Q. I understand how to prototype a program call, using EXTPGM and naming the program. But I want to allow the user to select an item from a subfile, then have the program use that selection to choose one from among several different possible programs to run. Is there a way to do this?

A. Yes. The EXTPGM value can be a character variable, as well as a literal or constant:

D Mypgm           PR                 Extpgm(Pgmname)
D                                5U 0 Const
D                               35    Const

D Parm1           S              5U 0
D Parm2           S             35
D Pgmname         S             10

 /Free
  Pgmname = 'AR101';
  Mypgm(Parm1:Parm2);     // Call AR101 (Implied Callp)

  Pgmname = 'PR457';
  Mypgm(Parm1:Parm2);     // Call PR457 (Implied Callp)

 /End-free

The parameter list must be consistent for any of the possible programs to be called; that is, AR101 and PR457 must accept the same parameters. But you can include the OPTIONS(*NOPASS) and/or OPTIONS(*OMIT) keywords to make some parameters optional or omissible.

The called programs (AR101 and PR457) need not be coded with prototypes, even though the calling program will use prototyping and CALLP.