# 18.2.51. MPI_Comm_create_group

MPI_Comm_create_group — Creates a new communicator.

## 18.2.51.1. SYNTAX

### 18.2.51.1.1. C Syntax

    #include <mpi.h>

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

### 18.2.51.1.2. 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

### 18.2.51.1.3. 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

## 18.2.51.2. INPUT PARAMETERS

- `comm` : Communicator (handle).

- `group` : Group, which is a subset of the group of comm (handle).

- `tag` : Tag (integer).

## 18.2.51.3. OUTPUT PARAMETERS

- `newcomm` : New communicator (handle).

- `ierror` : Fortran only: Error status (integer).

## 18.2.51.4. 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.

## 18.2.51.5. 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.

## 18.2.51.6. ERRORS

Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.

Open MPI includes three predefined error handlers that can be used:

- `MPI_ERRORS_ARE_FATAL` Causes the program to abort all connected MPI processes.

- `MPI_ERRORS_ABORT` An error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.

- `MPI_ERRORS_RETURN` Returns an error code to the application.

MPI applications can also implement their own error handlers by calling:

- MPI_Comm_create_errhandler then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

Note that MPI does not guarantee that an MPI program can continue past an error.

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

See also

- MPI_Comm_create