Re: [Caml-list] Using C threads

[Niki Yoshiuchi <aplusbi@gmail.com>]

I figured it out so I'm posting it here for reference.
caml_c_thread_register() attempts to get the master lock, which
involves dereferencing a pointer.  That pointer is initialized by the
Thread module.  However if the OCaml code doesn't use the Thread
module that pointer is never initialized resulting in a segfault.  You
can force the initialization with something as simple as:

    ignore (Thread.self ());

Additionally, if your OCaml code doesn't use any threads (or call any
c code with a call to caml_enter_blocking_section()) then it appears
that the master lock is never released by the main thread, so the C
thread hangs in the caml_c_thread_register() call.

-Niki Yoshiuchi

On Tue, Mar 1, 2011 at 7:14 AM, Dmitry Bely <dmitry.bely@gmail.com> wrote:
> On Tue, Mar 1, 2011 at 12:56 AM, Niki Yoshiuchi <aplusbi@gmail.com> wrote:
>> I'm trying to call some OCaml code from a thread created in C.  I know
>> I'm supposed to call caml_c_thread_{un}register, however this pretty
>> much guarantees a segfault.  If I leave them out, my code works fine
>> so long as I don't try and do much of anything.  Here's my code
>> stripped down to the bare minimum:
>>
>> C code:
>>
>> void *my_thread(void *ptr)
>> {
>>    caml_c_thread_register();
>>    for(;;)
>>    {
>>        caml_acquire_runtime_system();
>>        caml_callback(*caml_named_value("my_callback"), Val_unit);
>>        caml_release_runtime_system();
>>    }
>>    caml_c_thread_unregister(); // I realize this will never be
>> called.  I've tried without the for loop as well, same result
>> }
>>
>> CAMLprim value caml_create_callback(value unit)
>> {
>>    pthread_t thread1;
>>    pthread_create(&thread1, NULL, my_thread, NULL);
>>    return Val_unit;
>> }
>>
>> OCaml code:
>>
>> external create_callback : unit -> unit = "caml_create_callback"
>>
>> let my_thread () =
>>    ()
>>
>> let _ =
>>    Callback.register "my_callback" random_thread;
>>    create_callback ();
>>    while true do
>>        ()
>>    done
>>
>> This code will segfault immediately.  If I remove the thread_register
>> functions it will run as expected.  If I try and do anything more than
>> just return unit, it will segfault unless I add in some sleep
>> statements.  I'm assuming that the sleep function acts as a sort of
>> poor-man's thread synchronizer.  I've tried endless permutations
>
> Maybe you are trying to call caml_c_thread_register() inside a thread
> created by Caml runtime? Otherwise I cannot imagine how
> caml_callback() could ever succeed without registering C-thread first.
>
> - Dmitry Bely
>
>
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