MPI procedures sometimes assign a special meaning to a special value of a basic type argument; e.g. tag is an integer-valued argument of point-to-point communication operations, with a special wild-card value, MPI_ANY_TAG. Such arguments will have a range of regular values, which is a proper subrange of the range of values of the corresponding basic type; special values (such as MPI_ANY_TAG) will be outside the regular range. The range of regular values can be queried using environmental inquiry functions (Chapter MPI Environmental Management ).
MPI also provides predefined named constant handles, such as MPI_COMM_WORLD which is a handle to an object that represents all processes available at start-up time and allowed to communicate with any of them.
All named constants, with the exception of MPI_BOTTOM in Fortran, can be used in initialization expressions or assignments. These constants do not change values during execution. Opaque objects accessed by constant handles are defined and do not change value between MPI initialization ( MPI_INIT() call) and MPI completion ( MPI_FINALIZE() call).