What is the effect of extern "C" in C++?

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What exactly does putting
extern "C" into C++ code
do?

For
example:

extern "C"
            {
 void foo();
}

Answer

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.

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 "C" linkage to each individual declaration/definition explicitly or use a block
to group a sequence of declarations/definitions to have a certain
linkage:

extern "C" void
            foo(int);
extern "C"
{
 void g(char);
 int
            i;
}

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


• all function types, function names
  and variable names have a language linkage href="https://stackoverflow.com/questions/1041866/in-c-source-what-is-the-effect-of-extern-c#comment20842899_1041880">See
  Richard's Comment: 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)


• extern "C" forces a function to
  have external linkage (cannot make it static) href="https://stackoverflow.com/questions/1041866/what-is-the-effect-of-extern-c-in-c?rq=1#comment20842893_1041880">See
  Richard's comment: '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