diff --git a/otherlibs/threads/Makefile b/otherlibs/threads/Makefile
index ba72c7852..97f706214 100644
--- a/otherlibs/threads/Makefile
+++ b/otherlibs/threads/Makefile
@@ -17,7 +17,7 @@ LIB_OBJS=pervasives.cmo \
   $(LIB)/set.cmo $(LIB)/map.cmo $(LIB)/stack.cmo $(LIB)/queue.cmo \
   $(LIB)/stream.cmo $(LIB)/printf.cmo $(LIB)/format.cmo $(LIB)/arg.cmo \
   $(LIB)/printexc.cmo $(LIB)/gc.cmo $(LIB)/digest.cmo $(LIB)/random.cmo \
-  $(LIB)/oo.cmo $(LIB)/genlex.cmo
+  $(LIB)/oo.cmo $(LIB)/genlex.cmo $(LIB)/callback.cmo $(LIB)/weak.cmo
 
 all: libthreads.a threads.cma stdlib.cma
 
diff --git a/otherlibs/threads/scheduler.c b/otherlibs/threads/scheduler.c
index c51e80184..82a2606f3 100644
--- a/otherlibs/threads/scheduler.c
+++ b/otherlibs/threads/scheduler.c
@@ -480,26 +480,26 @@ value thread_delay(time)          /* ML */
 
 /* Suspend the current thread on a Unix file descriptor, with timeout */
 
-value thread_wait_timed_read(fd, time)        /* ML */
-     value fd, time;
+value thread_wait_timed_read(fd_time)        /* ML */
+     value fd_time;
 {
-  double date = timeofday() + Double_val(time);
+  double date = timeofday() + Double_val(Field(fd_time, 1));
   Assert(curr_thread != NULL);
   check_callback();
   curr_thread->status = BLOCKED_READ | BLOCKED_DELAY;
-  curr_thread->fd = fd;
+  curr_thread->fd = Field(fd_time, 0);
   Assign(curr_thread->delay, copy_double(date));
   return schedule_thread();
 }
 
-value thread_wait_timed_write(fd, time)        /* ML */
-     value fd, time;
+value thread_wait_timed_write(fd_time)        /* ML */
+     value fd_time;
 {
-  double date = timeofday() + Double_val(time);
+  double date = timeofday() + Double_val(Field(fd_time, 1));
   Assert(curr_thread != NULL);
   check_callback();
   curr_thread->status = BLOCKED_WRITE | BLOCKED_DELAY;
-  curr_thread->fd = fd;
+  curr_thread->fd = Field(fd_time, 0);
   Assign(curr_thread->delay, copy_double(date));
   return schedule_thread();
 }
diff --git a/otherlibs/threads/thread.ml b/otherlibs/threads/thread.ml
index 3d965a0fa..ec2e0ebca 100644
--- a/otherlibs/threads/thread.ml
+++ b/otherlibs/threads/thread.ml
@@ -30,7 +30,9 @@ type resumption_status =
    it takes sp from the new thread, but keeps pc from the old thread.
    But that's OK if all calls to rescheduling primitives are immediately
    followed by a RETURN operation, which will restore the correct pc
-   from the stack. *)
+   from the stack. Furthermore, the RETURNs must all have the same
+   frame size, which means that both the primitives and their ML wrappers
+   must take exactly one argument. *)
 
 external thread_initialize : unit -> unit = "thread_initialize"
 external thread_new : (unit -> unit) -> t = "thread_new"
@@ -39,10 +41,10 @@ external thread_sleep : unit -> unit = "thread_sleep"
 external thread_wait_read : Unix.file_descr -> unit = "thread_wait_read"
 external thread_wait_write : Unix.file_descr -> unit = "thread_wait_write"
 external thread_wait_timed_read
-            : Unix.file_descr -> float -> resumption_status
+            : Unix.file_descr * float -> resumption_status (* remeber: 1 arg *)
             = "thread_wait_timed_read"
 external thread_wait_timed_write
-            : Unix.file_descr -> float -> resumption_status
+            : Unix.file_descr * float -> resumption_status (* remeber: 1 arg *)
             = "thread_wait_timed_write"
 external thread_join : t -> unit = "thread_join"
 external thread_delay : float -> unit = "thread_delay"
@@ -67,19 +69,19 @@ let self () = thread_self()
 let kill pid = thread_kill pid
 let exit () = thread_kill(thread_self())
 
-let wait_timed_read_aux fd d = thread_wait_timed_read fd d
-let wait_timed_write_aux fd d = thread_wait_timed_write fd d
+let wait_timed_read_aux arg = thread_wait_timed_read arg
+let wait_timed_write_aux arg = thread_wait_timed_write arg
 let wait_pid_aux pid = thread_wait_pid pid
 
-let wait_timed_read fd d = wait_timed_read_aux fd d = Resumed_io
-let wait_timed_write fd d = wait_timed_write_aux fd d = Resumed_io
+let wait_timed_read fd d = wait_timed_read_aux (fd, d) = Resumed_io
+let wait_timed_write fd d = wait_timed_write_aux (fd, d) = Resumed_io
 let wait_pid pid = 
   match wait_pid_aux pid with
     Resumed_wait(pid, status) -> (pid, status)
   | _ -> invalid_arg "Thread.wait_pid"
 
-(* For new, make sure the function passed to thread_new always terminates
-   by calling exit. *)
+(* For Thread.create, make sure the function passed to thread_new
+   always terminates by calling Thread.exit. *)
 
 let create fn arg =
   thread_new