Getting Information

• MPI_Comm_size function $\Longrightarrow$ number of processes
• MPI_Comm_rank function $\Longrightarrow$ label of the calling process
• 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)
  

• Note that each process that calls either one of these functions must belong in the supplied communicator, otherwise an error will occur.
• 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 calling

  MPI_Comm_rank(MPI_COMM_WORLD, &rank)
  

  that takes two arguments:
  1. the communicator,
  2. an integer variable rank.
• Up on return, the variable rank stores the rank of the process.