# Name

`MPI_Comm_create_group` - Creates a new communicator.

# Syntax

## C Syntax

```c
#include <mpi.h>

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

## Fortran Syntax

```fortran
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

```fortran
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 sub`group` 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)](MPI_Comm_create.html)