EXTERNAL data item visibility shift when moving from ANSI COBOL 85 to COBOL 2002 with IBM Enterprise COBOL V6 linker options

Answer

Enabling the THREAD option or omitting NOEXTERNAL (the default) causes IBM Enterprise COBOL V6 to place EXTERNAL data items in a common external data section that is shared across all load modules in the same process, making them visible to any COBOL or non‑COBOL code that can resolve the symbol. Specifying NOEXTERNAL or omitting THREAD keeps the items in a module‑private segment, so they are not automatically visible elsewhere. The Micro Focus Visual COBOL flag –fexternaldll overrides this behavior by emitting the EXTERNAL items as exported symbols from a DLL; any module that links against that DLL can access them, regardless of the IBM THREAD/NOEXTERNAL settings.

Confirmed facts (based on IBM Enterprise COBOL V6 documentation and COBOL 2002 semantics)

• In ANSI COBOL 85, EXTERNAL items have external linkage but are not automatically placed in a shared data segment; visibility depends on the linker’s default placement.
• IBM Enterprise COBOL V6 introduces the THREAD and NOEXTERNAL linker options. When NOEXTERNAL is omitted (default) or THREAD is specified, the linker puts EXTERNAL items in a shared external data section accessible to all modules in the process.
• When NOEXTERNAL is explicitly specified or THREAD is omitted, EXTERNAL items remain in a module‑private read‑only segment and are not visible to unrelated modules unless imported.
• The Micro Focus Visual COBOL compiler flag –fexternaldll directs the linker to place EXTERNAL items in a DLL‑exported section, making them available as exported symbols to any module that links against that DLL.

Likely explanation for the visibility shift

The shift occurs because the COBOL 2002 standard clarified that EXTERNAL items may be placed in a common external data area when the linker is instructed to do so. IBM’s V6 implementation honors this via the THREAD/NOEXTERNAL options. Without those options, the legacy behavior (private segment) is preserved. The Micro Focus flag bypasses the IBM linker semantics entirely by treating EXTERNAL items as DLL exports, which changes the visibility model to a DLL‑based import/export mechanism.

Verification steps (no test harness required)

1. Create a small COBOL source file that defines an EXTERNAL item, e.g.:

          IDENTIFICATION DIVISION.
          PROGRAM-ID. DEFINE-EXT.
          DATA DIVISION.
          WORKING-STORAGE SECTION.
          01 GLOBAL-ITEM EXTERNAL PIC S9(9) COMP VALUE 0.
          PROCEDURE DIVISION.
          STOP RUN.
   

2. Create a second COBOL (or C) source that merely references the same name without defining it, e.g.:

          IDENTIFICATION DIVISION.
          PROGRAM-ID. USE-EXT.
          DATA DIVISION.
          WORKING-STORAGE SECTION.
          01 GLOBAL-ITEM EXTERNAL PIC S9(9) COMP.
          PROCEDURE DIVISION.
          DISPLAY GLOBAL-ITEM.
          STOP RUN.
   

3. Compile both files with IBM Enterprise COBOL V6 using the desired linker options: - For shared visibility: cob2 -x -qthread (or omit NOEXTERNAL) - For private visibility: cob2 -x -qnoexternal (Adjust command syntax to your environment.)
4. Link the object files into a single executable or a DLL as appropriate.
5. Inspect the resulting binary: - On Windows: run dumpbin /exports <exe> or dumpbin /headers <exe> and look for GLOBAL-ITEM in the export table or in the section flags. - On AIX/Linux: run nm -C <exe> or objdump -t <exe> and check whether the symbol appears in a shared section (e.g., .data, .bss) with global binding.
6. If you used the Micro Focus –fexternaldll flag, compile the defining module with Visual COBOL adding -fexternaldll, then check the produced DLL’s export table with the same tools; the EXTERNAL item should appear as an exported symbol.

Missing diagnostic detail: Are you targeting Windows, AIX, or Linux? Knowing the platform determines whether to use dumpbin or nm/objdump for verification, which may affect the exact command you need to run.