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[Caml-list] Segfault if you load threads.cma more than once
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Date: 2002-08-14 (19:00)
From: tim@f...
Subject: [Caml-list] Segfault if you load threads.cma more than once

Here's some terminal dialogue ending in a segfault that happened
because of bad options I passed in while linking.  I'm running ocaml
from the cvs as of yesterday except for the change discussed below.
The patch included below causes a meaningful error message to be
produced instead of a segfault.  I originally observed this behavior
in ocaml 3.0.4.

   lobus:/tmp/aggravate> cat Makefile
   OCAMLC = ocamlc
   .PHONY: all
   all: foo
   foo: foo1.ml foo1.mli foo3.ml foo3.mli foo2.ml Makefile
   	$(OCAMLC) -thread -c foo1.mli
   	$(OCAMLC) -thread -c foo1.ml
   	$(OCAMLC) -a unix.cma threads.cma -thread -o foo1.cma foo1.cmo
   	$(OCAMLC) -thread -c foo3.mli
   	$(OCAMLC) -thread -c foo3.ml
   	$(OCAMLC) -a unix.cma threads.cma -thread -o foo3.cma foo3.cmo
   	$(OCAMLC) -thread -c foo2.ml 
   	$(OCAMLC) unix.cma threads.cma -thread -custom -I . -g -o foo foo1.cma foo3.cma foo2.cmo

   .PHONY: clean
   	rm foo *.cmo *.cmi *.cma
   lobus:/tmp/aggravate> cat foo1.mli
   val mutexcreate: unit -> Mutex.t
   lobus:/tmp/aggravate> cat foo1.ml
   Format.printf "Hello, world from foo1!\n@?";;
   let mutexcreate () = Mutex.create ();;
   ignore (mutexcreate ());;
   lobus:/tmp/aggravate> cat foo2.mli
   cat: foo2.mli: No such file or directory
   lobus:/tmp/aggravate> cat foo3.mli
   val mutexcreate: unit -> Mutex.t
   lobus:/tmp/aggravate> cat foo3.ml
   Format.printf "Hello, world from foo3!\n@?";;
   let mutexcreate () = Mutex.create ();;
   ignore (mutexcreate ());;
   lobus:/tmp/aggravate> cat foo2.ml
   type foo = Foo of foo array;;

   let rec wastespace (power: int): foo =
     if power = 0 then
       Foo [||]
       let a: foo array = Array.make 10 (Foo [||])
         for i = 0 to 9 do
           a.(i) <- wastespace (power - 1)
         Foo a
     ignore (Foo1.mutexcreate ()); (* Use Foo1 and Foo3 to make sure *)
     ignore (Foo3.mutexcreate ()); (* they get linked in. *)
     ignore (wastespace 6); (* Force a garbage collection. *)

   lobus:/tmp/aggravate> make
   ocamlc -thread -c foo1.mli
   ocamlc -thread -c foo1.ml
   ocamlc -a unix.cma threads.cma -thread -o foo1.cma foo1.cmo
   ocamlc -thread -c foo3.mli
   ocamlc -thread -c foo3.ml
   ocamlc -a unix.cma threads.cma -thread -o foo3.cma foo3.cmo
   ocamlc -thread -c foo2.ml 
   ocamlc unix.cma threads.cma -thread -custom -I . -g -o foo foo1.cma foo3.cma foo2.cmo
   Hello, world from foo1!
   thread_initialize was called more than once.
   If we don't stop now, we'll fail during the next garbage collection.
   Hello, world from foo3!
   make: *** [all] Segmentation fault

You can get different outcomes depending on exactly what you do:

1. If you run what's in CVS right now, you'll get the segmentation
   fault without the "thread_initialize was called more than once..."
   message.  This happened to me and it took a while for me to figure
   out what was happening.
2. If you apply the patch below, you'll get a clean failure with an
   explanation of what went wrong.
3. If you apply the patch below and change "failwith" to "printf", you
   get exactly the behavior above.
4. If you eliminate unix.cma and threads.cma when building the
   libraries, the problem goes away as it should.

Tim Freeman       
GPG public key fingerprint ECDF 46F8 3B80 BB9E 575D  7180 76DF FE00 34B1 5C78 

--- scheduler.c	2001/12/07 13:40:22	1.54
+++ scheduler.c	2002/08/14 18:50:25
@@ -153,11 +153,24 @@
 static int stdin_initial_status, stdout_initial_status, stderr_initial_status;
 static void thread_restore_std_descr(void);
+/* Garbage collection will fail if we've been initialized twice.  It's possible
+   to do this by passing bad linker options, so crash with a message instead of
+   failing. */
+static int initialized = 0;
 /* Initialize the thread machinery */
 value thread_initialize(value unit)       /* ML */
   struct itimerval timer;
+  if (initialized) {
+    failwith
+      ("thread_initialize was called more than once.\n"
+       "If we don't stop now, we'll fail during the next garbage collection.\n"
+       "Did you link in threads.cma more than once?");
+  } else {
+    initialized = 1;
+  }
   /* Create a descriptor for the current thread */
   curr_thread =
     (thread_t) alloc_shr(sizeof(struct thread_struct) / sizeof(value), 0);
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