int MPI_Group_union(
  MPI_Group group1,
  MPI_Group group2,
  MPI_Group *newgroup
);

Parameters

group1

[in] first group (handle)

group2

[in] second group (handle)

newgroup

[out] union group (handle)

Remarks

Produces a group by combining two groups.

Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe.

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_INTERN

This error is returned when some part of the MPICH implementation is unable to acquire memory.

See Also

Example Code

The following sample code illustrates MPI_Group_union.

/*
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;
}