17.2.76. MPI_Comm_size

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

17.2.76.1. SYNTAX

17.2.76.1.1. C Syntax

#include <mpi.h>

int MPI_Comm_size(MPI_Comm comm, int *size)

17.2.76.1.2. Fortran Syntax

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

17.2.76.1.3. 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

17.2.76.2. INPUT PARAMETER

  • comm: Communicator (handle).

17.2.76.3. OUTPUT PARAMETERS

  • size: Number of processes in the group of comm (integer).

  • IERROR: Fortran only: Error status (integer).

17.2.76.4. 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.

17.2.76.5. NOTE

MPI_COMM_NULL is not considered a valid argument to this function.

17.2.76.6. ERRORS

Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.

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.