Stack overflow reported as segfault (?)

Learning
1

I believe that sometimes a stack overflow is reported as a segfault in native compiled ocaml code. (Is this right?)

Is this inevitable? Or is it possible to somehow to report it always as a proper stack overflow?

2

I think it will get reported as a segfault rather than an exception if it happens in C code (called from OCaml code). In that case, it is kind of inevitable.

3

If you’re on an ARM64 processor (like the new Macs), then reliable stack overflow detection was only added in OCaml 4.13.
On some other systems, there is a bug with stack overflow recovery that could cause segfaults after a stack overflow. This has been fixed, but not released yet.

It would be helpful to know whether your case matches one of these situations or is a different issue. Could you provide some details on your setup (OCaml version, OS, processor architecture, reproduction steps if possible) ?

4

Thanks for the replies. I am on ocaml4.12.0, Ubuntu 20.04 (kubuntu actually), x86-64. I don’t have a repro, but the error happens when a long running system encounters a very large list and tries to do List.map (or similar). If the list is made large to begin with, there is a stack overflow which is reported as such. But in the long running code, it appears as a segfault. And fixing the List.map (to a rev_map or similar) fixes the segfault, so I’m fairly sure the underlying bug is actually a stack overflow.

5

OK, so for C code, a stack overflow is always reported as a segfault? (I know very little about C.)

6

A similar issue on github, resolved in ocaml trunk: Segmentation fault on stack overflow - OCaml 4.12, Linux · Issue #10645 · ocaml/ocaml · GitHub

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7

As far as I know, yes. So, if your failing List.map happens to (indirectly) call a native function written in C (e.g., a system call or a foreign function), then OCaml might not be able to recover from it.

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8

The long-term goal is that stack overflows from OCaml code are properly reported as Stack_overflow exceptions, and stack overflows from C code are (1) avoided if at all possible in the OCaml run-time system, and (2) reported as SEGV signals otherwise. But catching stack overflows is tricky, both in Unix-like systems and under Windows, so the current implementation in OCaml is a best effort that is occasionally buggy.

So, with native code compilation, never rely on stack overflows being turned into exceptions. Just treat the Stack_overflow as a debugging aid.

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