int MPI_Comm_size(
  MPI_Comm comm,
  int *size
);

Parameters

comm

[in] communicator (handle)

size

[out] number of processes in the group of comm (integer)

Remarks

Rationale.

This function is equivalent to accessing the communicator's group with MPI_COMM_GROUP (see above), computing the size using MPI_GROUP_SIZE, and then freeing the temporary group via MPI_GROUP_FREE. However, this function is so commonly used, that this shortcut was introduced. ( End of rationale.)

Advice to users.

This function indicates the number of processes involved in a communicator. For MPI_COMM_WORLD, it indicates the total number of processes available (for this version of MPI, there is no standard way to change the number of processes once initialization has taken place).

Thread and Interrupt Safety

This routine is both thread- and interrupt-safe. This means that this routine may safely be used by multiple threads and from within a signal handler.

Notes for Fortran

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.

Errors

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). The MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS

No error; MPI routine completed successfully.

MPI_ERR_COMM

Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).

MPI_ERR_ARG

Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK).

Example Code

The following sample code illustrates MPI_Comm_size.

#include "mpi.h"
#include <stdio.h>
int main(int argc, char *argv[])
{
    int rank, nprocs;

    MPI_Init(&argc,&argv);
    MPI_Comm_size(MPI_COMM_WORLD,&nprocs);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    printf("Hello, world.  I am %d of %d\n", rank, nprocs);fflush(stdout);
    MPI_Finalize();
    return 0;
}