# Name

`MPI_Comm_dup` - Duplicates an existing communicator with all its
cached information.

# Syntax

## C Syntax

```c
#include <mpi.h>

int MPI_Comm_dup(MPI_Comm comm, MPI_Comm *newcomm)
```

## Fortran Syntax

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

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

## Fortran 2008 Syntax

```fortran
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
```

# Input Parameter

* `comm` : Communicator (handle).

# Output Parameters

* `newcomm` : Copy of comm (handle).
* `IERROR` : Fortran only: Error status (integer).

# 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 Section 5.7.1 of the MPI-1 Standard,
"Functionality").

# 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 Chapter 6, "Process Topologies," in the MPI-1
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.

# 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_dup_with_info`(3)](MPI_Comm_dup_with_info.html)
[`MPI_Comm_idup`(3)](MPI_Comm_idup.html)