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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Dual ABI</title><meta name="generator" content="DocBook XSL-NS Stylesheets V1.78.1" /><meta name="keywords" content="ISO C++, library" /><meta name="keywords" content="ISO C++, runtime, library" /><link rel="home" href="../index.html" title="The GNU C++ Library" /><link rel="up" href="using.html" title="Chapter 3. Using" /><link rel="prev" href="using_macros.html" title="Macros" /><link rel="next" href="using_namespaces.html" title="Namespaces" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">Dual ABI</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="using_macros.html">Prev</a> </td><th width="60%" align="center">Chapter 3. Using</th><td width="20%" align="right"> <a accesskey="n" href="using_namespaces.html">Next</a></td></tr></table><hr /></div><div class="section"><div class="titlepage"><div><div><h2 class="title" style="clear: both"><a id="manual.intro.using.abi"></a>Dual ABI</h2></div></div></div><p> In the GCC 5.1 release libstdc++ introduced a new library ABI that
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includes new implementations of <code class="classname">std::string</code> and
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<code class="classname">std::list</code>. These changes were necessary to conform
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to the 2011 C++ standard which forbids Copy-On-Write strings and requires
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lists to keep track of their size.
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</p><p> In order to maintain backwards compatibility for existing code linked
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to libstdc++ the library's soname has not changed and the old
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implementations are still supported in parallel with the new ones.
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This is achieved by defining the new implementations in an inline namespace
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so they have different names for linkage purposes, e.g. the new version of
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<code class="classname">std::list<int></code> is actually defined as
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<code class="classname">std::__cxx11::list<int></code>. Because the symbols
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for the new implementations have different names the definitions for both
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versions can be present in the same library.
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</p><p> The <span class="symbol">_GLIBCXX_USE_CXX11_ABI</span> macro (see
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<a class="xref" href="using_macros.html" title="Macros">Macros</a>) controls whether
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the declarations in the library headers use the old or new ABI.
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So the decision of which ABI to use can be made separately for each
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source file being compiled.
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Using the default configuration options for GCC the default value
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of the macro is <code class="literal">1</code> which causes the new ABI to be active,
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so to use the old ABI you must explicitly define the macro to
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<code class="literal">0</code> before including any library headers.
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(Be aware that some GNU/Linux distributions configure GCC 5 differently so
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that the default value of the macro is <code class="literal">0</code> and users must
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define it to <code class="literal">1</code> to enable the new ABI.)
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</p><p> Although the changes were made for C++11 conformance, the choice of ABI
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to use is independent of the <code class="option">-std</code> option used to compile
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your code, i.e. for a given GCC build the default value of the
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<span class="symbol">_GLIBCXX_USE_CXX11_ABI</span> macro is the same for all dialects.
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This ensures that the <code class="option">-std</code> does not change the ABI, so
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that it is straightforward to link C++03 and C++11 code together.
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</p><p> Because <code class="classname">std::string</code> is used extensively
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throughout the library a number of other types are also defined twice,
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including the stringstream classes and several facets used by
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<code class="classname">std::locale</code>. The standard facets which are always
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installed in a locale may be present twice, with both ABIs, to ensure that
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code like
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<code class="code">std::use_facet<std::time_get<char>>(locale);</code>
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will work correctly for both <code class="classname">std::time_get</code> and
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<code class="classname">std::__cxx11::time_get</code> (even if a user-defined
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facet that derives from one or other version of
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<code class="classname">time_get</code> is installed in the locale).
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</p><p> Although the standard exception types defined in
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<code class="filename"><stdexcept></code> use strings, they
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are not defined twice, so that a <code class="classname">std::out_of_range</code>
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exception thrown in one file can always be caught by a suitable handler in
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another file, even if the two files are compiled with different ABIs.
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</p><div class="section"><div class="titlepage"><div><div><h3 class="title"><a id="manual.intro.using.abi.trouble"></a>Troubleshooting</h3></div></div></div><p> If you get linker errors about undefined references to symbols
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that involve types in the <code class="code">std::__cxx11</code> namespace or the tag
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<code class="code">[abi:cxx11]</code> then it probably indicates that you are trying to
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link together object files that were compiled with different values for the
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<span class="symbol">_GLIBCXX_USE_CXX11_ABI</span> macro. This commonly happens when
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linking to a third-party library that was compiled with an older version
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of GCC. If the third-party library cannot be rebuilt with the new ABI then
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you will need to recompile your code with the old ABI.
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</p><p> Not all uses of the new ABI will cause changes in symbol names, for
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example a class with a <code class="classname">std::string</code> member variable
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will have the same mangled name whether compiled with the old or new ABI.
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In order to detect such problems the new types and functions are
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annotated with the <span class="property">abi_tag</span> attribute, allowing the
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compiler to warn about potential ABI incompatibilities in code using them.
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Those warnings can be enabled with the <code class="option">-Wabi-tag</code> option.
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</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="using_macros.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="using.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="using_namespaces.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Macros </td><td width="20%" align="center"><a accesskey="h" href="../index.html">Home</a></td><td width="40%" align="right" valign="top"> Namespaces</td></tr></table></div></body></html> |