.. _mpi_comm_create_keyval:

MPI_Comm_create_keyval
======================

.. include_body

:ref:`MPI_Comm_create_keyval` - Generates a new attribute key.

Syntax
------

C Syntax
^^^^^^^^

.. code:: c

   #include <mpi.h>

   int MPI_Comm_create_keyval(MPI_Comm_copy_attr_function
       *comm_copy_attr_fn, MPI_Comm_delete_attr_function
       *comm_delete_attr_fn, int *comm_keyval,
       void *extra_state)

Fortran Syntax (See Fortran 77 Notes)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code:: fortran

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

   MPI_COMM_CREATE_KEYVAL(COMM_COPY_ATTR_FN, COMM_DELETE_ATTR_FN,
       COMM_KEYVAL, EXTRA_STATE, IERROR)
       EXTERNAL COMM_COPY_ATTR_FN, COMM_DELETE_ATTR_FN
       INTEGER COMM_KEYVAL, IERROR

       INTEGER(KIND=MPI_ADDRESS_KIND) EXTRA_STATE

Fortran 2008 Syntax
^^^^^^^^^^^^^^^^^^^

.. code:: fortran

   USE mpi_f08

   MPI_Comm_create_keyval(comm_copy_attr_fn, comm_delete_attr_fn, comm_keyval,
           extra_state, ierror)
       PROCEDURE(MPI_Comm_copy_attr_function) :: comm_copy_attr_fn
       PROCEDURE(MPI_Comm_delete_attr_function) :: comm_delete_attr_fn
       INTEGER, INTENT(OUT) :: comm_keyval
       INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: extra_state
       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

Input Parameters
----------------

-  ``comm_copy_attr_fn`` : Copy callback function for ``comm_keyval``
   (function).
-  ``comm_delete_attr_fn`` : Delete callback function for
   ``comm_keyval`` (function).
-  ``extra_state`` : Extra state for callback functions.

Output Parameter
----------------

-  ``comm_keyval`` : Key value for future access (integer).
-  ``IERROR`` : Fortran only: Error status (integer).

Description
-----------

This function replaces :ref:`MPI_Keyval_create`, the use of which is
deprecated. The C binding is identical. The Fortran binding differs in
that ``extra_state`` is an address-sized integer. Also, the copy and
delete callback functions have Fortran bindings that are consistent with
address-sized attributes. The argument ``comm_copy_attr_fn`` may be
specified as MPI_COMM_NULL_COPY_FN or MPI_COMM_DUP_FN from C or
Fortran. MPI_COMM_NULL_COPY_FN is a function that does nothing more
than returning ``flag = 0`` and MPI_SUCCESS. MPI_COMM_DUP_FN is
a simple-minded copy function that sets ``flag = 1``, returns the value
of ``attribute_val_in`` in ``attribute_val_out``, and returns
MPI_SUCCESS. These replace the MPI-1 predefined callbacks
MPI_NULL_COPY_FN and MPI_DUP_FN, the use of which is deprecated.
The two C callback functions are:

.. code:: c

   typedef int MPI_Comm_copy_attr_function(MPI_Comm oldcomm, int comm_keyval,
                void *extra_state, void *attribute_val_in,
                void *attribute_val_out, int *flag);

   typedef int MPI_Comm_delete_attr_function(MPI_Comm comm, int comm_keyval,
                void *attribute_val, void *extra_state);

which are the same as the MPI-1.1 calls but with a new name. The old
names are deprecated. The two Fortran callback functions are:

.. code:: fortran

   SUBROUTINE COMM_COPY_ATTR_FN(OLDCOMM, COMM_KEYVAL, EXTRA_STATE,
                ATTRIBUTE_VAL_IN, ATTRIBUTE_VAL_OUT, FLAG, IERROR)
       INTEGER OLDCOMM, COMM_KEYVAL, IERROR

       INTEGER(KIND=MPI_ADDRESS_KIND) EXTRA_STATE, ATTRIBUTE_VAL_IN,
           ATTRIBUTE_VAL_OUT
       LOGICAL FLAG

   SUBROUTINE COMM_DELETE_ATTR_FN(COMM, COMM_KEYVAL, ATTRIBUTE_VAL, EXTRA_STATE,
                IERROR)
       INTEGER COMM, COMM_KEYVAL, IERROR

       INTEGER(KIND=MPI_ADDRESS_KIND) ATTRIBUTE_VAL, EXTRA_STATE

Fortran 77 Notes
^^^^^^^^^^^^^^^^

The MPI standard prescribes portable Fortran syntax for the
``EXTRA_STATE`` argument only for Fortran 90. FORTRAN 77 users may use
the non-portable syntax

.. code:: fortran

   INTEGER*MPI_ADDRESS_KIND EXTRA_STATE

where ``MPI_ADDRESS_KIND`` is a constant defined in mpif.h and gives the
length of the declared integer in bytes.

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
:ref:`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 the MPI man page for a full list of MPI error codes.


.. seealso:: :ref:`MPI`
