二〇二六年四月二十一日

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+---
+title: caml_release_runtime_system导致的死锁问题
+date: 2023-06-28 21:45:55
+tags: [Technique]
+---
+
+
+给出一个`test.ml`:
+```ocaml
+Printf.eprintf "hello from OCaml\n%!"
+```
+
+和`test-ocaml-5.c`:
+```c
+#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:
+```
+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：
+
+```ocaml
+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:
+```
+starting up ...
+hello from OCaml
+acquiring ...
+acquired
+```
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