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<tr class="t-nv"><td colspan="5"> <a href="static_assert.html" title="cpp/language/static assert"> static_assert declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="enum.html" title="cpp/language/enum"> opaque enum declaration</a><span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Other declarations </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="namespace.html" title="cpp/language/namespace"> namespace definition</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function.html" title="cpp/language/function"> function declaration</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> class template declaration</a> </td></tr>
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<tr class="t-nv"><td colspan="5"> <a href="class_template.html#Explicit_instantiation" title="cpp/language/class template"> explicit template instantiation</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> explicit template specialization</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> linkage specification</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> attribute declaration</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="declarations.html" title="cpp/language/declarations"> empty declaration</a> </td></tr>
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<p>Provides for linkage between modules written in different programming languages.
</p>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>extern</b></code> <span class="t-spar">string-literal</span> <code><b>{</b></code> <span class="t-spar">declaration-seq</span><span class="t-mark">(optional)</span> <code><b>} </b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>extern</b></code> <span class="t-spar">string-literal</span> <span class="t-spar">declaration</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> Applies the language specification <span class="t-spar">string-literal</span> to all function types, function names with external linkage and variables with external linkage declared in <span class="t-spar">declaration-seq</span></div>
<div class="t-li1"><span class="t-li">2)</span> Applies the language specification <span class="t-spar">string-literal</span> to a single declaration or definition.</div>
<table class="t-par-begin">
<tr class="t-par">
<td> <span class="t-spar">string-literal</span>
</td>
<td> -
</td>
<td> The name of the required language linkage
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">declaration-seq</span>
</td>
<td> -
</td>
<td> a sequence of declarations, which may include nested linkage specifications
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">declaration</span>
</td>
<td> -
</td>
<td> a declaration
</td></tr></table>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="language_linkage.html#Explanation"><span class="tocnumber">1</span> <span class="toctext">Explanation</span></a>
<ul>
<li class="toclevel-2 tocsection-2"><a href="language_linkage.html#Special_rules_for_.22C.22_linkage"><span class="tocnumber">1.1</span> <span class="toctext">Special rules for "C" linkage</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-3"><a href="language_linkage.html#Notes"><span class="tocnumber">2</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="language_linkage.html#References"><span class="tocnumber">3</span> <span class="toctext">References</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/language_linkage&action=edit&section=1" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>Every function type, every function name with <a href="storage_duration.html" title="cpp/language/storage duration">external linkage</a>, and every variable name with <a href="storage_duration.html" title="cpp/language/storage duration">external linkage</a>, has a property called <i>language linkage</i>. Language linkage encapsulates the set of requirements necessary to link with a module written in another programming language: <a href="http://en.wikipedia.com/wiki/Calling_convention" class="extiw" title="enwiki:Calling convention">calling convention</a>, <a href="http://en.wikipedia.com/wiki/Name_mangling" class="extiw" title="enwiki:Name mangling">name mangling</a> algorithm, etc.
</p><p>Only two language linkages are guaranteed to be supported:
</p>
<div class="t-li1"><span class="t-li">1)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"C++"</span></span></span>, the default language linkage.</div>
<div class="t-li1"><span class="t-li">2)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"C"</span></span></span>, which makes it possible to link with functions written in the C programming language, and to define, in a C++ program, functions that can be called from the modules written in C.</div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">extern</span> <span class="st0">"C"</span> <span class="br0">{</span>
<span class="kw4">int</span> open<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span> <span class="sy2">*</span>pathname, <span class="kw4">int</span> flags<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// C function declaration</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="kw4">int</span> fd <span class="sy1">=</span> open<span class="br0">(</span><span class="st0">"test.txt"</span>, <span class="nu0">0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls a C function from a C++ program</span>
<span class="br0">}</span>
 
<span class="co1">// This C++ function can be called from C code</span>
<span class="kw4">extern</span> <span class="st0">"C"</span> <span class="kw4">void</span> handler<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span> <span class="br0">{</span>
<a href="../io/cout.html"><span class="kw1446">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Callback invoked<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="co1">// It can use C++</span>
<span class="br0">}</span></pre></div></div>
<p>Since language linkage is part of every function type, pointers to functions maintain language linkage as well. Language linkage of function types (which represents calling convention) and language linkage of function names (which represents name mangling) are independent of each other:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">extern</span> <span class="st0">"C"</span> <span class="kw4">void</span> f1<span class="br0">(</span><span class="kw4">void</span><span class="br0">(</span><span class="sy2">*</span>pf<span class="br0">)</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declares a function f1 with C linkage,</span>
<span class="co1">// which returns void and takes a pointer to a C function</span>
<span class="co1">// which returns void and takes no parameters</span>
<span class="kw4">extern</span> <span class="st0">"C"</span> <span class="kw1">typedef</span> <span class="kw4">void</span> FUNC<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declares FUNC as a C function type that returns void</span>
<span class="co1">// and takes no parameters</span>
FUNC f2<span class="sy4">;</span> <span class="co1">// the name f2 has C++ linkage, but its type is C function</span>
<span class="kw4">extern</span> <span class="st0">"C"</span> FUNC f3<span class="sy4">;</span> <span class="co1">// the name f3 has C linkage and its type is C function void()</span>
<span class="kw4">void</span> <span class="br0">(</span><span class="sy2">*</span>pf2<span class="br0">)</span><span class="br0">(</span>FUNC<span class="sy2">*</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// the name pf2 has C++ linkage, and its type is</span>
<span class="co1">// "pointer to a C++ function which returns void and takes one</span>
<span class="co1">// argument of type 'pointer to the C function which returns void</span>
<span class="co1">// and takes no parameters'"</span>
<span class="kw4">extern</span> <span class="st0">"C"</span> <span class="br0">{</span>
<span class="kw4">static</span> <span class="kw4">void</span> f4<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// the name of the function f4 has internal linkage (no language)</span>
<span class="co1">// but the function's type has C language linkage</span>
<span class="br0">}</span></pre></div></div>
<p>Two functions with the same name and the same parameter list in the same namespace cannot have two different language linkages (note, however, that linkage of a parameter may permit such overloading, as in the case of <span class="t-lc"><a href="../algorithm/qsort.html" title="cpp/algorithm/qsort">std::qsort</a></span> and <span class="t-lc"><a href="../algorithm/bsearch.html" title="cpp/algorithm/bsearch">std::bsearch</a></span>). Likewise, two variables in the same namespace cannot have two different language linkages.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/language_linkage&action=edit&section=2" title="Edit section: Special rules for "C" linkage">edit</a>]</span> <span class="mw-headline" id="Special_rules_for_.22C.22_linkage">Special rules for "C" linkage</span></h4>
<div class="t-li1"><span class="t-li">1)</span> When class member declarations and member function type declarations appear in a <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"C"</span></span></span> language block, their linkage remains <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"C++"</span></span></span></div>
<div class="t-li1"><span class="t-li">2)</span> When two functions with the same unqualified name are declared in different namespaces, and both have <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"C"</span></span></span> language linkage, the declarations refer to the same function</div>
<div class="t-li1"><span class="t-li">3)</span> When two variables with <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"C"</span></span></span> language linkage and the same name that appear in different namespaces, they refer to the same variable.</div>
<div class="t-li1"><span class="t-li">4)</span> A <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"C"</span></span></span> variable and a <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"C"</span></span></span> function cannot have the same name, regardless if they are defined in the same or different namespaces.</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/language_linkage&action=edit&section=3" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>Language specifications can only appear in namespace scope
</p><p>The braces of the language specification do not establish a scope.
</p><p>When language specifications nest, the innermost specification is the one that is in effect.
</p><p>A function can be re-declared without a linkage specification after it was declared with a language specification, the second declaration will reuse the first language linkage. The opposite is not true: if the first declaration has no language linkage, it is assumed <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="st0">"C++"</span></span></span>, and redeclaring with another language is an error.
</p><p>A declaration directly contained in a language linkage specification is treated as if it contains the <a href="storage_duration.html" title="cpp/language/storage duration">extern specifier</a> for the purpose of determining the <a href="storage_duration.html#Linkage" title="cpp/language/storage duration">linkage</a> of the declared name and whether it is a <a href="definition.html" title="cpp/language/definition">definition</a>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">extern</span> <span class="st0">"C"</span> <span class="kw4">int</span> x<span class="sy4">;</span> <span class="co1">// a declaration and not a definition</span>
<span class="co1">// The above line is equivalent to extern "C" { extern int x; }</span>
 
<span class="kw4">extern</span> <span class="st0">"C"</span> <span class="br0">{</span> <span class="kw4">int</span> x<span class="sy4">;</span> <span class="br0">}</span> <span class="co1">// a declaration and definition</span></pre></div></div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/language_linkage&action=edit&section=4" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-c++14">
<ul><li> C++14 standard (ISO/IEC 14882:2014):
</li></ul>
<dl><dd><ul><li> 7.5 Linkage specifications [dcl.link]
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++11">
<ul><li> C++11 standard (ISO/IEC 14882:2011):
</li></ul>
<dl><dd><ul><li> 7.5 Linkage specifications [dcl.link]
</li></ul>
</dd></dl>
</div>
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