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

       USE MPI
       ! or the older form: INCLUDE 'mpif.h'
       MPI_COMM_SIZE(COMM, SIZE, IERROR)
            INTEGER   COMM, SIZE, IERROR

Fortran 2008 Syntax

       USE mpi_f08
       MPI_Comm_size(comm, size, ierror)
            TYPE(MPI_Comm), INTENT(IN) :: comm
            INTEGER, INTENT(OUT) :: size
            INTEGER, OPTIONAL, INTENT(OUT) :: 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 inter-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++ functions 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

4.0.3                                             Mar 03, 2020                                  MPI_Comm_size(3)