The following are all local operations.
MPI_COMM_SIZE(comm, size) IN comm communicator OUT size number of processes in the group of commMPI_Comm_size(MPI_Comm comm, int *size)
MPI_COMM_SIZE(COMM, SIZE, IERROR)INTEGER COMM, SIZE, IERROR
MPI_COMM_SIZE returns the size of the group associated with comm.
This function indicates the number of processes involved in an intracommunicator. For MPI_COMM_WORLD, it indicates the total number of processes MPI_COMM_WORLD available at initialization time. (For this version of MPI, this is also the total number of processes available throughout the computation).
MPI_Comm_rank(MPI_Comm comm, int *rank)
MPI_COMM_RANK(COMM, RANK, IERROR)INTEGER COMM, RANK, IERROR
MPI_COMM_RANK indicates the rank of the process that calls it, in the range from 0...size-1, where size is the return value of MPI_COMM_SIZE. This rank is relative to the group associated with the intracommunicator comm. Thus, MPI_COMM_RANK(MPI_COMM_WORLD, rank) returns in rank the ``absolute'' rank of the calling process in the global communication group of MPI_COMM_WORLD; MPI_COMM_RANK( MPI_COMM_SELF, rank) returns rank = 0.
MPI_Comm_compare(MPI_Comm comm1,MPI_Comm comm2, int *result)
MPI_COMM_COMPARE(COMM1, COMM2, RESULT, IERROR)INTEGER COMM1, COMM2, RESULT, IERROR
MPI_COMM_COMPARE is used to find the relationship between two intra-communicators. MPI_IDENT results if and only if MPI_IDENT comm1 and comm2 are handles for the same object (representing the same communication domain). MPI_CONGRUENT results if the underlying groups are identical MPI_CONGRUENT in constituents and rank order (the communicators represent two distinct communication domains with the same group attribute). MPI_SIMILAR results if the group members of both MPI_SIMILAR communicators are the same but the rank order differs. MPI_UNEQUAL results otherwise. The groups MPI_UNEQUAL associated with two different communicators could be gotten via MPI_COMM_GROUP and then used in a call to MPI_GROUP_COMPARE. If MPI_COMM_COMPARE gives MPI_CONGRUENT then MPI_GROUP_COMPARE will give MPI_IDENT. If MPI_COMM_COMPARE gives MPI_SIMILAR then MPI_GROUP_COMPARE will give MPI_SIMILAR.