openmpi/ompi/mpi/man/man3/MPI_Comm_create_group.md

Name

MPI_Comm_create_group - Creates a new communicator.

Syntax

C Syntax

#include <mpi.h>

int MPI_Comm_create_group(MPI_Comm comm, MPI_Group group, int tag, MPI_Comm *newcomm)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'

MPI_COMM_CREATE_GROUP(COMM, GROUP, TAG, NEWCOMM, IERROR)
    INTEGER	COMM, GROUP, TAG, NEWCOMM, IERROR

Fortran 2008 Syntax

USE mpi_f08

MPI_Comm_create_group(comm, group, tag, newcomm, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    TYPE(MPI_Group), INTENT(IN) :: group
    INTEGER, INTENT(IN) :: tag
    TYPE(MPI_Comm), INTENT(OUT) :: newcomm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

Input Parameters

• comm : Communicator (handle).
• group : Group, which is a subset of the group of comm (handle).
• tag : Tag (integer).

Output Parameters

• newcomm : New communicator (handle).
• IERROR : Fortran only: Error status (integer).

Description

MPI_Comm_create_group is similar to MPI_Comm_create; however, MPI_Comm_create must be called by all processes in the group of comm, whereas MPI_Comm_create_group must be called by all processes in group, which is a subgroup of the group of comm. In addition, MPI_Comm_create_group requires that comm is an intracommunicator. MPI_Comm_create_group returns a new intracommunicator, newcomm, for which the group argument defines the communication group. No cached information propagates from comm to newcomm. Each process must provide a group argument that is a subgroup of the group associated with comm; this could be MPI_GROUP_EMPTY. If a non-empty group is specified, then all processes in that group must call the function, and each of these processes must provide the same arguments, including a group that contains the same members with the same ordering. Otherwise the call is erroneous. If the calling process is a member of the group given as the group argument, then newcomm is a communicator with group as its associated group. If the calling process is not a member of group, e.g., group is MPI_GROUP_EMPTY, then the call is a local operation and MPI_COMM_NULL is returned as newcomm.

Notes

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

Errors

Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

See Also

MPI_Comm_create(3)