Create a new attribute key


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

Copy callback function for keyval
Delete callback function for keyval
Extra state for callback functions

Output Parameters

key value for future access (integer)


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

Default copy and delete functions are available. These are

empty copy function
empty delete function
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.


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.

No error; MPI routine completed successfully.

See Also MPI_Comm_free_keyval