In JCL for a COBOL application, how do you define the files that are referenced only inside a called subroutine rather than in the main program?

Introduction / Context:
In COBOL applications, both main programs and called subroutines can perform file I O. On the mainframe, actual physical data sets are associated with logical file names through JCL DD statements. A common question is how to handle files that are referenced only by a subroutine and not by the main program. This question examines how JCL defines such files and how subroutines gain access to them.

Given Data / Assumptions:

• The main program calls a COBOL subroutine using CALL.
• The subroutine has its own FILE SECTION entries and OPEN and CLOSE logic for its files.
• The physical data sets for these files must be allocated in the job step where the load module runs.
• COBOL and the operating system associate files with DD names defined in JCL.

Concept / Approach:
In z OS, DD statements in a job step define data sets that are available to all programs running in that step, including main programs and called subroutines. When a subroutine references a file with a given SELECT and ASSIGN name, the runtime resolves that name against the DD statements in the JCL. There is no special JCL requirement for subroutines; you simply ensure that the DD names match what the subroutine expects. As long as the DD is present in the step, the subroutine can open and use the file just like the main program could.

Step-by-Step Solution:
Step 1: Identify the file names and DD names used in the subroutine FILE SECTION and ENVIRONMENT DIVISION. Step 2: In the JCL step that runs the combined main and subroutine load module, allocate DD statements with those same DD names and appropriate data set attributes. Step 3: Allow the subroutine to issue OPEN, READ, WRITE, and CLOSE statements against its files, which will now resolve correctly to the JCL defined data sets. Step 4: Confirm during testing that the subroutine can access its files without requiring any special separate job or additional step definitions.

Verification / Alternative check:
Mainframe COBOL and JCL documentation state that files are bound to JCL DD statements by name, not by whether they are used in a main program or subroutine. Examples show subprograms that handle their own files and rely on DD names defined in the same step as the main program. There is no requirement to allocate those files in a separate job, which validates that a single step definition with matching DD names is sufficient.

Why Other Options Are Wrong:
Option B is wrong because subroutines do not automatically create data sets at runtime without JCL definitions. Option C incorrectly suggests that subroutine files must be defined in a separate job, which is not how COBOL and z OS work. Option D falsely claims that subroutines cannot perform file I O, which is not a language restriction. Option E ignores the requirement for JCL DD statements, which are necessary for connecting logical file names to physical data sets.

Common Pitfalls:
A common pitfall is forgetting to add DD statements for subroutine files when promoting jobs to higher environments, causing file open failures. Another issue is reusing the same DD names for different purposes in a step, leading to unexpected sharing or contention. Developers should document which DD names each module uses and coordinate with operations teams to ensure that JCL definitions match the program expectations. Correctly defining files in JCL for both main programs and subroutines leads to predictable and maintainable batch processing.

Final Answer:
You define the files in the JCL step with DD statements using the same DD names that the subroutine expects, and those DD names are available to both the main program and its called subroutines