extern "C"

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extern “C”

introduction

  • 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 (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.
  • 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.
  • Just so you know, you can specify extern "C" linkage to each individual declaration/definition explicitly or use a block to group a sequence of declarations/definitions to have a certain linkage:
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extern "C" void foo(int);
extern "C"
{
   void g(char);
   int i;
}

details

  • 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"):
  • extern "C" is a linkage-specification
  • Every compiler is required to provide “C” linkage
  • A linkage specification shall occur only in namespace scope
  • Only function names and variable names with external linkage have a language linkage
  • Two function types with distinct language linkages are distinct types even if otherwise identical
  • Linkage specs nest, inner one determines the final linkage
  • extern "C" is ignored for class members
  • At most one function with a particular name can have “C” linkage (regardless of namespace)
  • static inside extern "C" is valid; an entity so declared has internal linkage, and so does not have a language linkage
  • 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

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