Handles are passed between Fortran and C or C++ by using an explicit C wrapper to convert Fortran handles to C handles. There is no direct access to C or C++ handles in Fortran. Handles are passed between C and C++ using overloaded C++ operators called from C++ code. There is no direct access to C++ objects from C. The type definition \fIMPI_Fint\fP is provided in C/C++ for an integer of the size that matches a Fortran \fIINTEGER\fP; usually, \fIMPI_Fint\fP will be equivalent to \fIint\fP. The handle translation functions are provided in C to convert from a Fortran handle (which is an integer) to a C handle, and vice versa.
.PP
For example, if \fIcomm\fP is a valid Fortran handle to a communicator, then MPI_Comm_f2c returns a valid C handle to that same communicator; if \fIcomm\fP = MPI_COMM_NULL (Fortran value), then MPI_Comm_f2c returns a null C handle; if \fIcomm\fP is an invalid Fortran handle, then MPI_Comm_f2c returns an invalid C handle.
.SH NOTE
This function does not return an error value. Consequently, the result of calling it before MPI_Init or after MPI_Finalize is undefined.
