Basically, it gives you the ability to store a procedure reference in a pointer variable ... and then invoke that procedure in your code.
The following is a basic example of how to use a procedure pointer:
// Possible procedures to call D Addcust PR D 5U 0 Value D Updcust PR D 5U 0 Value D Delcust PR D 5U 0 Value // Prototype for generic procedure D Process PR Extproc(Processptr) D 5U 0 Value // Pointer to hold procedure address D Processptr S * Procptr D Action S 1 Inz('A') D Parameter S 5U 0 /Free Select; // Assign appropriate procedure address to pointer When Action = 'A'; Processptr = %Paddr(Addcust); When Action = 'U'; Processptr = %Paddr(Updcust); When Action = 'D'; Processptr = %Paddr(Delcust); Endsl; Process(Parameter); // Call appropriate procedure /End-free
The prototype for each procedure must be consistent with the prototype associated with the procedure pointer; that is, they must accept the same parameters. But you can include the OPTIONS(*NOPASS) and/or OPTIONS(*OMIT) keywords to make some parameters optional or omissible.
(Portions copied from http://www.bmeyers.net/2005/09/using-callp-with-a-procedure-pointer.php)