diff --git a/README.md b/README.md
index c6a53326..285b34bf 100644
--- a/README.md
+++ b/README.md
@@ -172,6 +172,15 @@ In the module `Containers_advanced`:
 - `CCCat`, a few categorical structures
 - `CCBatch`, to combine operations on collections into one traversal
 
+### Thread
+
+In the library `containers.thread`, for preemptive system threads:
+
+- `CCFuture`, a set of tools for preemptive threading, including a thread pool,
+  monadic futures, and MVars (concurrent boxes)
+- `CCLock`, values protected by locks
+- `CCSemaphore`, a simple implementation of semaphores
+
 ### Misc
 
 See [doc](http://cedeela.fr/~simon/software/containers/misc). This list
@@ -191,9 +200,6 @@ is not necessarily up-to-date.
 
 ### Others
 
-- `Future`, a set of tools for preemptive threading, including a thread pool,
-monadic futures, and MVars (concurrent boxes)
-
 - `containers.lwt` contains [Lwt](http://ocsigen.org/lwt/)-related modules (experimental)
 
 There is a QuickCheck-like library called `QCheck` (now in its own repo).
diff --git a/_oasis b/_oasis
index f9c58f97..08da485a 100644
--- a/_oasis
+++ b/_oasis
@@ -130,7 +130,7 @@ Library "containers_misc"
 
 Library "containers_thread"
   Path:             src/threads/
-  Modules:          CCFuture, CCLock
+  Modules:          CCFuture, CCLock, CCSemaphore
   FindlibName:      thread
   FindlibParent:    containers
   Build$:           flag(thread)
diff --git a/doc/intro.txt b/doc/intro.txt
index df66e5fd..922d9d36 100644
--- a/doc/intro.txt
+++ b/doc/intro.txt
@@ -156,6 +156,7 @@ Lwt_pipe
 {!modules:
 CCFuture
 CCLock
+CCSemaphore
 }
 
 
diff --git a/src/threads/CCSemaphore.ml b/src/threads/CCSemaphore.ml
new file mode 100644
index 00000000..582e04a6
--- /dev/null
+++ b/src/threads/CCSemaphore.ml
@@ -0,0 +1,119 @@
+
+(** {1 Semaphores} *)
+
+type t = {
+  mutable n : int;
+  mutex : Mutex.t;
+  cond : Condition.t;
+}
+
+let create n = {
+  n;
+  mutex=Mutex.create();
+  cond=Condition.create();
+}
+
+let get t = t.n
+
+(* assume [t.mutex] locked, try to acquire [t] *)
+let acquire_once_locked_ m t =
+  while t.n < m do
+    Condition.wait t.cond t.mutex;
+  done;
+  assert (t.n >= m);
+  t.n <- t.n - m;
+  Condition.broadcast t.cond;
+  Mutex.unlock t.mutex
+
+let acquire m t =
+  Mutex.lock t.mutex;
+  acquire_once_locked_ m t
+
+(* assume [t.mutex] locked, try to release [t] *)
+let release_once_locked_ m t =
+  t.n <- t.n + m;
+  Condition.broadcast t.cond;
+  Mutex.unlock t.mutex
+
+let release m t =
+  Mutex.lock t.mutex;
+  release_once_locked_ m t;
+  ()
+
+(*$R
+  let s = create 1 in
+  let r = CCLock.create false in
+  let _ = Thread.create (fun s -> acquire 5 s; CCLock.set r true) s in
+  Thread.yield ();
+  assert_equal false (CCLock.get r);
+  release 4 s;
+  Thread.delay 0.2;
+  assert_equal true (CCLock.get r);
+  assert_equal 0 (get s)
+*)
+
+let with_acquire ~n t ~f =
+  Mutex.lock t.mutex;
+  acquire_once_locked_ n t;
+  try
+    let x = f() in
+    release_once_locked_ n t;
+    x
+  with e ->
+    release_once_locked_ n t;
+    raise e
+
+(*$R
+  let s = create 5 in
+  let n = CCLock.create 0 in
+  let a = Array.init 100 (fun i ->
+    Thread.create (fun _ ->
+      with_acquire ~n:(1 + (i mod 5)) s
+        ~f:(fun () -> CCLock.incr n)
+    ) ())
+  in
+  Array.iter Thread.join a;
+  assert_equal ~printer:CCInt.to_string 5 (get s);
+  assert_equal ~printer:CCInt.to_string 100 (CCLock.get n)
+*)
+
+let wait_until_at_least ~n t ~f =
+  Mutex.lock t.mutex;
+  while t.n < n do
+    Condition.wait t.cond t.mutex;
+  done;
+  assert (t.n >= n);
+  Mutex.unlock t.mutex;
+  f ()
+
+(*$R
+  let output s = () in
+  let s = create 2 in
+  let res = CCLock.create false in
+  let id = Thread.create
+    (fun () ->
+      output "start";
+      wait_until_at_least ~n:5 s
+        ~f:(fun () ->
+          assert (get s >= 5);
+          output "modify now";
+          CCLock.set res true)
+    ) ()
+  in
+  output "launched thread";
+  Thread.yield();
+  assert_bool "start" (not (CCLock.get res));
+  output "release 2";
+  release 2 s;
+  Thread.yield();
+  assert_bool "after release 2" (not (CCLock.get res));
+  output "release 1";
+  release 1 s;
+  (* should work now *)
+  Thread.delay 0.2;
+  Thread.join id;
+  output "check";
+  assert_bool "after release 1" (CCLock.get res)
+*)
+
+
diff --git a/src/threads/CCSemaphore.mli b/src/threads/CCSemaphore.mli
new file mode 100644
index 00000000..11831cc9
--- /dev/null
+++ b/src/threads/CCSemaphore.mli
@@ -0,0 +1,33 @@
+
+(* This file is free software, part of containers. See file "license" for more details. *)
+
+(** {1 Semaphores}
+
+    @since NEXT_RELEASE *)
+
+type t
+(** A semaphore *)
+
+val create : int -> t
+(** [create n] creates a semaphore with initial value [n]
+    @raise Invalid_argument if [n < 0] *)
+
+val get : t -> int
+(** Current value *)
+
+val acquire : int -> t -> unit
+(** [acquire n s] blocks until [get s > n], then atomically
+    sets [s := !s - n] *)
+
+val release : int -> t -> unit
+(** [release n s] atomically sets [s := !s + n] *)
+
+val with_acquire : n:int -> t -> f:(unit -> 'a) -> 'a
+(** [with_acquire ~n s ~f] first acquires [s] with [n] units,
+    calls [f ()], and then release [s] with [n] units.
+    Safely release the semaphore even if [f ()] fails *)
+
+val wait_until_at_least : n:int -> t -> f:(unit -> 'a) -> 'a
+(** [wait_until_at_least ~n s ~f] waits until [get s >= n], then calls [f ()]
+    and returns its result. Doesn't modify the semaphore. *)
+