E.13. Linkage Specifications

It is possible from a C++ program to call functions written and compiled with a C compiler. As stated in Section 6.17, C++ specially encodes function names for type-safe linkage. C, however, does not encode its function names. Thus, a function compiled in C will not be recognized when an attempt is made to link C code with C++ code, because the C++ code expects a specially encoded function name. C++ enables the programmer to provide linkage specifications to inform the compiler that a function was compiled on a C compiler and to prevent the name of the function from being encoded by the C++ compiler. Linkage specifications are useful when large libraries of specialized functions have been developed, and the user either does not have access to the source code for recompilation into C++ or does not have time to convert the library functions from C to C++.

To inform the compiler that one or several functions have been compiled in C, write the function prototypes as follows:

extern "C" function prototype // single function extern "C" // multiple functions { function prototypes }

These declarations inform the compiler that the specified functions are not compiled in C++, so name encoding should not be performed on the functions listed in the linkage specification. These functions can then be linked properly with the program. C++ environments normally include the standard C libraries and do not require the programmer to use linkage specifications for those functions.