osx man page for mpi_comm_size

MPI_Comm_size(3OpenMPI) 												   MPI_Comm_size(3OpenMPI)

NAME
       MPI_Comm_size  - Returns the size of the group associated with a communicator.

SYNTAX
C Syntax
       #include <mpi.h>
       int MPI_Comm_size(MPI_Comm comm, int *size)

Fortran Syntax
       INCLUDE 'mpif.h'
       MPI_COMM_SIZE(COMM, SIZE, IERROR)
	    INTEGER   COMM, SIZE, IERROR

C++ Syntax
       #include <mpi.h>
       int Comm::Get_size() const

INPUT PARAMETER
       comm	 Communicator (handle).

OUTPUT PARAMETERS
       size	 Number of processes in the group of comm (integer).

       IERROR	 Fortran only: Error status (integer).

DESCRIPTION
       This  function indicates the number of processes involved in a communicator. For MPI_COMM_WORLD, it indicates the total number of processes
       available. This function is equivalent to accessing the communicator's group with MPI_Comm_group, computing the size using  MPI_Group_size,
       and  then  freeing  the temporary group via MPI_Group_free. If the communicator is an intra-communicator (enables communication between two
       groups), this function returns the size of the local group.  To return the size of the remote group, use the MPI_Comm_remote_size function.

       This call is often used with MPI_Comm_rank  to  determine  the  amount  of  concurrency	available  for	a  specific  library  or  program.
       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.

NOTE
       MPI_COMM_NULL is not considered a valid argument to this function.

ERRORS
       Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ func-
       tions  do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism
       will be used to throw an MPI:Exception object.

       Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except  for
       I/O  function  errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be
       used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

SEE ALSO
       MPI_Comm_group
       MPI_Comm_rank
       MPI_Comm_compare

Open MPI 1.2							  September 2006					   MPI_Comm_size(3OpenMPI)