MPI_Comm_size function

Retrieves the number of processes involved in a communicator, or the total number of processes available.

Syntax

int MPIAPI MPI_Comm_size(
        MPI_Comm comm,
  _Out_ int      *size
);

Parameters

• comm
  The communicator to evaluate. Specify the MPI_COMM_WORLD constant to retrieve the total number of processes available.

• size [out]
  On return, indicates the number of processes in the group for the communicator.

Return value

Returns MPI_SUCCESS on success. Otherwise, the return value is an error code.

In Fortran, the return value is stored in the IERROR parameter.

Fortran

    MPI_COMM_SIZE(COMM,SIZE,IERROR)
        INTEGER COMM, SIZE, IERROR

Remarks

This function enables the user to retrieve the group size with a single function call. Otherwise, it would be necessary to create a temporary group by using the MPI_Comm_group function, get the size of the group by using the MPI_Group_size function, and then free the temporary group by using the MPI_Group_free function.

This function is often used with the MPI_Comm_rank function to determine the amount of concurrency that is available for a specific library or program. The MPI_Comm_rank function indicates the rank of the process that calls it in the range from 0 to size-1, where size is retrieved by using the MPI_Comm_size function.

Note

There is no standard way to change the number of processes after initialization has taken place.

 

Requirements

Product	HPC Pack 2012 MS-MPI Redistributable Package, HPC Pack 2008 R2 MS-MPI Redistributable Package, HPC Pack 2008 MS-MPI Redistributable Package or HPC Pack 2008 Client Utilities
Header	Mpi.h; Mpif.h
Library	Msmpi.lib
DLL	Msmpi.dll

See also

MPI Communicator Functions

MPI_Comm_rank