To understand the COBOL RENT compiler option, we first need to know what a reentrant program is.
What Is a Reentrant Program in COBOL?
A reentrant program is a program that does not modify its own code while running.
If you’ve heard of multithreading, the concept is similar—both involve sharing a single copy of program instructions across multiple users or processes.
Key idea:
A reentrant COBOL program allows multiple users to run the same program simultaneously, sharing the same program code but keeping separate dynamic storage for each user.
Three Logical Parts of a COBOL Program
We can logically divide a COBOL program into three parts:
- Program Code – The instructions executed when the program runs.
- Constants – Fixed values that never change after compilation.
- Working Storage Area – Variables that store user-specific data while the program runs.
How COBOL Handles Working Storage in Reentrant Programs
A reentrant COBOL program gives each user their own copy of the working storage area.
- For each user, MVS allocates memory dynamically using the GETMAIN and STORAGE macros.
- These copies are initialized from the program’s original storage template.
- When a user interacts with the program, they only modify their own copy of variables.
If the program is interrupted to let another user execute it, the user’s data area is saved and restored when control returns to that user.
What Happens If a CICS Program Is Not Reentrant?
If you write a CICS program as non-reentrant:
- When one transaction pauses, another transaction might overwrite the same working storage variables.
- This can cause unstable behavior, data corruption, or incorrect results.
Why Reentrancy Matters in COBOL
Reentrant programs:
- Allow a single copy of a program or routine to be shared concurrently by multiple users or processes.
- Are essential for multi-user environments like CICS, IMS, or batch systems with concurrent execution.
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