Low Priority! Environment variable support

Stefan, George,

Recursive expansion requires recursive code. You don't like writing such code, George - you know it and I know it. Besides, the danger of recursive symbol expansion is looping, like (pseudo code):

%define %ABC = %ABC
CHANGE XYZ %ABC

Errors like this CAN be detected at run time, but it requires a "results stack" that must be checked to avoid getting stuck in a loop (and, "loop" means "crash"). That careful code is needed to avoid trickier problems like:

%define %ABC = %DEF
%define %DEF = %ABC
CHANGE XYZ %ABC

Stefan, my recommendation is to create user environment variables at startup time, with some small utility of your own. That way, at every reboot, the most current variables' values are obtained to create your own 'special' variables. Then, you only need single-level expansion.

R

p.s. IBM compilers and assemblers generally allow nested COPY statements, but they typically limit the nesting to some maximum level. Once they reach the max, the COPY is halted. This is an easy way to prevent recursive copying errors like:

member ABC:
      COPY ABC

This is one way to prevent recursion errors, by limiting the number of recursion levels to a hard limit, without having to maintain and check a results stack.

Sorry, I meant no offense, I just seem to recall you disliking the idea of recursion.

RENT routines mainly add the RENT attribute to the object file, and for assembler, it does a small amount of checking to try to see if you violated RENT by storing something into the current object file. It also is used as a cross-check for PL/1 programs that have OPTIONS (MAIN, REENTRANT) in their high level procedure.

And yes, a very small example like this is easy to see it's wrong, but the problem comes when you have many definitions that have been edited over and over, and you didn't realize you made a mistake.

I believe the reason to use RENT in assembler was not that the code was going to call itself, but that other programs (maybe other tasks or threads in CICS, possibly) might call the same code, so you had to make sure you didn't modify any memory in the module itself, because it could get restarted in another thread before the first one was finished. Real reentrant code is rarely used.

R

Stefan: Are symbols to be resolved when 1st entered, or are they to remain in their native form and only resolved when actually put in play?

Big difference.

George

Yes, there are several ways to simulate recursion using loops and arrays, this is one of them. Most simulation techniques have much higher overhead than true recursion does, but the strings you are processing are short. I am sure you could convert this logic to PB.

R