Re: [Caml-list] Using C threads

[Dmitry Bely <dmitry.bely@gmail.com>]

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