# 17.2.52. MPI_Comm_create_keyval

MPI_Comm_create_keyval — Generates a new attribute key.

## 17.2.52.1. SYNTAX

### 17.2.52.1.1. C Syntax

    #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)

### 17.2.52.1.2. Fortran Syntax

    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

### 17.2.52.1.3. Fortran 2008 Syntax

    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

## 17.2.52.2. 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.

## 17.2.52.3. OUTPUT PARAMETER

- `comm_keyval` : Key value for future access (integer).

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

## 17.2.52.4. DESCRIPTION

This function replaces 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:

    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:

    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

## 17.2.52.5. 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_free_keyval