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MPI_Comm_create_group(3) Open MPI MPI_Comm_create_group(3)

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_cre-
ate_group requires that comm is an intracommunicator. MPI_Comm_cre-
ate_group returns a new intracommunicator, newcomm, for which the group
argument defines the communication group. No cached information propa-
gates 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 argu-
ments, 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 com-
municator 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 communica-
tion space. newcomm, which is created by MPI_Comm_create_group, can be
used in subsequent calls to MPI_Comm_create_group (or other communica-
tor 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.