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MPI_Comm_size.3.rst

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MPI_Comm_size

:ref:`MPI_Comm_size` |mdash| 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

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 :ref:`MPI_Comm_group`, computing the size using :ref:`MPI_Group_size`, and then freeing the temporary group via :ref:`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 :ref:`MPI_Comm_remote_size` function.

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

NOTE

MPI_COMM_NULL is not considered a valid argument to this function.

ERRORS

.. seealso::
   * :ref:`MPI_Comm_group`
   * :ref:`MPI_Comm_rank`
   * :ref:`MPI_Comm_compare`