The Great and Terrible implementation of MPI-2

function index


Creates an intercommuncator from two intracommunicators
int MPI_Intercomm_create(
  MPI_Comm local_comm,
  int local_leader,
  MPI_Comm peer_comm,
  int remote_leader,
  int tag,
  MPI_Comm *newintercomm


[in] Local (intra)communicator
[in] Rank in local_comm of leader (often 0)
[in] Communicator used to communicate between a designated process in the other communicator. Significant only at the process in local_comm with rank local_leader.
[in] Rank in peer_comm of remote leader (often 0)
[in] Message tag to use in constructing intercommunicator; if multiple MPI_Intercomm_creates are being made, they should use different tags (more precisely, ensure that the local and remote leaders are using different tags for each MPI_intercomm_create).
[out] Created intercommunicator


This call creates an inter-communicator. It is collective over the union of the local and remote groups. Processes should provide identical local_comm and local_leader arguments within each group. Wildcards are not permitted for remote_leader, local_leader, and tag.

This call uses point-to-point communication with communicator peer_comm, and with tag tag between the leaders. Thus, care must be taken that there be no pending communication on peer_comm that could interfere with this communication.

peer_comm is significant only for the process designated the local_leader in the local_comm.

The MPI 1.1 Standard contains two mutually exclusive comments on the input intracommunicators. One says that their repective groups must be disjoint; the other that the leaders can be the same process. After some discussion by the MPI Forum, it has been decided that the groups must be disjoint. Note that the reason given for this in the standard is not the reason for this choice; rather, the other operations on intercommunicators (like MPI_Intercomm_merge) do not make sense if the groups are not disjoint.

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).
Invalid tag argument. Tags must be non-negative; tags in a receive (MPI_Recv, MPI_Irecv, MPI_Sendrecv, etc.) may also be MPI_ANY_TAG. The largest tag value is available through the the attribute MPI_TAG_UB.
This error is returned when some part of the MPICH implementation is unable to acquire memory.
Invalid source or destination rank. Ranks must be between zero and the size of the communicator minus one; ranks in a receive (MPI_Recv, MPI_Irecv, MPI_Sendrecv, etc.) may also be MPI_ANY_SOURCE.

See Also

MPI_Intercomm_merge, MPI_Comm_free, MPI_Comm_remote_group,

Example Code

The following sample code illustrates MPI_Intercomm_create.

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

#define BUFSIZE 2000

main( int argc, char *argv[] )
    MPI_Status status;
    MPI_Comm comm,scomm;
int a[10], b[10];
int buf[BUFSIZE], *bptr, bl, i, j, rank, size, color, errs=0;

    MPI_Init( 0, 0 );
    MPI_Comm_rank( MPI_COMM_WORLD, &rank );
    color = rank % 2;
    MPI_Comm_split( MPI_COMM_WORLD, color, rank, &scomm );
    MPI_Intercomm_create( scomm, 0, MPI_COMM_WORLD, 1-color, 52, &comm);
    MPI_Comm_rank( comm, &rank );
    MPI_Comm_remote_size( comm, &size );
    MPI_Buffer_attach( buf, BUFSIZE );

    for (j=0; j<10; j++) {
for (i=0; i<10; i++) {
            a[i] = (rank + 10 * j) * size + i;
        MPI_Bsend( a, 10, MPI_INT, 0, 27+j, comm );
if (rank == 0) {
        for (i=0; i<size; i++) {
for (j=0; j<10; j++) {
int k;
                status.MPI_TAG = -10;
                status.MPI_SOURCE = -20;
                MPI_Recv( b, 10, MPI_INT, i, 27+j, comm, &status );

                if (status.MPI_TAG != 27+j) {
                    printf( "Wrong tag = %d\n", status.MPI_TAG );fflush(stdout);
if (status.MPI_SOURCE != i) {
                    printf( "Wrong source = %d\n", status.MPI_SOURCE );fflush(stdout);
for (k=0; k<10; k++) {
if (b[k] != (i + 10 * j) * size + k) {
                        printf( "received b[%d] = %d from %d tag %d\n", k, b[k], i, 27+j );fflush(stdout);
    MPI_Buffer_detach( &bptr, &bl );
return errs;