> I'm having trouble incorporating multithreading support into my OCaml-Win32
> library.  Basically, any program that uses OCaml threading and also invokes
> a Win32 message loop will crash after a few seconds with an illegal attempt
> to access memory at "0x00000001".  This behavior is demonstrated by the
> enclosed sample program, which is very simple and does not require my
> OCaml-Win32 library.
> I don't really know how to investigate this problem further.  I've
> determined that the problem goes away if I disable the tick thread

Several users reported similar problems with threads under Windows in
OCaml 3.04.  After investigation, I found that I managed to break the
timer-based preemption while trying to work around another issue in
the Windows port.  

If you're willing to recompile from sources, here is the fix: in
byterun/signals.c, replace

  CAMLexport void leave_blocking_section(void)
  {
  #ifdef _WIN32
    /* Under Win32, asynchronous signals such as ctrl-C are not processed
       immediately (see ctrl_handler in win32.c), but simply set
       pending_signal and let the system call run to completion.
       Hence, test pending_signal here and act upon it, before we get
       a chance to process the result of the system call. */
    int signal_number = pending_signal;
    pending_signal = 0;
    if (signal_number) execute_signal(signal_number, 1);
  #endif
    if (leave_blocking_section_hook != NULL) leave_blocking_section_hook();
    Assert(async_signal_mode);
    async_signal_mode = 0;
  }

by

  CAMLexport void leave_blocking_section(void)
  {
  #ifdef _WIN32
    int signal_number;
  #endif

    if (leave_blocking_section_hook != NULL) leave_blocking_section_hook();
  #ifdef _WIN32
    /* Under Win32, asynchronous signals such as ctrl-C are not processed
       immediately (see ctrl_handler in win32.c), but simply set
       pending_signal and let the system call run to completion.
       Hence, test pending_signal here and act upon it, before we get
       a chance to process the result of the system call. */
    signal_number = pending_signal;
    pending_signal = 0;
    if (signal_number) execute_signal(signal_number, 1);
  #endif
    Assert(async_signal_mode);
    async_signal_mode = 0;
  }

Hope this helps,

- Xavier Leroy
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