Using user-defined functions
COBOL user-defined functions allow you to write your own function definitions that may be
invoked by using the FUNCTION keyword. User-defined functions are always recursive
and may invoke themselves.
User-defined functions can be invoked statically, dynamically, or via dynamic-link library (DLL). The ENTRY-INTERFACE phrase of the FUNCTION-ID paragraph determines the kind of invocation associated with the function. The CALLINTERFACE directive has no effect on the interface convention for user-defined functions.
Function pointers to user-defined functions are not supported. If a user-defined
function is invoked via the CALL statement the behavior is unpredictable.