_4.1.0</p></li><li><p>gcc-4.2.0: GCC_4.2.0</p></li><li><p>gcc-4.3.0: GCC_4.3.0</p></li></ul></div></li><li><p>
	Release versioning on the libstdc++.so binary, implemented in
	the same was as the libgcc_s.so binary above. Listed is the
	filename: <code class="constant">DT_SONAME</code> can be deduced from
	the filename by removing the last two period-delimited numbers. For
	example, filename <code class="filename">libstdc++.so.5.0.4</code>
	corresponds to a <code class="constant">DT_SONAME</code> of
	<code class="constant">libstdc++.so.5</code>. Binaries with equivalent
	<code class="constant">DT_SONAME</code>s are forward-compatibile: in
	the table below, releases incompatible with the previous
	one are explicitly noted.
      </p><p>It is versioned as follows:
    </p><div class="itemizedlist"><ul type="disc"><li><p>gcc-3.0.0: libstdc++.so.3.0.0</p></li><li><p>gcc-3.0.1: libstdc++.so.3.0.1</p></li><li><p>gcc-3.0.2: libstdc++.so.3.0.2</p></li><li><p>gcc-3.0.3: libstdc++.so.3.0.2 (See Note 1)</p></li><li><p>gcc-3.0.4: libstdc++.so.3.0.4</p></li><li><p>gcc-3.1.0: libstdc++.so.4.0.0 <span class="emphasis"><em>(Incompatible with previous)</em></span></p></li><li><p>gcc-3.1.1: libstdc++.so.4.0.1</p></li><li><p>gcc-3.2.0: libstdc++.so.5.0.0 <span class="emphasis"><em>(Incompatible with previous)</em></span></p></li><li><p>gcc-3.2.1: libstdc++.so.5.0.1</p></li><li><p>gcc-3.2.2: libstdc++.so.5.0.2</p></li><li><p>gcc-3.2.3: libstdc++.so.5.0.3 (See Note 2)</p></li><li><p>gcc-3.3.0: libstdc++.so.5.0.4</p></li><li><p>gcc-3.3.1: libstdc++.so.5.0.5</p></li><li><p>gcc-3.3.2: libstdc++.so.5.0.5</p></li><li><p>gcc-3.3.3: libstdc++.so.5.0.5</p></li><li><p>gcc-3.4.0: libstdc++.so.6.0.0 <span class="emphasis"><em>(Incompatible with previous)</em></span></p></li><li><p>gcc-3.4.1: libstdc++.so.6.0.1</p></li><li><p>gcc-3.4.2: libstdc++.so.6.0.2</p></li><li><p>gcc-3.4.3: libstdc++.so.6.0.3</p></li><li><p>gcc-3.4.4: libstdc++.so.6.0.3</p></li><li><p>gcc-3.4.5: libstdc++.so.6.0.3</p></li><li><p>gcc-3.4.6: libstdc++.so.6.0.3</p></li><li><p>gcc-4.0.0: libstdc++.so.6.0.4</p></li><li><p>gcc-4.0.1: libstdc++.so.6.0.5</p></li><li><p>gcc-4.0.2: libstdc++.so.6.0.6</p></li><li><p>gcc-4.0.3: libstdc++.so.6.0.7</p></li><li><p>gcc-4.1.0: libstdc++.so.6.0.7</p></li><li><p>gcc-4.1.1: libstdc++.so.6.0.8</p></li><li><p>gcc-4.1.2: libstdc++.so.6.0.8</p></li><li><p>gcc-4.2.0: libstdc++.so.6.0.9</p></li><li><p>gcc-4.2.1: libstdc++.so.6.0.9 (See Note 3)</p></li><li><p>gcc-4.2.2: libstdc++.so.6.0.9</p></li><li><p>gcc-4.2.3: libstdc++.so.6.0.9</p></li><li><p>gcc-4.2.4: libstdc++.so.6.0.9</p></li><li><p>gcc-4.3.0: libstdc++.so.6.0.10</p></li><li><p>gcc-4.3.1: libstdc++.so.6.0.10</p></li><li><p>gcc-4.3.2: libstdc++.so.6.0.10</p></li></ul></div><p>
      Note 1: Error should be libstdc++.so.3.0.3.
    </p><p>
      Note 2: Not strictly required.
    </p><p>
      Note 3: This release (but not previous or subsequent) has one
      known incompatibility, see <a class="ulink" href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33678" target="_top">33678</a>
      in the GCC bug database.
    </p></li><li><p>Symbol versioning on the libstdc++.so binary.</p><p>mapfile: libstdc++/config/linker-map.gnu</p><p>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.
   </p><div class="itemizedlist"><ul type="disc"><li><p>gcc-3.0.0: (Error, not versioned)</p></li><li><p>gcc-3.0.1: (Error, not versioned)</p></li><li><p>gcc-3.0.2: (Error, not versioned)</p></li><li><p>gcc-3.0.3: (Error, not versioned)</p></li><li><p>gcc-3.0.4: (Error, not versioned)</p></li><li><p>gcc-3.1.0: GLIBCPP_3.1, CXXABI_1</p></li><li><p>gcc-3.1.1: GLIBCPP_3.1, CXXABI_1</p></li><li><p>gcc-3.2.0: GLIBCPP_3.2, CXXABI_1.2</p></li><li><p>gcc-3.2.1: GLIBCPP_3.2.1, CXXABI_1.2</p></li><li><p>gcc-3.2.2: GLIBCPP_3.2.2, CXXABI_1.2</p></li><li><p>gcc-3.2.3: GLIBCPP_3.2.2, CXXABI_1.2</p></li><li><p>gcc-3.3.0: GLIBCPP_3.2.2, CXXABI_1.2.1</p></li><li><p>gcc-3.3.1: GLIBCPP_3.2.3, CXXABI_1.2.1</p></li><li><p>gcc-3.3.2: GLIBCPP_3.2.3, CXXABI_1.2.1</p></li><li><p>gcc-3.3.3: GLIBCPP_3.2.3, CXXABI_1.2.1</p></li><li><p>gcc-3.4.0: GLIBCXX_3.4, CXXABI_1.3</p></li><li><p>gcc-3.4.1: GLIBCXX_3.4.1, CXXABI_1.3</p></li><li><p>gcc-3.4.2: GLIBCXX_3.4.2</p></li><li><p>gcc-3.4.3: GLIBCXX_3.4.3</p></li><li><p>gcc-4.0.0: GLIBCXX_3.4.4, CXXABI_1.3.1</p></li><li><p>gcc-4.0.1: GLIBCXX_3.4.5</p></li><li><p>gcc-4.0.2: GLIBCXX_3.4.6</p></li><li><p>gcc-4.0.3: GLIBCXX_3.4.7</p></li><li><p>gcc-4.1.1: GLIBCXX_3.4.8</p></li><li><p>gcc-4.2.0: GLIBCXX_3.4.9</p></li><li><p>gcc-4.3.0: GLIBCXX_3.4.10, CXXABI_1.3.2</p></li></ul></div></li><li><p>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.)
    </p><p>
    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.
    </p><p>
    It is versioned as follows, where 'n' is given by '-fabi-version=n':
    </p><div class="itemizedlist"><ul type="disc"><li><p>gcc-3.0.x: 100</p></li><li><p>gcc-3.1.x: 100 (Error, should be 101)</p></li><li><p>gcc-3.2.x: 102</p></li><li><p>gcc-3.3.x: 102</p></li><li><p>gcc-3.4.x, gcc-4.[0-3].x: 102 (when n=1)</p></li><li><p>gcc-3.4.x, gcc-4.[0-3].x: 1000 + n (when n&gt;1) </p></li><li><p>gcc-3.4.x, gcc-4.[0-3].x: 999999 (when n=0)</p></li></ul></div><p></p></li><li><p>Changes to the default compiler option for
    <code class="code">-fabi-version</code>.
    </p><p>
    It is versioned as follows:
    </p><div class="itemizedlist"><ul type="disc"><li><p>gcc-3.0.x: (Error, not versioned) </p></li><li><p>gcc-3.1.x: (Error, not versioned) </p></li><li><p>gcc-3.2.x: <code class="code">-fabi-version=1</code></p></li><li><p>gcc-3.3.x: <code class="code">-fabi-version=1</code></p></li><li><p>gcc-3.4.x, gcc-4.[0-3].x: <code class="code">-fabi-version=2</code> <span class="emphasis"><em>(Incompatible with previous)</em></span></p></li></ul></div><p></p></li><li><p>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.
    </p><p>
    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.)
    </p><p>
    It is versioned as follows:
    </p><div class="itemizedlist"><ul type="disc"><li><p>gcc-3.0.0: 20010615</p></li><li><p>gcc-3.0.1: 20010819</p></li><li><p>gcc-3.0.2: 20011023</p></li><li><p>gcc-3.0.3: 20011220</p></li><li><p>gcc-3.0.4: 20020220</p></li><li><p>gcc-3.1.0: 20020514</p></li><li><p>gcc-3.1.1: 20020725</p></li><li><p>gcc-3.2.0: 20020814</p></li><li><p>gcc-3.2.1: 20021119</p></li><li><p>gcc-3.2.2: 20030205</p></li><li><p>gcc-3.2.3: 20030422</p></li><li><p>gcc-3.3.0: 20030513</p></li><li><p>gcc-3.3.1: 20030804</p></li><li><p>gcc-3.3.2: 20031016</p></li><li><p>gcc-3.3.3: 20040214</p></li><li><p>gcc-3.4.0: 20040419</p></li><li><p>gcc-3.4.1: 20040701</p></li><li><p>gcc-3.4.2: 20040906</p></li><li><p>gcc-3.4.3: 20041105</p></li><li><p>gcc-3.4.4: 20050519</p></li><li><p>gcc-3.4.5: 20051201</p></li><li><p>gcc-3.4.6: 20060306</p></li><li><p>gcc-4.0.0: 20050421</p></li><li><p>gcc-4.0.1: 20050707</p></li><li><p>gcc-4.0.2: 20050921</p></li><li><p>gcc-4.0.3: 20060309</p></li><li><p>gcc-4.1.0: 20060228</p></li><li><p>gcc-4.1.1: 20060524</p></li><li><p>gcc-4.1.2: 20070214</p></li><li><p>gcc-4.2.0: 20070514</p></li><li><p>gcc-4.2.1: 20070719</p></li><li><p>gcc-4.2.2: 20071007</p></li><li><p>gcc-4.2.3: 20080201</p></li><li><p>gcc-4.2.4: 20080519</p></li><li><p>gcc-4.3.0: 20080306</p></li><li><p>gcc-4.3.1: 20080606</p></li><li><p>gcc-4.3.2: 20080827</p></li></ul></div><p></p></li><li><p>
    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).
    </p><p>
    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.
    </p><p>
    It is versioned as follows:
    </p><div class="itemizedlist"><ul type="disc"><li><p>gcc-3.0.0: "3.0.0"</p></li><li><p>gcc-3.0.1: "3.0.0" (Error, should be "3.0.1")</p></li><li><p>gcc-3.0.2: "3.0.0" (Error, should be "3.0.2")</p></li><li><p>gcc-3.0.3: "3.0.0" (Error, should be "3.0.3")</p></li><li><p>gcc-3.0.4: "3.0.0" (Error, should be "3.0.4")</p></li><li><p>gcc-3.1.0: "3.1.0"</p></li><li><p>gcc-3.1.1: "3.1.1"</p></li><li><p>gcc-3.2.0: "3.2"</p></li><li><p>gcc-3.2.1: "3.2.1"</p></li><li><p>gcc-3.2.2: "3.2.2"</p></li><li><p>gcc-3.2.3: "3.2.3"</p></li><li><p>gcc-3.3.0: "3.3"</p></li><li><p>gcc-3.3.1: "3.3.1"</p></li><li><p>gcc-3.3.2: "3.3.2"</p></li><li><p>gcc-3.3.3: "3.3.3"</p></li><li><p>gcc-3.4.x: "version-unused"</p></li><li><p>gcc-4.[0-3].x: "version-unused"</p></li></ul></div><p></p></li><li><p>
    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.
    </p><p>
    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).
    </p><p>
    C++ includes are versioned as follows:
    </p><div class="itemizedlist"><ul type="disc"><li><p>gcc-3.0.0: include/g++-v3</p></li><li><p>gcc-3.0.1: include/g++-v3</p></li><li><p>gcc-3.0.2: include/g++-v3</p></li><li><p>gcc-3.0.3: include/g++-v3</p></li><li><p>gcc-3.0.4: include/g++-v3</p></li><li><p>gcc-3.1.0: include/g++-v3</p></li><li><p>gcc-3.1.1: include/c++/3.1.1</p></li><li><p>gcc-3.2.0: include/c++/3.2</p></li><li><p>gcc-3.2.1: include/c++/3.2.1</p></li><li><p>gcc-3.2.2: include/c++/3.2.2</p></li><li><p>gcc-3.2.3: include/c++/3.2.3</p></li><li><p>gcc-3.3.0: include/c++/3.3</p></li><li><p>gcc-3.3.1: include/c++/3.3.1</p></li><li><p>gcc-3.3.2: include/c++/3.3.2</p></li><li><p>gcc-3.3.3: include/c++/3.3.3</p></li><li><p>gcc-3.4.0: include/c++/3.4.0</p></li><li><p>gcc-3.4.1: include/c++/3.4.1</p></li><li><p>gcc-3.4.2: include/c++/3.4.2</p></li><li><p>gcc-3.4.3: include/c++/3.4.3</p></li><li><p>gcc-3.4.4: include/c++/3.4.4</p></li><li><p>gcc-3.4.5: include/c++/3.4.5</p></li><li><p>gcc-3.4.6: include/c++/3.4.6</p></li><li><p>gcc-4.0.0: include/c++/4.0.0</p></li><li><p>gcc-4.0.1: include/c++/4.0.1</p></li><li><p>gcc-4.0.2: include/c++/4.0.2</p></li><li><p>gcc-4.0.3: include/c++/4.0.3</p></li><li><p>gcc-4.1.0: include/c++/4.1.0</p></li><li><p>gcc-4.1.1: include/c++/4.1.1</p></li><li><p>gcc-4.1.2: include/c++/4.1.2</p></li><li><p>gcc-4.2.0: include/c++/4.2.0</p></li><li><p>gcc-4.2.1: include/c++/4.2.1</p></li><li><p>gcc-4.2.2: include/c++/4.2.2</p></li><li><p>gcc-4.2.3: include/c++/4.2.3</p></li><li><p>gcc-4.2.4: include/c++/4.2.4</p></li><li><p>gcc-4.3.0: include/c++/4.3.0</p></li><li><p>gcc-4.3.1: include/c++/4.3.1</p></li><li><p>gcc-4.3.2: include/c++/4.3.2</p></li></ul></div><p></p></li></ol></div><p>
  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.
</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="abi.versioning.prereq"></a>Prerequisites</h4></div></div></div><p>
      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.
    </p><p>
      On top of all that, an additional constraint: libstdc++ did not
      attempt to version symbols (or age gracefully, really) until
      version 3.1.0.
    </p><p>
      Most modern Linux and BSD versions, particularly ones using
      gcc-3.1.x tools and more recent vintages, will meet the
      requirements above.
    </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="abi.versioning.config"></a>Configuring</h4></div></div></div><p>
      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.
    </p><p>
      For more information on configure options, including ABI
      impacts, see:
      http://gcc.gnu.org/onlinedocs/libstdc++/configopts.html
    </p><p>
      There is one flag that explicitly deals with symbol versioning:
      --enable-symvers.
    </p><p>
      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.
    </p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="abi.versioning.active"></a>Checking Active</h4></div></div></div><p>
      When the GNU C++ library is being built with symbol versioning
      on, you should see the following at configure time for
      libstdc++:
    </p><pre class="screen">
<code class="computeroutput">
  checking versioning on shared library symbols... gnu
</code>
</pre><p>
  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.
</p><p>
  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:
</p><pre class="programlisting">
#include &lt;iostream&gt;

int main()
{ std::cout &lt;&lt; "hello" &lt;&lt; std::endl; return 0; }

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

%ldd hello.out
        libstdc++.so.5 =&gt; /usr/lib/libstdc++.so.5 (0x00764000)
        libm.so.6 =&gt; /lib/tls/libm.so.6 (0x004a8000)
        libgcc_s.so.1 =&gt; /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x40016000)
        libc.so.6 =&gt; /lib/tls/libc.so.6 (0x0036d000)
        /lib/ld-linux.so.2 =&gt; /lib/ld-linux.so.2 (0x00355000)

%nm hello.out
</pre><p>
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:
</p><p>
   <code class="code">U _ZNSt8ios_base4InitC1Ev@@GLIBCXX_3.4</code>
</p></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="abi.changes_allowed"></a>Allowed Changes</h3></div></div></div><p>
The following will cause the library minor version number to
increase, say from "libstdc++.so.3.0.4" to "libstdc++.so.3.0.5".
</p><div class="orderedlist"><ol type="1"><li><p>Adding an exported global or static data member</p></li><li><p>Adding an exported function, static or non-virtual member function</p></li><li><p>Adding an exported symbol or symbols by additional instantiations</p></li></ol></div><p>
Other allowed changes are possible.
</p></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="abi.changes_no"></a>Prohibited Changes</h3></div></div></div><p>
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".
</p><div class="orderedlist"><ol type="1"><li><p>Changes in the gcc/g++ compiler ABI</p></li><li><p>Changing size of an exported symbol</p></li><li><p>Changing alignment of an exported symbol</p></li><li><p>Changing the layout of an exported symbol</p></li><li><p>Changing mangling on an exported symbol</p></li><li><p>Deleting an exported symbol</p></li><li><p>Changing the inheritance properties of a type by adding or removing
    base classes</p></li><li><p>
  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.
</p></li><li><p> 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 <a class="ulink" href="http://www.codesourcery.com/cxx-abi/abi.html#calls" target="_top"> this part</a>
 of the C++ ABI documentation for further details. 
 </p></li></ol></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="abi.impl"></a>Implementation</h3></div></div></div><div class="orderedlist"><ol type="1"><li><p>
     Separation of interface and implementation
   </p><p>
     This is accomplished by two techniques that separate the API from
     the ABI: forcing undefined references to link against a library
     binary for definitions.
   </p><div class="variablelist"><dl><dt><span class="term">Include files have declarations, source files have defines</span></dt><dd><p>
	For non-templatized types, such as much of <code class="code">class
	locale</code>, the appropriate standard C++ include, say
	<code class="code">locale</code>, can contain full declarations, while
	various source files (say <code class="code"> locale.cc, locale_init.cc,
	localename.cc</code>) contain definitions.
      </p></dd><dt><span class="term">Extern template on required types</span></dt><dd><p>
       For parts of the standard that have an explicit list of
       required instantiations, the GNU extension syntax <code class="code"> extern
       template </code> can be used to control where template
       definitions reside. By marking required instantiations as
       <code class="code"> extern template </code> 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 <code class="code">
       char</code> and <code class="code"> wchar_t</code> instantiations, and
       includes <code class="code"> basic_string</code>, the locale facets, and the
       types in <code class="code"> iostreams</code>.
     </p></dd></dl></div><p> 
   In addition, these techniques have the additional benefit that they
   reduce binary size, which can increase runtime performance.
 </p></li><li><p>
     Namespaces linking symbol definitions to export mapfiles
   </p><p>
     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.
   </p><p>The following namespaces are transformed by the mapfile:</p><div class="variablelist"><dl><dt><span class="term"><code class="code">namespace std</code></span></dt><dd><p> Defaults to exporting all symbols in label
<code class="code">GLIBCXX</code> that do not begin with an underscore, i.e.,
<code class="code">__test_func</code> would not be exported by default. Select
exceptional symbols are allowed to be visible.</p></dd><dt><span class="term"><code class="code">namespace __gnu_cxx</code></span></dt><dd><p> Defaults to not exporting any symbols in label
<code class="code">GLIBCXX</code>, select items are allowed to be visible.</p></dd><dt><span class="term"><code class="code">namespace __gnu_internal</code></span></dt><dd><p> Defaults to not exported, no items are allowed to be visible.</p></dd><dt><span class="term"><code class="code">namespace __cxxabiv1</code>, aliased to <code class="code"> namespace abi</code></span></dt><dd><p> Defaults to not exporting any symbols in label
<code class="code">CXXABI</code>, select items are allowed to be visible.</p></dd></dl></div><p>
</p></li><li><p>Freezing the API</p><p>Disallowed changes, as above, are not made on a stable release
branch. Enforcement tends to be less strict with GNU extensions that
standard includes.</p></li></ol></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="abi.testing"></a>Testing</h3></div></div></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="abi.testing.single"></a>Single ABI Testing</h4></div></div></div><p>
      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.
    </p><p>
      Testing the C++ compiler ABI can be done various ways.
    </p><p>
      One.  Intel ABI checker. More information can be obtained <a class="ulink" href="http://developer.intel.com/software/products/opensource/" target="_top">here.</a>
    </p><p>
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.
</p><p>
Three.
Involves using the vlad.consistency test framework. This has also been
discussed on the gcc mailing lists.
</p><p>
Testing the C++ library ABI can also be done various ways.
</p><p>
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)
</p><p>
Details on how to set this kind of test up can be found here:
http://gcc.gnu.org/ml/gcc/2002-08/msg00142.html
</p><p>
Two.  
Use the 'make check-abi' rule in the libstdc++ Makefile. 
</p><p>
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 <span class="emphasis"><em>default</em></span>
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.
</p><p>
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. 
</p><p>
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.
</p><p>
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.)
</p><p>
Perhaps there are other C++ ABI checkers. If so, please notify
us. We'd like to know about them!
</p></div><div class="sect3" lang="en" xml:lang="en"><div class="titlepage"><div><div><h4 class="title"><a id="abi.testing.multi"></a>Multiple ABI Testing</h4></div></div></div><p>
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.
</p><p> As above, libone is constructed as follows: </p><pre class="programlisting">
%$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 
</pre><p> And, libtwo is constructed as follows: </p><pre class="programlisting">
%$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 
</pre><p> ...with the resulting libraries looking like </p><pre class="screen">
<code class="computeroutput">
%ldd libone.so.1.0.0
        libstdc++.so.6 =&gt; /usr/lib/libstdc++.so.6 (0x40016000)
        libm.so.6 =&gt; /lib/tls/libm.so.6 (0x400fa000)
        libgcc_s.so.1 =&gt; /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x4011c000)
        libc.so.6 =&gt; /lib/tls/libc.so.6 (0x40125000)
        /lib/ld-linux.so.2 =&gt; /lib/ld-linux.so.2 (0x00355000)

%ldd libtwo.so.1.0.0
        libstdc++.so.5 =&gt; /usr/lib/libstdc++.so.5 (0x40027000)
        libm.so.6 =&gt; /lib/tls/libm.so.6 (0x400e1000)
        libgcc_s.so.1 =&gt; /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x40103000)
        libc.so.6 =&gt; /lib/tls/libc.so.6 (0x4010c000)
        /lib/ld-linux.so.2 =&gt; /lib/ld-linux.so.2 (0x00355000)
</code>
</pre><p> 
  Then, the "C" compiler is used to compile a source file that uses
  functions from each library.
</p><pre class="programlisting">
gcc test.c -g -O2 -L. -lone -ltwo /usr/lib/libstdc++.so.5 /usr/lib/libstdc++.so.6
</pre><p>
  Which gives the expected:
</p><pre class="screen">
<code class="computeroutput">
%ldd a.out
        libstdc++.so.5 =&gt; /usr/lib/libstdc++.so.5 (0x00764000)
        libstdc++.so.6 =&gt; /usr/lib/libstdc++.so.6 (0x40015000)
        libc.so.6 =&gt; /lib/tls/libc.so.6 (0x0036d000)
        libm.so.6 =&gt; /lib/tls/libm.so.6 (0x004a8000)
        libgcc_s.so.1 =&gt; /mnt/hd/bld/gcc/gcc/libgcc_s.so.1 (0x400e5000)
        /lib/ld-linux.so.2 =&gt; /lib/ld-linux.so.2 (0x00355000)
</code>
</pre><p>
  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.
</p></div></div><div class="sect2" lang="en" xml:lang="en"><div class="titlepage"><div><div><h3 class="title"><a id="abi.issues"></a>Outstanding Issues</h3></div></div></div><p> 
  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.
</p><p>
  For more background on this issue, see these bugzilla entries:
</p><p>
<a class="ulink" href="http://gcc.gnu.org/PR24660" target="_top">24660: versioning weak symbols in libstdc++</a>
</p><p>
<a class="ulink" href="http://gcc.gnu.org/PR19664" target="_top">19664: libstdc++ headers should have pop/push of the visibility around the declarations</a>
</p></div><div class="bibliography"><div class="titlepage"><div><div><h3 class="title"><a id="abi.biblio"></a>Bibliography</h3></div></div></div><div class="biblioentry"><a id="id430252"></a><p><span class="title"><i>
      ABIcheck, a vague idea of checking ABI compatibility
    </i>. </span><span class="biblioid">
      <a class="ulink" href="http://abicheck.sourceforge.net/" target="_top">
      </a>
    . </span></p></div><div class="biblioentry"><a id="id430269"></a><p><span class="title"><i>
      C++ ABI Reference
    </i>. </span><span class="biblioid">
      <a class="ulink" href="http://www.codesourcery.com/cxx-abi" target="_top">
      </a>
    . </span></p></div><div class="biblioentry"><a id="id430287"></a><p><span class="title"><i>
      Intel® Compilers for Linux* -Compatibility with the GNU Compilers
    </i>. </span><span class="biblioid">
      <a class="ulink" href="http://developer.intel.com/software/products/compilers/techtopics/LinuxCompilersCompatibility.htm" target="_top">
      </a>
    . </span></p></div><div class="biblioentry"><a id="id430304"></a><p><span class="title"><i>
      Intel® Compilers for Linux* -Compatibility with the GNU Compilers
    </i>. </span><span class="biblioid">
      <a class="ulink" href="http://developer.intel.com/software/products/compilers/techtopics/LinuxCompilersCompatibility.htm" target="_top">
      </a>
    . </span></p></div><div class="biblioentry"><a id="id430322"></a><p><span class="title"><i>
      Sun Solaris 2.9 : Linker and Libraries Guide (document 816-1386)
    </i>. </span><span class="biblioid">
      <a class="ulink" href="http://docs.sun.com/?p=/doc/816-1386&amp;a=load" target="_top">
      </a>
    . </span></p></div><div class="biblioentry"><a id="id430339"></a><p><span class="title"><i>
      Sun Solaris 2.9 : C++ Migration Guide (document 816-2459)
    </i>. </span><span class="biblioid">
      <a class="ulink" href="http://docs.sun.com/db/prod/solaris.9" target="_top">
      </a>
    . </span></p></div><div class="biblioentry"><a id="id430356"></a><p><span class="title"><i>
      ELF Symbol Versioning
    </i>. </span><span class="author"><span class="firstname">Ulrich</span> <span class="surname">Drepper</span>. </span><span class="biblioid">
      <a class="ulink" href="http://people.redhat.com/drepper/symbol-versioning" target="_top">
      </a>
    . </span></p></div><div class="biblioentry"><a id="id430385"></a><p><span class="title"><i>
      C++ ABI for the ARM Architecture
    </i>. </span><span class="biblioid">
      <a class="ulink" href="http://www.arm.com/miscPDFs/8033.pdf" target="_top">
      </a>
    . </span></p></div><div class="biblioentry"><a id="id430402"></a><p><span class="title"><i>
      Dynamic Shared Objects: Survey and Issues
    </i>. </span><span class="subtitle">
      ISO C++ J16/06-0046
    . </span><span class="author"><span class="firstname">Benjamin</span> <span class="surname">Kosnik</span>. </span><span class="biblioid">
      <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n1976.html" target="_top">
      </a>
    . </span></p></div><div class="biblioentry"><a id="id430434"></a><p><span class="title"><i>
      Versioning With Namespaces
    </i>. </span><span class="subtitle">
      ISO C++ J16/06-0083
    . </span><span class="author"><span class="firstname">Benjamin</span> <span class="surname">Kosnik</span>. </span><span class="biblioid">
      <a class="ulink" href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2006/n2013.html" target="_top">
      </a>
    . </span></p></div></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="internals.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="appendix_porting.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="api.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Porting to New Hardware or Operating Systems </td><td width="20%" align="center"><a accesskey="h" href="../spine.html">Home</a></td><td width="40%" align="right" valign="top"> API Evolution and Deprecation History</td></tr></table></div></body></html>