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