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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>ABI Policy and Guidelines</title><meta name="generator" content="DocBook XSL Stylesheets V1.73.2" /><meta name="keywords" content=" C++ , ABI , version , dynamic , shared " /><meta name="keywords" content=" ISO C++ , library " /><link rel="start" href="../spine.html" title="The GNU C++ Library Documentation" /><link rel="up" href="appendix_porting.html" title="Appendix B. Porting and Maintenance" /><link rel="prev" href="internals.html" title="Porting to New Hardware or Operating Systems" /><link rel="next" href="api.html" title="API Evolution and Deprecation History" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">ABI Policy and Guidelines</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="internals.html">Prev</a> </td><th width="60%" align="center">Appendix B. 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="ulink" href="http://gcc.gnu.org/onlinedocs/libstdc++/configopts.html" target="_top">
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 (Ie, 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 stable
releases series libraries the ability to add new symbols and add
functionality, all the while retaining backwards compatibility with
the previous releases in the series. Note: the reverse is not true. It
is not possible to take binaries linked with the latest version of a
release series (if symbols have been added) and expect the initial
release of the series to remain link compatible.
</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 DT_SONAME
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.x, gcc-4.1.x, gcc-4.2.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 preceeding
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></ul></div></li><li><p>Release versioning on the libstdc++.so binary, implemented in the same was as the libgcc_s.so binary, above.</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 (Error should be libstdc++.so.3.0.3)</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</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</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 (Not strictly required)</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</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></ul></div></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 preceeding 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></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.x, gcc-4.1.x, gcc-4.2.x: 102 (when n=1)</p></li><li><p>gcc-3.4.x, gcc-4.0.x, gcc-4.1.x, gcc-4.2.x: 1000 + n (when n>1)</p></li><li><p>gcc-3.4.x, gcc-4.0.x, gcc-4.1.x, gcc-4.2.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.x, gcc-4.1.x, gcc-4.2.x: <code class="code">-fabi-version=2</code></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></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.x: "version-unused"</p></li><li><p>gcc-4.1.x: "version-unused"</p></li><li><p>gcc-4.2.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></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 <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
</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, ie
<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 => /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)
</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 => /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)
</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++ ABI's 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="id510592"></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="id510609"></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="id510627"></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="id510644"></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="id510662"></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&a=load" target="_top">
</a>
. </span></p></div><div class="biblioentry"><a id="id510679"></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="id510696"></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="id510725"></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="id510742"></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="id499374"></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>
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