Getting Information

• The MPI_Comm_size and MPI_Comm_rank functions are used to determine the number of processes and the label of the calling process, respectively. The calling sequences of these routines are as follows:

  int MPI_Comm_size(MPI_Comm comm, int *size) 
  int MPI_Comm_rank(MPI_Comm comm, int *rank)
  

• The function MPI_Comm_size returns in the variable size the number of processes that belong to the communicator comm.
• So, when there is a single process per processor, the call
  MPI_Comm_size(MPI_COMM_WORLD, &size) will return in size the number of processors used by the program.
• Every process that belongs to a communicator is uniquely identified by its rank. The rank of a process is an integer that ranges from zero up to the size of the communicator minus one.
• A process can determine its rank in a communicator by using the MPI_Comm_rank function that takes two arguments: the communicator and an integer variable rank. Up on return, the variable rank stores the rank of the process. Note that each process that calls either one of these functions must belong in the supplied communicator, otherwise an error will occur.