An extern linkage specification is required for C++ to call Fortran and for Fortran to call C++. All entry declarations are made in the C++ code; no special declaration is required in the Fortran routine. The specification allows the C++ routine to pass arguments and return values to and from the Fortran routine using an internal argument list compatible with Fortran.

The extern linkage specification has the following format:

extern "FORTRAN" {declaration}

declaration is a valid C++ prototype of the Fortran program being called by C++, or the C++ routine being called by Fortran.