S0C4 protection exception

S0C4 is a protection exception: the program tried to read or write a storage address it does not own. It usually means a wild subscript, pointer or branch.

What it means

• S0C4 (protection exception) means the program touched memory outside its own region.
• In COBOL the usual suspects are a table subscript out of range or an uninitialized pointer.
• It can also come from branching to a bad address after storage gets overlaid.

Common causes

• OCCURS subscript larger than the table size, or zero/negative.
• Reference modification with a start position past the end of the field.
• Uninitialized pointer used in SET ADDRESS or CALL.
• Storage overlay: a previous MOVE wrote past a field and corrupted a save area or pointer.
• Calling a subprogram that was never loaded (bad load module).

How to diagnose

• From the dump, note the failing address and the registers; map the PSW to the compiler listing offset.
• Check every subscript and reference-modification near the failing statement.
• Look at the registers for addresses that look like data (e.g. packed fields) rather than code addresses.
• If it fails intermittently, suspect a subscript driven by input data.

How to fix it

• Bounds-check subscripts before use: IF WS-IDX > 0 AND WS-IDX <= TABLE-SIZE.
• Initialize pointers to NULL and test them before dereferencing.
• Fix the overlaying MOVE — the S0C4 is often the victim, not the culprit.
• Recompile with SSRANGE to catch subscript errors in test before production.

Example: Bounds-check every subscript

Compile test builds with the SSRANGE option to trap these before production.

Interview tip

• Q: How would you troubleshoot a S0C4 protection exception in production?
• A: I would map the failing PSW address to the compiler listing, inspect subscripts and pointers near that statement, check the dump registers for a wild address, and fix the out-of-range subscript or the storage overlay causing it.

Prevention checklist

• Always validate subscripts against the OCCURS size.
• Use SSRANGE in test compiles.
• Initialize pointers and check for NULL before use.