diff --git a/lib/polly.ml b/lib/polly.ml
index bd05aac..a59c03b 100644
--- a/lib/polly.ml
+++ b/lib/polly.ml
@@ -96,16 +96,31 @@ module Events = struct
   let test x y = x land y <> empty
 end
 
-type t = Unix.file_descr (* epoll fd *)
-
-external caml_polly_add : t -> Unix.file_descr -> Events.t -> unit
-  = "caml_polly_add"
-
-external caml_polly_del : t -> Unix.file_descr -> Events.t -> unit
-  = "caml_polly_del"
-
-external caml_polly_mod : t -> Unix.file_descr -> Events.t -> unit
-  = "caml_polly_mod"
+type t = int (* epoll fd *)
+
+external caml_polly_add :
+  (   (t[@untagged])
+   -> (int[@untagged])
+   -> (Events.t[@untagged])
+   -> (int[@untagged])
+  [@noalloc]
+  ) = "caml_polly_add" "caml_untagged_polly_add"
+
+external caml_polly_del :
+  (   (t[@untagged])
+   -> (int[@untagged])
+   -> (Events.t[@untagged])
+   -> (int[@untagged])
+  [@noalloc]
+  ) = "caml_polly_del" "caml_untagged_polly_del"
+
+external caml_polly_mod :
+  (   (t[@untagged])
+   -> (int[@untagged])
+   -> (Events.t[@untagged])
+   -> (int[@untagged])
+  [@noalloc]
+  ) = "caml_polly_mod" "caml_untagged_polly_mod"
 
 external caml_polly_create1 : unit -> t = "caml_polly_create1"
 
@@ -113,7 +128,7 @@ external caml_polly_wait :
      t (* epoll fd *)
   -> int (* max number of fds handled *)
   -> int (* timeout in ms *)
-  -> (Unix.file_descr -> Unix.file_descr -> Events.t -> unit)
+  -> (t -> Unix.file_descr -> Events.t -> unit)
   -> int (* actual number of ready fds; 0 = timeout *) = "caml_polly_wait"
 
 external caml_polly_wait_fold :
@@ -121,18 +136,32 @@ external caml_polly_wait_fold :
   -> int (* max number of fds handled *)
   -> int (* timeout in ms *)
   -> 'a (* initial value *)
-  -> (Unix.file_descr -> Unix.file_descr -> Events.t -> 'a -> 'a)
+  -> (t -> Unix.file_descr -> Events.t -> 'a -> 'a)
   -> 'a (* final value *) = "caml_polly_wait_fold"
 
 let create = caml_polly_create1
 
-let close t = Unix.close t
+let close t = Unix.close (Obj.magic t : Unix.file_descr)
+
+external uerror : string -> 'a = "caml_raise_unix_error"
 
-let add = caml_polly_add
+let add : t -> Unix.file_descr -> Events.t -> unit =
+ fun t fd evt ->
+  let __FUNCTION__ = "Polly.add" in
+  let r = caml_polly_add t (Obj.magic fd) evt in
+  if r = -1 then uerror __FUNCTION__
 
-let del t fd = caml_polly_del t fd Events.empty
+let del : t -> Unix.file_descr -> unit =
+ fun t fd ->
+  let __FUNCTION__ = "Polly.del" in
+  let r = caml_polly_del t (Obj.magic fd) Events.empty in
+  if r = -1 then uerror __FUNCTION__
 
-let upd = caml_polly_mod
+let upd : t -> Unix.file_descr -> Events.t -> unit =
+ fun t fd evt ->
+  let __FUNCTION__ = "Polly.upd" in
+  let r = caml_polly_mod t (Obj.magic fd) evt in
+  if r = -1 then uerror __FUNCTION__
 
 let wait = caml_polly_wait
 
diff --git a/lib/polly_stubs.c b/lib/polly_stubs.c
index 24393fb..f3a17c8 100644
--- a/lib/polly_stubs.c
+++ b/lib/polly_stubs.c
@@ -43,6 +43,13 @@ CONSTANT(EPOLLET);
 CONSTANT(EPOLLEXCLUSIVE);
 #endif
 
+/* necessary because of changes from 4.X to 5.X in ocaml,
+   uerror is a macro in 5.0 */
+CAMLprim void caml_raise_unix_error(value funname) {
+  CAMLparam1(funname);
+  uerror(String_val(funname),Nothing);
+}
+
 CAMLprim value caml_polly_create1(value val_unit)
 {
 	CAMLparam1(val_unit);
@@ -60,16 +67,13 @@ CAMLprim value caml_polly_create1(value val_unit)
 static value
 caml_polly_ctl(value val_epfd, value val_fd, value val_events, int op)
 {
-	CAMLparam3(val_epfd, val_fd, val_events);
-	struct epoll_event event = {
-		.events = (uint32_t) Int_val(val_events),
-		.data.fd = Int_val(val_fd)
-	};
+  CAMLparam0(); /* no need to register int */
+  struct epoll_event event = {
+    .events = (uint32_t) Int_val(val_events),
+    .data.fd = Int_val(val_fd)
+  };
 
-	if (epoll_ctl(Int_val(val_epfd), op, Int_val(val_fd), &event) == -1)
-		uerror(__FUNCTION__, Nothing);
-
-	CAMLreturn(Val_unit);
+  CAMLreturn(Val_int(epoll_ctl(Int_val(val_epfd), op, Int_val(val_fd), &event)));
 }
 
 CAMLprim value caml_polly_add(value val_epfd, value val_fd, value val_events)
@@ -77,16 +81,40 @@ CAMLprim value caml_polly_add(value val_epfd, value val_fd, value val_events)
 	return caml_polly_ctl(val_epfd, val_fd, val_events, EPOLL_CTL_ADD);
 }
 
+int caml_untagged_polly_add(int epfd, int fd, int events) {
+  struct epoll_event event = {
+    .events = (uint32_t) events,
+    .data.fd = fd
+  };
+  return epoll_ctl(epfd, EPOLL_CTL_ADD, fd, &event);
+}
+
 CAMLprim value caml_polly_mod(value val_epfd, value val_fd, value val_events)
 {
 	return caml_polly_ctl(val_epfd, val_fd, val_events, EPOLL_CTL_MOD);
 }
 
+int caml_untagged_polly_mod(int epfd, int fd, int events) {
+  struct epoll_event event = {
+    .events = (uint32_t) events,
+    .data.fd = fd
+  };
+  return epoll_ctl(epfd, EPOLL_CTL_MOD, fd, &event);
+}
+
 CAMLprim value caml_polly_del(value val_epfd, value val_fd, value val_events)
 {
 	return caml_polly_ctl(val_epfd, val_fd, val_events, EPOLL_CTL_DEL);
 }
 
+int caml_untagged_polly_del(int epfd, int fd, int events) {
+  struct epoll_event event = {
+    .events = (uint32_t) events,
+    .data.fd = fd
+  };
+  return epoll_ctl(epfd, EPOLL_CTL_DEL, fd, &event);
+}
+
 CAMLprim value
 caml_polly_wait(value val_epfd, value val_max, value val_timeout, value val_f)
 {