The Great and Terrible implementation of MPI-2

function index


Creates a new communicator
int MPI_Comm_create(
  MPI_Comm comm,
  MPI_Group group,
  MPI_Comm *newcomm


[in] communicator (handle)
[in] group, which is a subset of the group of comm (handle)
[out] new communicator (handle)


This function creates a new communicator newcomm with communication group defined by group and a new context. No cached information propagates from comm to newcomm. The function returns MPI_COMM_NULL to processes that are not in group. The call is erroneous if not all group arguments have the same value, or if group is not a subset of the group associated with comm. Note that the call is to be executed by all processes in comm, even if they do not belong to the new group. This call applies only to intra-communicators.


The requirement that the entire group of comm participate in the call stems from the following considerations:

  • It allows the implementation to layer MPI_COMM_CREATE on top of regular collective communications.
  • It provides additional safety, in particular in the case where partially overlapping groups are used to create new communicators.
  • It permits implementations sometimes to avoid communication related to context creation.
Advice to users.

MPI_COMM_CREATE provides a means to subset a group of processes for the purpose of separate MIMD computation, with separate communication space. newcomm, which emerges from MPI_COMM_CREATE can be used in subsequent calls to MPI_COMM_CREATE (or other communicator constructors) further to subdivide a computation into parallel sub-computations. A more general service is provided by MPI_COMM_SPLIT.

If comm_in is an intercommunicator, then the output communicator is also an intercommunicator where the local group consists only of those processes contained in group. The group argument should only contain those processes in the local group of the input intercommunicator that are to be a part of comm_out. If either group does not specify at least one process in the local group of the intercommunicator, or if the calling process is not included in the group, MPI_COMM_NULL is returned.

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).
Null or invalid group passed to function.

See Also


Example Code

The following sample code illustrates MPI_Comm_create.

#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;