int MPI_Group_size(
  MPI_Group group,
  int *size
);

Parameters

group

[in] group (handle) Output Parameter:

size

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

Remarks

Returns the size of a group

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_GROUP

Null or invalid group passed to function.

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_Group_size.

/*
Test the group routines
MPI_Group_compare
MPI_Group_excl
MPI_Group_intersection
MPI_Group_range_excl
MPI_Group_rank
MPI_Group_size
MPI_Group_translate_ranks
MPI_Group_union
*/
#include "mpi.h"
#include <stdio.h>
#include <stdlib.h>
 
int main( int argc, char **argv )
{
    int errs=0, toterr;
    MPI_Group basegroup;
    MPI_Group g1, g2, g3, g4, g5, g6, g7, g8, g9, g10;
    MPI_Group g3a, g3b;
    MPI_Comm comm, newcomm, splitcomm, dupcomm;
    int i, grp_rank, rank, grp_size, size, result;
    int nranks, *ranks, *ranks_out;
    int range[1][3];
    int worldrank;
 
    MPI_Init( &argc, &argv );
    MPI_Comm_rank( MPI_COMM_WORLD, &worldrank );
    comm = MPI_COMM_WORLD;
    MPI_Comm_group( comm, &basegroup );
    MPI_Comm_rank( comm, &rank );
    MPI_Comm_size( comm, &size );
 
    /* Get the basic information on this group */
    MPI_Group_rank( basegroup, &grp_rank );
    if (grp_rank != rank) {
        errs++;
        fprintf( stdout, "group rank %d != comm rank %d\n", grp_rank, rank );fflush(stdout);
    }
    MPI_Group_size( basegroup, &grp_size );
    if (grp_size != size) {
        errs++;
        fprintf( stdout, "group size %d != comm size %d\n", grp_size, size );
    }
 
    /* Form a new communicator with inverted ranking */
    MPI_Comm_split( comm, 0, size - rank, &newcomm );
    MPI_Comm_group( newcomm, &g1 );
    ranks = (int *)malloc( size * sizeof(int) );
    ranks_out = (int *)malloc( size * sizeof(int) );
    for (i=0; i<size; i++) ranks[i] = i;
    nranks = size;
    MPI_Group_translate_ranks( g1, nranks, ranks, basegroup, ranks_out );
    for (i=0; i<size; i++) {
        if (ranks_out[i] != (size - 1) - i) {
            errs++;
            fprintf( stdout, "Translate ranks got %d expected %d\n", ranks_out[i], (size - 1) - i );fflush(stdout);
        }
    }
 
    /* Check Compare */
    MPI_Group_compare( basegroup, g1, &result );
    if (result != MPI_SIMILAR) {
        errs++;
        fprintf( stdout, "Group compare should have been similar, was %d\n", result );fflush(stdout);
    }
    MPI_Comm_dup( comm, &dupcomm );
    MPI_Comm_group( dupcomm, &g2 );
    MPI_Group_compare( basegroup, g2, &result );
    if (result != MPI_IDENT) {
        errs++;
        fprintf( stdout, "Group compare should have been ident, was %d\n", result );fflush(stdout);
    }
    MPI_Comm_split( comm, rank < size/2, rank, &splitcomm );
    MPI_Comm_group( splitcomm, &g3 );
    MPI_Group_compare( basegroup, g3, &result );
    if (result != MPI_UNEQUAL) {
        errs++;
        fprintf( stdout, "Group compare should have been unequal, was %d\n", result );fflush(stdout);
    }
 
    /* Build two groups that have this process and one other, but do not have the same processes */
    ranks[0] = rank;
    ranks[1] = (rank + 1) % size;
    MPI_Group_incl( basegroup, 2, ranks, &g3a );
    ranks[1] = (rank + size - 1) % size;
    MPI_Group_incl( basegroup, 2, ranks, &g3b );
    MPI_Group_compare( g3a, g3b, &result );
    if (result != MPI_UNEQUAL) {
        errs++;
        fprintf( stdout, "Group compare of equal sized but different groups should have been unequal, was %d\n", result );fflush(stdout);
    }

    /* Build two new groups by excluding members; use Union to put them together again */
    /* Exclude 0 */
    for (i=0; i<size; i++) ranks[i] = i;
    MPI_Group_excl( basegroup, 1, ranks, &g4 );
    /* Exclude 1-(size-1) */
    MPI_Group_excl( basegroup, size-1, ranks+1, &g5 );
    MPI_Group_union( g5, g4, &g6 );
    MPI_Group_compare( basegroup, g6, &result );
    if (result != MPI_IDENT) {
        int usize;
        errs++;
        /* See ordering requirements on union */
        fprintf( stdout, "Group excl and union did not give ident groups\n" );
        fprintf( stdout, "[%d] result of compare was %d\n", rank, result );
        MPI_Group_size( g6, &usize );
        fprintf( stdout, "Size of union is %d, should be %d\n", usize, size );
        fflush(stdout);
    }
    MPI_Group_union( basegroup, g4, &g7 );
    MPI_Group_compare( basegroup, g7, &result );
    if (result != MPI_IDENT) {
        int usize;
        errs++;
        fprintf( stdout, "Group union of overlapping groups failed\n" );
        fprintf( stdout, "[%d] result of compare was %d\n", rank, result );
        MPI_Group_size( g7, &usize );
        fprintf( stdout, "Size of union is %d, should be %d\n", usize, size );
        fflush(stdout);
    }
 
    /* Use range_excl instead of ranks */
    /* printf ("range excl\n" ); fflush( stdout ); */
    range[0][0] = 1;
    range[0][1] = size-1;
    range[0][2] = 1;
    MPI_Group_range_excl( basegroup, 1, range, &g8 );
    /* printf( "out of range excl\n" ); fflush( stdout ); */
    MPI_Group_compare( g5, g8, &result );
    /* printf( "out of compare\n" ); fflush( stdout ); */
    if (result != MPI_IDENT) {
        errs++;
        fprintf( stdout, "Group range excl did not give ident groups\n" );
    }
 
    /* printf( "intersection\n" ); fflush( stdout ); */
    MPI_Group_intersection( basegroup, g4, &g9 );
    MPI_Group_compare( g9, g4, &result );
    if (result != MPI_IDENT) {
        errs++;
        fprintf( stdout, "Group intersection did not give ident groups\n" );
    }
 
    /* Exclude EVERYTHING and check against MPI_GROUP_EMPTY */
    /* printf( "range excl all\n" ); fflush( stdout ); */
    range[0][0] = 0;
    range[0][1] = size-1;
    range[0][2] = 1;
    MPI_Group_range_excl( basegroup, 1, range, &g10 );
 
    MPI_Group_compare( g10, MPI_GROUP_EMPTY, &result );
 
    if (result != MPI_IDENT) {
        errs++;
        fprintf( stdout, "MPI_GROUP_EMPTY didn't compare against empty group\n");fflush(stdout);
    }
 
    MPI_Group_free( &basegroup );
    MPI_Group_free( &g1 );
    MPI_Group_free( &g2 );
    MPI_Group_free( &g3 );
    MPI_Group_free( &g3a );
    MPI_Group_free( &g3b );
    MPI_Group_free( &g4 );
    MPI_Group_free( &g5 );
    MPI_Group_free( &g6 );
    MPI_Group_free( &g7 );
    MPI_Group_free( &g8 );
    MPI_Group_free( &g9 );
    MPI_Group_free( &g10 );
    MPI_Comm_free( &dupcomm );
    MPI_Comm_free( &splitcomm );
    MPI_Comm_free( &newcomm );
 
    MPI_Finalize();
    return errs;
}