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copied!<p>extern "C" makes a function-name in C++ have 'C' linkage (compiler does not mangle the name) so that client C code can link to (i.e use) your function using a 'C' compatible header file that contains just the declaration of your function. Your function definition is contained in a binary format (that was compiled by your C++ compiler) that the client 'C' linker will then link to using the 'C' name.</p> <p>Since C++ has overloading of function names and C does not, the C++ compiler cannot just use the function name as a unique id to link to, so it mangles the name by adding information about the arguments. A C compiler does not need to mangle the name since you can not overload function names in C. When you state that a function has extern "C" linkage in C++, the C++ compiler does not add argument/parameter type information to the name used for linkage.</p> <p>Just so you know, you can specify "C" linkage to each individual declaration/definition explicitly or use a block to group a sequence of declarations/definitions to have a certain linkage:</p> <pre><code>extern "C" void foo(int); extern "C" { void g(char); int i; } </code></pre> <p>If you care about the technicalities, they are listed in section 7.5 of the C++03 standard, here is a brief summary (with emphasis on extern "C"):</p> <ul> <li>extern "C" is a linkage-specification</li> <li>Every compiler is <em>required</em> to provide "C" linkage</li> <li>a linkage specification shall occur only in namespace scope</li> <li><strike> all function types, function names and variable names have a language linkage </strike> <b><a href="https://stackoverflow.com/questions/1041866/in-c-source-what-is-the-effect-of-extern-c#comment20842899_1041880">See Richard's Comment:</a></b> Only function names and variable names with external linkage have a language linkage</li> <li>two function types with distinct language linkages are distinct types even if otherwise identical</li> <li>linkage specs nest, inner one determines the final linkage</li> <li>extern "C" is ignored for class members </li> <li>at most one function with a particular name can have "C" linkage (regardless of namespace)</li> <li><strike> extern "C" forces a function to have external linkage (cannot make it static) </strike> <b> See Richard's comment: </b> 'static' inside 'extern "C"' is valid; an entity so declared has internal linkage, and so does not have a language linkage </li> <li>Linkage from C++ to objects defined in other languages and to objects defined in C++ from other languages is implementation-defined and language-dependent. Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved </li> </ul>

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