给出一个test.ml:

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Printf.eprintf "hello from OCaml\n%!"

和test-ocaml-5.c:

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#include <stdio.h>
#include <stdlib.h>
#include <caml/misc.h>
#include <caml/callback.h>
#include <caml/threads.h>

int
main (int argc, char *argv[])
  {
    fprintf (stderr, "starting up ...\n");
    caml_startup (argv);
    fprintf (stderr, "acquiring ...\n");
    caml_acquire_runtime_system ();
    fprintf (stderr, "acquired\n");
    // here is where I would be calling an OCaml callback
    // omitted for simplicity
    caml_release_runtime_system ();
    exit (0);
  }

然后用OCaml5的ocaml native compiler编译一下（这里我用的是ocaml-variants.5.0.0+options）：
ocamlopt -g test-ocaml-5.c test.ml -o test-ocaml-5

运行test-ocaml-5 会出现:

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starting up ...
hello from OCaml
acquiring ...
Fatal error: Fatal error during lock: Resource deadlock avoided

[1]    8998 IOT instruction (core dumped)  ./test-ocaml-5

这里我尝试了一下 4.14.0 和 4.14.1 ， 都没有出现这个情况，而如果用threads编译的话:
ocamlopt -g -I +unix unix.cmxa -I +threads threads.cmxa test-ocaml-5.c test.ml -o test-ocaml-5

无论在5.0还是4.14.x，都会挂起。

出现这个问题的一个可能原因是，在test-ocaml-5.c中，我在开头调用了caml_startup()，这会让当前线程获取锁，而caml_acquire_runtime_system()会再次获取它，caml_acquire_runtime_system() 应该在caml_release_runtime_system（）之后调用：

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int
main (int argc, char *argv[])
  {
    fprintf (stderr, "starting up ...\n");
    caml_startup (argv);
    fprintf (stderr, "acquiring ...\n");
    caml_release_runtime_system ();
    caml_acquire_runtime_system ();
    fprintf (stderr, "acquired\n");
    exit (0);
  }

运行结果为:

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starting up ...
hello from OCaml
acquiring ...
acquired