Release 10.1A: OpenEdge Development:
Programming Interfaces

Passing the Unknown value and NULL values

If you have to pass a NULL value to a shared library routine, do not pass a null MEMPTR variable. Instead, define the INPUT or INPUT-OUTPUT parameter as a LONG, and pass it 0 when you run the routine. If this conflicts with calling the shared library entry point another way, you can create a second declaration using the ORDINAL option of the PROCEDURE statement. (Note that this option is not available on UNIX.)

One exception to the rule of not passing a null MEMPTR variable is a MEMPTR array, in which you can pass a MEMPTR of size 0.

You cannot pass the Unknown value (?), regardless of the parameter’s data type.