# 17.2.55. MPI_Comm_dup

MPI_Comm_dup — Duplicates an existing communicator with all its cached information.

## 17.2.55.1. SYNTAX

### 17.2.55.1.1. C Syntax

    #include <mpi.h>

    int MPI_Comm_dup(MPI_Comm comm, MPI_Comm *newcomm)

### 17.2.55.1.2. Fortran Syntax

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

    MPI_COMM_DUP(COMM, NEWCOMM, IERROR)
        INTEGER COMM, NEWCOMM, IERROR

### 17.2.55.1.3. Fortran 2008 Syntax

    USE mpi_f08

    MPI_Comm_dup(comm, newcomm, ierror)
        TYPE(MPI_Comm), INTENT(IN) :: comm
        TYPE(MPI_Comm), INTENT(OUT) :: newcomm
        INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.55.2. INPUT PARAMETER

- `comm` : Communicator (handle).

## 17.2.55.3. OUTPUT PARAMETERS

- `newcomm` : Copy of comm (handle).

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

## 17.2.55.4. DESCRIPTION

MPI_Comm_dup duplicates the existing communicator comm with associated key values. For each key value, the respective copy callback function determines the attribute value associated with this key in the new communicator; one particular action that a copy callback may take is to delete the attribute from the new communicator. Returns in newcomm a new communicator with the same group, any copied cached information, but a new context (see the “Functionality” subsection of the “Caching” section in the “Groups, Contexts, and Communicators” chapter in the MPI Standard).

## 17.2.55.5. NOTES

This operation is used to provide a parallel library call with a duplicate communication space that has the same properties as the original communicator. This includes any attributes (see below) and topologies (see the “Process Topologies” chapter in the MPI Standard). This call is valid even if there are pending point-to-point communications involving the communicator comm. A typical call might involve an MPI_Comm_dup at the beginning of the parallel call, and an MPI_Comm_free of that duplicated communicator at the end of the call. Other models of communicator management are also possible. This call applies to both intra- and intercommunicators. Note that it is not defined by the MPI standard what happens if the attribute copy callback invokes other MPI functions. In Open MPI, it is not valid for attribute copy callbacks (or any of their children) to add or delete attributes on the same object on which the attribute copy callback is being invoked.

## 17.2.55.6. 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_dup_with_info