Still strange GC problems with OCaml and C: OCaml 2.04 bug?

From: David Mentré (David.Mentre@irisa.fr)
Date: Mon Feb 21 2000 - 15:09:30 MET

  • Next message: David Mentré: "Re: Calling C from OCaml, GC problems"

    Hi all Camlists,

    I'm still fighting against my bug while trying to interface OCaml with
    CMU bdd library (in C).

    Following Xavier comments, I've protected my values (with CAMLparamXX
    and CAMLlocalXX macros).

    Actual code is available at:
      http://www.irisa.fr/paris/pages-perso/David-Mentre/bdd.ml
      http://www.irisa.fr/paris/pages-perso/David-Mentre/bdd_interface.c

     1. I still obtain a segfault, however, the gdb backtrace has changed:
    (gdb) bt
    #0 0x80655b5 in mark_slice ()
    #1 0x80659eb in major_collection_slice ()
    #2 0x8065fd3 in minor_collection ()
    #3 0x8063859 in interprete ()
    #4 0x8065043 in caml_main ()
    #5 0x805ab45 in main ()
    #6 0x2ab457e2 in __libc_start_main () from /lib/libc.so.6

        It is still a GC bug, however the problem no longer arises while our
        C interface allocates CAML memory. So I think the bug has moved (or
        this is another bug).

     2. More interestingly, by compiling this code with OCaml 2.99 under
        sun4 architecture, this bug no longer occurs. I obtain in place a:

    Fatal error: uncaught exception Invalid_argument("Array.get")

        So, it would be possible that some Invalid_argument exceptions are
        not properly caught in OCaml 2.04.

        For CAML team, I was not able to produce a small/simple example that
        can trigger this bug. Only my gas-plant program seems to
        segfault. Sorry.

     3. I've found an interesting message on Damien Doliguez site[1]:

    http://pauillac.inria.fr/~doligez/caml-guts/Fahndrich99.txt

        This message tells that you can have strange CAML GC related
        problems while deallocating C structures. However, I think my code
        is in case 2 of Manuel proposed fixes (i.e. box every C pointer
        inside an abstract CAML block). So, in my opinion, my bug is not
        related to this problem. But, as am I paranoid now :), I've set
        pointers to 1 after deallocating them. This not fixed the bug.

    Anyway, if somebody has an advice on how to track down this bug, I'll
    glady accept it.

    David -- once happy in the Caml-only world

    [1]http://pauillac.inria.fr/~doligez/caml-guts/

    -- 
     David.Mentre@irisa.fr -- http://www.irisa.fr/prive/dmentre/
     Opinions expressed here are only mine.
    



    This archive was generated by hypermail 2b29 : Mon Feb 21 2000 - 18:28:58 MET