MPI programs require that library routines that are part of the basic language environment (such as date and write in Fortran and printf and malloc in ANSI C) and are executed after MPI_INIT and before MPI_FINALIZE operate independently and that their completion is independent of the action of other processes in an MPI program.
Note that this in no way prevents the creation of library routines that provide parallel services whose operation is collective. However, the following program is expected to complete in an ANSI C environment regardless of the size of MPI_COMM_WORLD (assuming that I/O is available at the executing nodes).
The corresponding Fortran 77 program is also expected to complete.
An example of what is not required is any particular ordering of the action of these routines when called by several tasks. For example, MPI makes neither requirements nor recommendations for the output from the following program (again assuming that I/O is available at the executing nodes).
MPI_Comm_rank( MPI_COMM_WORLD, &rank ); printf( "Output from task rank %d\n", rank );
In addition, calls that fail because of resource exhaustion or other error are not considered a violation of the requirements here (however, they are required to complete, just not to complete successfully).