The Call Stack

The call stack is a list of call stack entries, in a last-in-first-out (LIFO) order. A call stack entry is a call to a program or procedure. There is one call stack per job.

When an ILE program is called, the program entry procedure is first added to the call stack. The system then automatically performs a procedure call, and the associated user's procedure (the main procedure) is added. When a procedure is called, only the user's procedure (a main procedure or subprocedure) is added; there is no overhead of a program entry procedure.

Figure 1 shows a call stack for an application consisting of an OPM program which calls an ILE program. The RPG main procedure of the ILE program calls an RPG subprocedure, which in turn calls a C procedure. Note that in the diagrams in this book, the most recent entry is at the bottom of the stack.

Figure 1. Program and Procedure Calls on the Call Stack
Program and Procedure Calls on the Call Stack
Note: In a program call, the calls to the program entry procedure and the user entry procedure (UEP) occur together, since the call to the UEP is automatic. Therefore, from now on, the two steps of a program call will be combined in later diagrams involving the call stack in this and remaining chapters.