# MPI_Comm_create_keyval

Create a new attribute key

## Synopsis

    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)

## Input Parameters

comm_copy_attr_fn
  Copy callback function for `keyval`
comm_delete_attr_fn
  Delete callback function for `keyval`
extra_state
  Extra state for callback functions

## Output Parameters

comm_keyval
  key value for future access (integer)

## Notes

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

Default copy and delete functions are available. These are

MPI_COMM_NULL_COPY_FN
  empty copy function
MPI_COMM_NULL_DELETE_FN
  empty delete function
MPI_COMM_DUP_FN
  simple dup function

There are subtle differences between C and Fortran that require that the copy_fn be written in the same language from which `MPI_Comm_create_keyval` is called. 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.

## Return value from attribute callbacks

The MPI-2 versions of the attribute callbacks should return either `MPI_SUCCESS` on success or a valid MPI error code or class on failure. The MPI standard is ambiguous on this point, but as MPI-2 provides the routines `MPI_Add_error_class` and `MPI_Add_error_code` that allow the user to define and use MPI error codes and classes.

## 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.

## 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 guarantee 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.

## See Also

MPI_Comm_free_keyval