The Great and Terrible implementation of MPI-2

function index


Accesses the group associated with given communicator
int MPI_Comm_group(
  MPI_Comm comm,
  MPI_Group *group


[in] Communicator (handle)
[out] Group in communicator (handle)


MPI_COMM_GROUP returns in group a handle to the group of comm.

Because MPI specifies that null objects (e.g., MPI_COMM_NULL) are invalid as input to MPI routines unless otherwise specified, using MPI_COMM_NULL as input to this routine is an error.

Group constructors are used to subset and superset existing groups. These constructors construct new groups from existing groups. These are local operations, and distinct groups may be defined on different processes; a process may also define a group that does not include itself. Consistent definitions are required when groups are used as arguments in communicator-building functions. MPI does not provide a mechanism to build a group from scratch, but only from other, previously defined groups. The base group, upon which all other groups are defined, is the group associated with the initial communicator MPI_COMM_WORLD (accessible through the function MPI_COMM_GROUP).


In what follows, there is no group duplication function analogous to MPI_COMM_DUP. There is no need for a group duplicator. A group, once created, can have several references to it by making copies of the handle. The following constructors address the need for subsets and supersets of existing 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 routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

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


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.

No error; MPI routine completed successfully.
Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank).

Example Code

The following sample code illustrates MPI_Comm_group.

#include <mpi.h>
#include <stdio.h>

int main(int argc, char* argv[] )
    MPI_Comm dup_comm_world, world_comm;
    MPI_Group world_group;
int world_rank, world_size, rank, size;

    MPI_Init(&argc, &argv);
    MPI_Comm_rank( MPI_COMM_WORLD, &world_rank );
    MPI_Comm_size( MPI_COMM_WORLD, &world_size );
    MPI_Comm_dup( MPI_COMM_WORLD, &dup_comm_world );
    /* Exercise Comm_create by creating an equivalent to dup_comm_world (sans attributes) */
    MPI_Comm_group( dup_comm_world, &world_group );
    MPI_Comm_create( dup_comm_world, world_group, &world_comm );
    MPI_Comm_rank( world_comm, &rank );
    if (rank != world_rank) {
        printf( "incorrect rank in world comm: %d\n", rank );fflush(stdout);
        MPI_Abort(MPI_COMM_WORLD, 3001 );
    return 0;