# MPI_Keyval_create

Greates a new attribute key

## Synopsis

    int MPI_Keyval_create(MPI_Copy_function * copy_fn,
                          MPI_Delete_function * delete_fn, int *keyval, void *extra_state)

## Input Parameters

copy_fn
  Copy callback function for `keyval`
delete_fn
  Delete callback function for `keyval`
extra_state
  Extra state for callback functions

## Output Parameters

keyval
  key value for future access (integer)

## Notes

Key values are global (available for any and all communicators).

There are subtle differences between C and Fortran that require that the copy_fn be written in the same language that `MPI_Keyval_create` is called from. This should not be a problem for most users; only programmers using both Fortran and C in the same program need to be sure that they follow this rule.

## Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as `malloc` or other non-MPICH runtime routines that are themselves not interrupt-safe.

## Deprecated Function

The MPI-2 standard deprecated a number of routines because MPI-2 provides better versions. This routine is one of those that was deprecated. The routine may continue to be used, but new code should use the replacement routine. The replacement for this routine is `MPI_Comm_create_keyval`.

## Notes for Fortran

All MPI routines in Fortran (except for `MPI_WTIME` and `MPI_WTICK`) have an additional argument `ierr` at the end of the argument list. `ierr` is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the `call` statement.

All MPI objects (e.g., `MPI_Datatype`, `MPI_Comm`) are of type `INTEGER` in Fortran.

## Errors

All MPI routines (except `MPI_Wtime` and `MPI_Wtick`) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed with `MPI_Comm_set_errhandler` (for communicators), `MPI_File_set_errhandler` (for files), and `MPI_Win_set_errhandler` (for RMA windows). The MPI-1 routine `MPI_Errhandler_set` may be used but its use is deprecated. The predefined error handler `MPI_ERRORS_RETURN` may be used to cause error values to be returned. Note that MPI does not guarentee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS
  No error; MPI routine completed successfully.

MPI_ERR_INTERN
  This error is returned when some part of the MPICH implementation is unable to acquire memory.

MPI_ERR_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).

## See Also

MPI_Keyval_free, MPI_Comm_create_keyval