10.2.2.3 Special Constants

MPI-/ requires a number of special ``constants'' that cannot be implemented as normal Fortran constants, including MPI_BOTTOM, MPI_STATUS_IGNORE, MPI_IN_PLACE, MPI_STATUSES_IGNORE and MPI_ERRCODES_IGNORE. In C, these are implemented as constant pointers, usually as NULL and are used where the function prototype calls for a pointer to a variable, not the variable itself.

In Fortran the implementation of these special constants may require the use of language constructs that are outside the Fortran standard. Using special values for the constants (e.g., by defining them through parameter statements) is not possible because an implementation cannot distinguish these values from legal data. Typically these constants are implemented as predefined static variables (e.g., a variable in an MPI-/-declared COMMON block), relying on the fact that the target compiler passes data by address. Inside the subroutine, this address can be extracted by some mechanism outside the Fortran standard (e.g., by Fortran extensions or by implementing the function in C).

MPI-Standard for MARMOT