---
name: "MPI_Comm_create_group.openmpi"
section: "3"
platform: "linux"
package: "openmpi-doc"
version: "5.0.7-1"
release: "Debian 13 (Trixie)"
platforms: ["linux"]
url: "https://new.carta.tech/MPI_Comm_create_group.openmpi"
---

# MPI\_Comm\_create\_group.openmpi(3)

*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 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*

## COPYRIGHT

2003-2025, The Open MPI Community