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Date: Mon, 28 Feb 2011 16:56:33 -0500
Message-ID: <AANLkTim-aovgVd4uKvjDbq8J-8VeTh=S5CLYz3QcEMxL@mail.gmail.com>
From: Niki Yoshiuchi <aplusbi@gmail.com>
To: caml-list@inria.fr
Content-Type: text/plain; charset=UTF-8
Subject: [Caml-list] Using C threads

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

Other important info - I'm using OCaml 3.12 and gcc 4.3.2 on Linux.
The return value of caml_c_thread_register is 1, which means that it
is successfully returning.

Thanks,
-Niki Yoshiuchi