main(int argc, char **argv)
{
int me, count, count2;
void *send_buf, *recv_buf, *send_buf2, *recv_buf2;
MPI_Group MPI_GROUP_WORLD, grprem;
MPI_Comm commslave;
static int ranks[] = {0};
...
MPI_Init(&argc, &argv);
MPI_Comm_group(MPI_COMM_WORLD, &MPI_GROUP_WORLD);
MPI_Comm_rank(MPI_COMM_WORLD, &me); /* local */
MPI_Group_excl(MPI_GROUP_WORLD, 1, ranks, &grprem); /* local */
MPI_Comm_create(MPI_COMM_WORLD, grprem, &commslave);
if(me != 0)
{
/* compute on slave */
...
MPI_Reduce(send_buf,recv_buff,count, MPI_INT, MPI_SUM, 1, commslave);
...
}
/* zero falls through immediately to this reduce, others do later... */
MPI_Reduce(send_buf2, recv_buff2, count2,
MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD);
MPI_Comm_free(&commslave);
MPI_Group_free(&MPI_GROUP_WORLD);
MPI_Group_free(&grprem);
MPI_Finalize();
}
This example illustrates how a group consisting of all but the zeroth
process of the ``all'' group is created, and then how a communicator
is formed ( commslave) for that new group. The new communicator is used in
a collective call, and all processes execute a collective call
in the MPI_COMM_WORLD context. This example illustrates
how the two communicators (that inherently possess distinct contexts) protect
communication. That is, communication in MPI_COMM_WORLD is
insulated from communication in commslave, and vice versa.
In summary, ``group safety'' is achieved via communicators because distinct contexts within communicators are enforced to be unique on any process.