mirror of git://gcc.gnu.org/git/gcc.git
896 lines
29 KiB
HTML
896 lines
29 KiB
HTML
<?xml version="1.0" encoding="ISO-8859-1"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<head>
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<meta name="AUTHOR" content="bkoz@gcc.gnu.org (Benjamin Kosnik)" />
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<meta name="KEYWORDS" content="C++, libstdc++, dynamic, shared, library, ABI, version" />
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<meta name="DESCRIPTION" content="C++ Standard Library ABI" />
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<meta name="GENERATOR" content="emacs and ten fingers" />
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<title>Standard C++ Library ABI</title>
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<link rel="StyleSheet" href="lib3styles.css" type="text/css" />
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<link rel="Start" href="documentation.html" type="text/html"
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title="GNU C++ Standard Library" />
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<link rel="Copyright" href="17_intro/license.html" type="text/html" />
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</head>
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<body>
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<h1 class="centered"><a name="top">C++ Standard Library ABI</a></h1>
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<p class="fineprint"><em>
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The latest version of this document is always available at
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<a href="http://gcc.gnu.org/onlinedocs/libstdc++/abi.html">
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http://gcc.gnu.org/onlinedocs/libstdc++/abi.html</a>.
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</em></p>
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<p><em>
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To the <a href="http://gcc.gnu.org/libstdc++/">libstdc++-v3 homepage</a>.
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</em></p>
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<!-- ####################################################### -->
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<hr />
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<h3 class="left">
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<a name="CXXinterface">The C++ interface</a>
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</h3>
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<p> C++ applications often dependent on specific language support
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routines, say for throwing exceptions, or catching exceptions, and
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perhaps also dependent on features in the C++ Standard Library.
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</p>
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<p> The C++ Standard Library has many include files, types defined in
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those include files, specific named functions, and other behavior. The
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text of these behaviors, as written in source include files, is called
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the Application Programing Interface, or API.
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</p>
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<p> Furthermore, C++ source that is compiled into object files is
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transformed by the compiler: it arranges objects with specific
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alignment and in a particular layout, mangling names according to a
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well-defined algorithm, has specific arrangements for the support of
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virtual functions, etc. These details are defined as the compiler
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Application Binary Interface, or ABI. The GNU C++ compiler uses an
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||
industry-standard C++ ABI starting with version 3. Details can be
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||
found in the <a href="http://www.codesourcery.com/cxx-abi/abi.html">
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||
ABI specification</a>.
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</p>
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||
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<p>
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The GNU C++ compiler, g++, has a compiler command line option to
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switch between various different C++ ABIs. This explicit version
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switch is the flag <code> -fabi-version</code>. In addition, some
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g++ command line options may change the ABI as a side-effect of
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use. Such flags include <code>-fpack-struct</code> and
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<code>-fno-exceptions</code>, but include others: see the complete
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||
list in the GCC manual under the heading <a
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||
href="http://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#Code%20Gen%20Options">Options
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||
for Code Generation Conventions</a>.
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||
</p>
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||
|
||
<p> The configure options used when building a specific libstdc++
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||
version may also impact the resulting library ABI. The available
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||
configure options, and their impact on the library ABI, are documented
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<a href="http://gcc.gnu.org/onlinedocs/libstdc++/configopts.html">
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here</a>.
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</p>
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<p> Putting all of these ideas together results in the C++ Standard
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library ABI, which is the compilation of a given library API by a
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given compiler ABI. In a nutshell:
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</p>
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<code> library API + compiler ABI = library ABI</code>
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<p>
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The library ABI is mostly of interest for end-users who have
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unresolved symbols and are linking dynamically to the C++ Standard
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||
library, and who thus must be careful to compile their application
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with a compiler that is compatible with the available C++ Standard
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||
library binary. In this case, compatible is defined with the equation
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above: given an application compiled with a given compiler ABI and
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library API, it will work correctly with a Standard C++ Library
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created with the same constraints.
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</p>
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||
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<p>
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To use a specific version of the C++ ABI, one must use a
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corresponding GNU C++ toolchain (Ie, g++ and libstdc++) that
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implements the C++ ABI in question.
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</p>
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<h3 class="left">
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<a name="ABI_versioning">Versioning</a>
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</h3>
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<p> The C++ interface has evolved throughout the history of the GNU
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C++ toolchain. With each release, various details have been changed so
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as to give distinct versions to the C++ interface.
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</p>
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<h5 class="left">
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<a name="goals">Goals of versioning</a>
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</h5>
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<p>Extending existing, stable ABIs. Versioning gives subsequent stable
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releases series libraries the ability to add new symbols and add
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functionality, all the while retaining backwards compatibility with
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the previous releases in the series. Note: the reverse is not true. It
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||
is not possible to take binaries linked with the latest version of a
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release series (if symbols have been added) and expect the initial
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||
release of the series to remain link compatible.
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||
</p>
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<p>Allows multiple, incompatible ABIs to coexist at the same time.
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</p>
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<p>
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</p>
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<h5 class="left">
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<a name="details"> Version History </a>
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||
</h5>
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<p>
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How can this complexity be managed? What does C++ versioning mean?
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Because library and compiler changes often make binaries compiled
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with one version of the GNU tools incompatible with binaries
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compiled with other (either newer or older) versions of the same GNU
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tools, specific techniques are used to make managing this complexity
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||
easier.
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||
</p>
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<p>
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The following techniques are used:
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</p>
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<ul>
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<li> <p>Release versioning on the libgcc_s.so binary. This is
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implemented via file names and the ELF DT_SONAME mechanism (at least
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on ELF systems).</p>
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<p>It is versioned as follows:
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</p>
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<ul>
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<li>gcc-3.0.0: libgcc_s.so.1</li>
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<li>gcc-3.0.1: libgcc_s.so.1</li>
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<li>gcc-3.0.2: libgcc_s.so.1</li>
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<li>gcc-3.0.3: libgcc_s.so.1</li>
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<li>gcc-3.0.4: libgcc_s.so.1</li>
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<li>gcc-3.1.0: libgcc_s.so.1</li>
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<li>gcc-3.1.1: libgcc_s.so.1</li>
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<li>gcc-3.2.0: libgcc_s.so.1</li>
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<li>gcc-3.2.1: libgcc_s.so.1</li>
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<li>gcc-3.2.2: libgcc_s.so.1</li>
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<li>gcc-3.2.3: libgcc_s.so.1</li>
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<li>gcc-3.3.0: libgcc_s.so.1</li>
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<li>gcc-3.3.1: libgcc_s.so.1</li>
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<li>gcc-3.3.2: libgcc_s.so.1</li>
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<li>gcc-3.3.3: libgcc_s.so.1</li>
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<li>gcc-3.4.0: on m68k-linux and hppa-linux this is either libgcc_s.so.1
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(when configuring <code>--with-sjlj-exceptions</code>) or
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libgcc_s.so.2. For all others, this is libgcc_s.so.1.
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</li>
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</ul>
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<p></p>
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</li>
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<li>Release versioning on the libstdc++.so binary, implemented in the same was as the libgcc_s.so binary, above.
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<p>It is versioned as follows:
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</p>
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<ul>
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<li>gcc-3.0.0: libstdc++.so.3.0.0</li>
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<li>gcc-3.0.1: libstdc++.so.3.0.1</li>
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<li>gcc-3.0.2: libstdc++.so.3.0.2</li>
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<li>gcc-3.0.3: libstdc++.so.3.0.2 (Error should be libstdc++.so.3.0.3)</li>
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<li>gcc-3.0.4: libstdc++.so.3.0.4</li>
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<li>gcc-3.1.0: libstdc++.so.4.0.0</li>
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<li>gcc-3.1.1: libstdc++.so.4.0.1</li>
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<li>gcc-3.2.0: libstdc++.so.5.0.0</li>
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<li>gcc-3.2.1: libstdc++.so.5.0.1</li>
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<li>gcc-3.2.2: libstdc++.so.5.0.2</li>
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<li>gcc-3.2.3: libstdc++.so.5.0.3 (Not strictly required)</li>
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<li>gcc-3.3.0: libstdc++.so.5.0.4</li>
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<li>gcc-3.3.1: libstdc++.so.5.0.5</li>
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<li>gcc-3.3.2: libstdc++.so.5.0.5</li>
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<li>gcc-3.3.3: libstdc++.so.5.0.5</li>
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<li>gcc-3.4.0: libstdc++.so.6.0.0</li>
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<li>gcc-3.4.1: libstdc++.so.6.0.1</li>
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</ul>
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<p></p>
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</li>
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<li>Symbol versioning on the libgcc_s.so binary.
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<p>mapfile: gcc/libgcc-std.ver</p>
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<p>It is versioned with the following labels and version definitions:</p>
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<ul>
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<li>gcc-3.0.0: GCC_3.0</li>
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<li>gcc-3.0.1: GCC_3.0</li>
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<li>gcc-3.0.2: GCC_3.0</li>
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<li>gcc-3.0.3: GCC_3.0</li>
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<li>gcc-3.0.4: GCC_3.0</li>
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<li>gcc-3.1.0: GCC_3.0</li>
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<li>gcc-3.1.1: GCC_3.0</li>
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<li>gcc-3.2.0: GCC_3.0</li>
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<li>gcc-3.2.1: GCC_3.0</li>
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<li>gcc-3.2.2: GCC_3.0</li>
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<li>gcc-3.2.3: GCC_3.0</li>
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<li>gcc-3.3.0: GCC_3.0</li>
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<li>gcc-3.3.1: GCC_3.0</li>
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<li>gcc-3.3.2: GCC_3.0</li>
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<li>gcc-3.3.3: GCC_3.0</li>
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<li>gcc-3.4.0: GCC_3.0</li>
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</ul>
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<p></p>
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</li>
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<li>Symbol versioning on the libstdc++.so binary.
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<p>mapfile: libstdc++-v3/config/linker-map.gnu</p>
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<p>It is versioned with the following labels and version
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definitions, where the version definition is the maximum for a
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particular release. Note, only symbol which are newly introduced
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will use the maximum version definition. Thus, for release series
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with the same label, but incremented version definitions, the later
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||
release has both versions. (An example of this would be the
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gcc-3.2.1 release, which has GLIBCPP_3.2.1 for new symbols and
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GLIBCPP_3.2 for symbols that were introduced in the gcc-3.2.0
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release.)
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</p>
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<ul>
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<li>gcc-3.0.0: (Error, not versioned)</li>
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<li>gcc-3.0.1: (Error, not versioned)</li>
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<li>gcc-3.0.2: (Error, not versioned)</li>
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<li>gcc-3.0.3: (Error, not versioned)</li>
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<li>gcc-3.0.4: (Error, not versioned)</li>
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<li>gcc-3.1.0: GLIBCPP_3.1, CXXABI_1</li>
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<li>gcc-3.1.1: GLIBCPP_3.1, CXXABI_1</li>
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<li>gcc-3.2.0: GLIBCPP_3.2, CXXABI_1.2</li>
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||
<li>gcc-3.2.1: GLIBCPP_3.2.1, CXXABI_1.2</li>
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<li>gcc-3.2.2: GLIBCPP_3.2.2, CXXABI_1.2</li>
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<li>gcc-3.2.3: GLIBCPP_3.2.2, CXXABI_1.2</li>
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<li>gcc-3.3.0: GLIBCPP_3.2.2, CXXABI_1.2.1</li>
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<li>gcc-3.3.1: GLIBCPP_3.2.3, CXXABI_1.2.1</li>
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<li>gcc-3.3.2: GLIBCPP_3.2.3, CXXABI_1.2.1</li>
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<li>gcc-3.3.3: GLIBCPP_3.2.3, CXXABI_1.2.1</li>
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<li>gcc-3.4.0: GLIBCXX_3.4, CXXABI_1.3</li>
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<li>gcc-3.4.1: GLIBCXX_3.4.1, CXXABI_1.3</li>
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</ul>
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<p></p>
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</li>
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<li>
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<p>Incremental bumping of a compiler pre-defined macro,
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__GXX_ABI_VERSION. This macro is defined as the version of the
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compiler v3 ABI, with g++ 3.0.x being version 100. This macro will
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be automatically defined whenever g++ is used (the curious can
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test this by invoking g++ with the '-v' flag.)
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</p>
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<p>
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This macro was defined in the file "lang-specs.h" in the gcc/cp directory.
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Later versions defined it in "c-common.c" in the gcc directory, and from
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G++ 3.4 it is defined in c-cppbuiltin.c and its value determined by the
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'-fabi-version' command line option.
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</p>
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<p>
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It is versioned as follows, where 'n' is given by '-fabi-version=n':
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</p>
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<ul>
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<li>gcc-3.0.x: 100</li>
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<li>gcc-3.1.x: 100 (Error, should be 101)</li>
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<li>gcc-3.2.x: 102</li>
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<li>gcc-3.3.x: 102</li>
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<li>gcc-3.4.x: 102 (when n=1)</li>
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<li>gcc-3.4.x: 1000 + n (when n>1)</li>
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<li>gcc-3.4.x: 999999 (when n=0)</li>
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</ul>
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<p></p>
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</li>
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<li>
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<p>Changes to the default compiler option for
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<code>-fabi-version</code>.
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</p>
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<p>
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It is versioned as follows:
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</p>
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<ul>
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<li>gcc-3.0.x: (Error, not versioned) </li>
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<li>gcc-3.1.x: (Error, not versioned) </li>
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<li>gcc-3.2.x: <code>-fabi-version=1</code></li>
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<li>gcc-3.3.x: <code>-fabi-version=1</code></li>
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<li>gcc-3.4.x: <code>-fabi-version=2</code></li>
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</ul>
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<p></p>
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||
</li>
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<li>
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<p>Incremental bumping of a library pre-defined macro. For releases
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before 3.4.0, the macro is __GLIBCPP__. For later releases, it's
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__GLIBCXX__. (The libstdc++ project generously changed from CPP to
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CXX throughout its source to allow the "C" pre-processor the CPP
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macro namespace.) These macros are defined as the date the library
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||
was released, in compressed ISO date format, as an unsigned long.
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</p>
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<p>
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In addition, the pre-defined macro is defined in the file
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"c++config" in the "libstdc++-v3/include/bits" directory and is
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changed every night by an automated script.
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</p>
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<p>
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It is versioned as follows:
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</p>
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<ul>
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<li>gcc-3.0.0: 20010615</li>
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<li>gcc-3.0.1: 20010819</li>
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||
<li>gcc-3.0.2: 20011023</li>
|
||
<li>gcc-3.0.3: 20011220</li>
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||
<li>gcc-3.0.4: 20020220</li>
|
||
<li>gcc-3.1.0: 20020514</li>
|
||
<li>gcc-3.1.1: 20020725</li>
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||
<li>gcc-3.2.0: 20020814</li>
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||
<li>gcc-3.2.1: 20021119</li>
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||
<li>gcc-3.2.2: 20030205</li>
|
||
<li>gcc-3.2.3: 20030422</li>
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||
<li>gcc-3.3.0: 20030513</li>
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||
<li>gcc-3.3.1: 20030804</li>
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||
<li>gcc-3.3.2: 20031016</li>
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||
<li>gcc-3.3.3: 20040214</li>
|
||
<li>gcc-3.4.0: 20040419</li>
|
||
<li>gcc-3.4.1: 20040701</li>
|
||
</ul>
|
||
<p></p>
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||
</li>
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||
|
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|
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<li>
|
||
<p>
|
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Incremental bumping of a library pre-defined macro,
|
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_GLIBCPP_VERSION. This macro is defined as the released version of
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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
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||
is called _GLIBCXX_VERSION).
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||
</p>
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<p>
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This macro is defined in the file "c++config" in the
|
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"libstdc++-v3/include/bits" directory and is generated
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automatically by autoconf as part of the configure-time generation
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||
of config.h.
|
||
</p>
|
||
|
||
<p>
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||
It is versioned as follows:
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||
</p>
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||
<ul>
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||
<li>gcc-3.0.0: "3.0.0"</li>
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||
<li>gcc-3.0.1: "3.0.0" (Error, should be "3.0.1")</li>
|
||
<li>gcc-3.0.2: "3.0.0" (Error, should be "3.0.2")</li>
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||
<li>gcc-3.0.3: "3.0.0" (Error, should be "3.0.3")</li>
|
||
<li>gcc-3.0.4: "3.0.0" (Error, should be "3.0.4")</li>
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||
<li>gcc-3.1.0: "3.1.0"</li>
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||
<li>gcc-3.1.1: "3.1.1"</li>
|
||
<li>gcc-3.2.0: "3.2"</li>
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<li>gcc-3.2.1: "3.2.1"</li>
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||
<li>gcc-3.2.2: "3.2.2"</li>
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||
<li>gcc-3.2.3: "3.2.3"</li>
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||
<li>gcc-3.3.0: "3.3"</li>
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||
<li>gcc-3.3.1: "3.3.1"</li>
|
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<li>gcc-3.3.2: "3.3.2"</li>
|
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<li>gcc-3.3.3: "3.3.3"</li>
|
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<li>gcc-3.4.0: "version-unused"</li>
|
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<li>gcc-3.4.1: "version-unused"</li>
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</ul>
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<p></p>
|
||
</li>
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<li>
|
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<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++-v3/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>
|
||
<ul>
|
||
<li>gcc-3.0.0: include/g++-v3</li>
|
||
<li>gcc-3.0.1: include/g++-v3</li>
|
||
<li>gcc-3.0.2: include/g++-v3</li>
|
||
<li>gcc-3.0.3: include/g++-v3</li>
|
||
<li>gcc-3.0.4: include/g++-v3</li>
|
||
<li>gcc-3.1.0: include/g++-v3</li>
|
||
<li>gcc-3.1.1: include/c++/3.1.1</li>
|
||
<li>gcc-3.2.0: include/c++/3.2</li>
|
||
<li>gcc-3.2.1: include/c++/3.2.1</li>
|
||
<li>gcc-3.2.2: include/c++/3.2.2</li>
|
||
<li>gcc-3.2.3: include/c++/3.2.3</li>
|
||
<li>gcc-3.3.0: include/c++/3.3</li>
|
||
<li>gcc-3.3.1: include/c++/3.3.1</li>
|
||
<li>gcc-3.3.2: include/c++/3.3.2</li>
|
||
<li>gcc-3.3.3: include/c++/3.3.3</li>
|
||
<li>gcc-3.4.0: include/c++/3.4.0</li>
|
||
<li>gcc-3.4.1: include/c++/3.4.1</li>
|
||
</ul>
|
||
<p></p>
|
||
</li>
|
||
</ul>
|
||
<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>
|
||
|
||
|
||
|
||
<h5 class="left">
|
||
<a name="requirements"> Minimum requirements for a versioned ABI </a>
|
||
</h5>
|
||
<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++-v3) 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>
|
||
|
||
|
||
<h5 class="left">
|
||
<a name="config"> What configure options impact symbol versioning? </a>
|
||
</h5>
|
||
<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++-v3/acinclude.m4 has a macro called
|
||
GLIBCXX_ENABLE_SYMVERS that defaults to yes (or the argument passed
|
||
in via --enable-symvers=foo). At that point, the macro attempts to
|
||
make sure that all the requirement for symbol versioning are in
|
||
place. For more information, please consult acinclude.m4.
|
||
</p>
|
||
|
||
|
||
<h5 class="left">
|
||
<a name="active"> How to tell if symbol versioning is, indeed, active? </a>
|
||
</h5>
|
||
<p>
|
||
When the GNU C++ library is being built with symbol versioning on,
|
||
you should see the following at configure time for libstdc++-v3:
|
||
</p>
|
||
|
||
|
||
<code> checking versioning on shared library symbols... gnu</code>
|
||
|
||
<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>
|
||
#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>
|
||
|
||
<code> U _ZNSt8ios_base4InitC1Ev@@GLIBCXX_3.4 </code>
|
||
|
||
<h3 class="left">
|
||
<a name="ABI_allowed">Library allowed ABI changes</a>
|
||
</h3>
|
||
<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>
|
||
<ul>
|
||
<li>adding an exported global or static data member</li>
|
||
<li>adding an exported function, static or non-virtual member function</li>
|
||
<li>adding an exported symbol or symbols by additional instantiations</li>
|
||
</ul>
|
||
<p>
|
||
</p>
|
||
<p>
|
||
Other allowed changes are possible.
|
||
</p>
|
||
|
||
|
||
<h3 class="left">
|
||
<a name="ABI_disallowed">Library disallowed ABI changes</a>
|
||
</h3>
|
||
|
||
<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>
|
||
<ul>
|
||
<li>changes in the gcc/g++ compiler ABI</li>
|
||
<li>changing size of an exported symbol</li>
|
||
<li>changing alignment of an exported symbol</li>
|
||
<li>changing the layout of an exported symbol</li>
|
||
<li>changing mangling on an exported symbol</li>
|
||
<li>deleting an exported symbol</li>
|
||
<li>changing the inheritance properties of a type by adding or removing
|
||
base classes</li>
|
||
<li>
|
||
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.
|
||
</li>
|
||
|
||
<li> 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
|
||
href="http://www.codesourcery.com/cxx-abi/abi.html#calls"> this part</a>
|
||
of the C++ ABI documentation for further details.
|
||
</li>
|
||
|
||
</ul>
|
||
|
||
<h3 class="left">
|
||
<a name="implementation">Library implementation strategy</a> </h3>
|
||
|
||
<ul>
|
||
<li>Separation of interface and implementation
|
||
<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>
|
||
|
||
<dl>
|
||
<dt>Include files have declarations, source files have defines</dt>
|
||
|
||
<dd> For non-templatized types, such as much of <code>class
|
||
locale</code>, the appropriate standard C++ include, say
|
||
<code>locale</code>, can contain full declarations, while various
|
||
source files (say <code> locale.cc, locale_init.cc,
|
||
localename.cc</code>) contain definitions.</dd>
|
||
|
||
<dt>Extern template on required types</dt>
|
||
|
||
<dd>For parts of the standard that have an explicit list of required
|
||
instantiations, the GNU extension syntax <code> extern template
|
||
</code> can be used to control where template definitions
|
||
reside. By marking required instantiations as <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> char</code> and <code>
|
||
wchar_t</code> instantiations, and includes <code>
|
||
basic_string</code>, the locale facets, and the types in <code>
|
||
iostreams</code>.</dd>
|
||
|
||
</dl>
|
||
<p> In addition, these techniques have the additional benefit that
|
||
they reduce binary size, which can increase runtime performance.
|
||
</p>
|
||
</li>
|
||
|
||
<li>Namespaces linking symbol definitions to export mapfiles
|
||
|
||
<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>
|
||
|
||
<dl>
|
||
<dt><code>namespace std</code></dt>
|
||
<dd> Defaults to exporting all symbols in label
|
||
<code>GLIBCXX</code> that do not begin with an underscore, ie
|
||
<code>__test_func</code> would not be exported by default. Select
|
||
exceptional symbols are allowed to be visible.</dd>
|
||
|
||
<dt><code>namespace __gnu_cxx</code></dt>
|
||
<dd> Defaults to not exporting any symbols in label
|
||
<code>GLIBCXX</code>, select items are allowed to be visible.</dd>
|
||
|
||
<dt><code>namespace __gnu_internal</code></dt>
|
||
<dd> Defaults to not exported, no items are allowed to be visible.</dd>
|
||
|
||
<dt><code>namespace __cxxabiv1</code>, aliased to <code> namespace abi</code></dt>
|
||
<dd> Defaults to not exporting any symbols in label
|
||
<code>CXXABI</code>, select items are allowed to be visible.</dd>
|
||
</dl>
|
||
<p>
|
||
</p>
|
||
</li>
|
||
|
||
<li>Freezing the API
|
||
<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>
|
||
</ul>
|
||
|
||
<h3 class="left">
|
||
<a name="ABI_testing">Testing ABI changes</a>
|
||
</h3>
|
||
|
||
<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 href="http://developer.intel.com/software/products/opensource/">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++-v3 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 3.2.0
|
||
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.
|
||
</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>
|
||
|
||
<h3 class="left">
|
||
<a name="ABI_multi_testing">Testing Multi-ABI binaries</a>
|
||
</h3>
|
||
|
||
<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>
|
||
%$bld/H-x86-gcc-3.4.0/bin/g++ -fPIC -DPIC -c a.cc
|
||
|
||
%$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
|
||
|
||
%$bld/H-x86-gcc-3.4.0/bin/g++ -c a.cc
|
||
|
||
%ar cru libone.a a.o
|
||
</pre>
|
||
|
||
<p> And, libtwo is constructed as follows: </p>
|
||
|
||
<pre>
|
||
%$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>
|
||
%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)
|
||
|
||
</pre>
|
||
|
||
<p> Then, the "C" compiler is used to compile a source file that uses
|
||
functions from each library.</p>
|
||
<pre>
|
||
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>
|
||
%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)
|
||
</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>
|
||
|
||
<h3 class="left">
|
||
<a name="references">Bibliography / Further Reading</a>
|
||
</h3>
|
||
|
||
<p>
|
||
ABIcheck, a vague idea of checking ABI compatibility
|
||
<br />
|
||
<a href="http://abicheck.sourceforge.net/">http://abicheck.sourceforge.net/</a>
|
||
</p>
|
||
|
||
<p>
|
||
C++ ABI reference
|
||
<br />
|
||
<a href="http://www.codesourcery.com/cxx-abi/">http://www.codesourcery.com/cxx-abi/</a>
|
||
</p>
|
||
|
||
<p>
|
||
Intel ABI documentation, "Intel<65> Compilers for Linux* -Compatibility with the GNU Compilers"
|
||
<br />
|
||
<a href="http://developer.intel.com/software/products/compilers/techtopics/LinuxCompilersCompatibility.htm">http://developer.intel.com/software/products/compilers/techtopics/LinuxCompilersCompatibility.htm</a>
|
||
</p>
|
||
|
||
<p>
|
||
Sun Solaris 2.9 docs
|
||
<br />
|
||
Linker and Libraries Guide (document 816-1386)
|
||
<br />
|
||
C++ Migration Guide (document 816-2459)
|
||
<br />
|
||
<a href="http://docs.sun.com/db/prod/solaris.9">http://docs.sun.com/db/prod/solaris.9</a>
|
||
<br />
|
||
<a href="http://docs.sun.com/?p=/doc/816-1386&a=load">http://docs.sun.com/?p=/doc/816-1386&a=load</a>
|
||
</p>
|
||
|
||
<p>
|
||
Ulrich Drepper, "ELF Symbol Versioning"
|
||
<br />
|
||
<a href="http://people.redhat.com/drepper/symbol-versioning">http://people.redhat.com/drepper/symbol-versioning</a>
|
||
</p>
|
||
|
||
</body>
|
||
</html>
|
||
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