diff --git a/src/core/nanoev.ml b/src/core/nanoev.ml
index f0ea5c7..d574367 100644
--- a/src/core/nanoev.ml
+++ b/src/core/nanoev.ml
@@ -1,13 +1,22 @@
 module Trace_ = Trace_
 
+module FD = struct
+  type t = {
+    fd: Unix.file_descr;
+    closed: bool Atomic.t;
+  }
+
+  let close self =
+    if not (Atomic.exchange self.closed true) then Unix.close self.fd
+end
+
 module Impl = struct
   type 'st ops = {
     clear: 'st -> unit;
+    get_fd: 'st -> Unix.file_descr -> FD.t;
     wakeup_from_outside: 'st -> unit;
-    on_readable:
-      'a 'b. 'st -> Unix.file_descr -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
-    on_writable:
-      'a 'b. 'st -> Unix.file_descr -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
+    on_readable: 'a 'b. 'st -> FD.t -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
+    on_writable: 'a 'b. 'st -> FD.t -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
     run_after_s: 'a 'b. 'st -> float -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
     step: 'st -> unit;
   }
@@ -22,6 +31,7 @@ open Impl
 type t = Impl.t
 
 let[@inline] clear (Ev (ops, st)) = ops.clear st
+let[@inline] get_fd (Ev (ops, st)) fd = ops.get_fd st fd
 let[@inline] wakeup_from_outside (Ev (ops, st)) = ops.wakeup_from_outside st
 
 let[@inline] on_readable (Ev (ops, st)) fd x y f : unit =
@@ -34,11 +44,3 @@ let[@inline] run_after_s (Ev (ops, st)) time x y f : unit =
   ops.run_after_s st time x y f
 
 let[@inline] step (Ev (ops, st)) : unit = ops.step st
-
-(*
-let rec read (self:t) fd buf i len : int =
-  match Unix.read fd buf i len with
-    | n -> n
-  | exception Unix.Unix_error (Unix, _, _) ->
-    read self fd buf i len
-*)
diff --git a/src/core/nanoev.mli b/src/core/nanoev.mli
index 2224efa..fb32320 100644
--- a/src/core/nanoev.mli
+++ b/src/core/nanoev.mli
@@ -2,14 +2,22 @@
 
 type t
 
+module FD : sig
+  type t = private {
+    fd: Unix.file_descr;
+    closed: bool Atomic.t;
+  }
+
+  val close : t -> unit
+end
+
 module Impl : sig
   type 'st ops = {
     clear: 'st -> unit;
+    get_fd: 'st -> Unix.file_descr -> FD.t;
     wakeup_from_outside: 'st -> unit;
-    on_readable:
-      'a 'b. 'st -> Unix.file_descr -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
-    on_writable:
-      'a 'b. 'st -> Unix.file_descr -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
+    on_readable: 'a 'b. 'st -> FD.t -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
+    on_writable: 'a 'b. 'st -> FD.t -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
     run_after_s: 'a 'b. 'st -> float -> 'a -> 'b -> ('a -> 'b -> unit) -> unit;
     step: 'st -> unit;
   }
@@ -21,12 +29,13 @@ val clear : t -> unit
 (** Reset the state *)
 
 val wakeup_from_outside : t -> unit
+val get_fd : t -> Unix.file_descr -> FD.t
 
 val step : t -> unit
 (** Run one step of the event loop until something happens *)
 
-val on_readable : t -> Unix.file_descr -> 'a -> 'b -> ('a -> 'b -> unit) -> unit
-val on_writable : t -> Unix.file_descr -> 'a -> 'b -> ('a -> 'b -> unit) -> unit
+val on_readable : t -> FD.t -> 'a -> 'b -> ('a -> 'b -> unit) -> unit
+val on_writable : t -> FD.t -> 'a -> 'b -> ('a -> 'b -> unit) -> unit
 val run_after_s : t -> float -> 'a -> 'b -> ('a -> 'b -> unit) -> unit
 
 (**/**)
diff --git a/src/picos/nanoev_picos.ml b/src/picos/nanoev_picos.ml
index 45f5cc6..54b7011 100644
--- a/src/picos/nanoev_picos.ml
+++ b/src/picos/nanoev_picos.ml
@@ -4,6 +4,8 @@ open struct
   let ( let@ ) = ( @@ )
 end
 
+module FD = Nanoev.FD
+
 module Global_ = struct
   type st =
     | None
@@ -67,6 +69,10 @@ let[@inline] unwrap_ = function
   | None -> ()
   | Some (exn, bt) -> Printexc.raise_with_backtrace exn bt
 
+let get_fd (fd : Unix.file_descr) : FD.t =
+  let ev = get_loop_exn_ () in
+  Nanoev.get_fd ev fd
+
 let retry_read_ fd f =
   let ev = get_loop_exn_ () in
   let[@unroll 1] rec loop () =
@@ -99,22 +105,24 @@ let retry_write_ fd f =
   in
   loop ()
 
-let read fd buf i len : int =
+let read (fd : FD.t) buf i len : int =
   retry_read_ fd (fun () ->
       Trace_.message "read";
-      Unix.read fd buf i len)
+      Unix.read fd.fd buf i len)
 
-let accept fd =
+let accept (fd : FD.t) =
   retry_read_ fd (fun () ->
       Trace_.message "accept";
-      Unix.accept fd)
+      let client_sock, client_addr = Unix.accept fd.fd in
+      get_fd client_sock, client_addr)
 
-let write fd buf i len : int =
+let write (fd : FD.t) buf i len : int =
   retry_write_ fd (fun () ->
       Trace_.message "write";
-      Unix.write fd buf i len)
+      Unix.write fd.fd buf i len)
 
-let connect fd addr = retry_write_ fd (fun () -> Unix.connect fd addr)
+let connect (fd : FD.t) addr =
+  retry_write_ fd (fun () -> Unix.connect fd.fd addr)
 
 let sleep t =
   if t > 0. then (
diff --git a/src/picos/nanoev_picos.mli b/src/picos/nanoev_picos.mli
index 290e655..9736a7f 100644
--- a/src/picos/nanoev_picos.mli
+++ b/src/picos/nanoev_picos.mli
@@ -1,5 +1,7 @@
 (** Basic interface with picos *)
 
+module FD = Nanoev.FD
+
 val setup_bg_thread : Nanoev.t -> unit
 (** Install this event loop in a background thread *)
 
@@ -8,8 +10,9 @@ val has_bg_thread : unit -> bool
 
 (** {2 Non blocking IO primitives} *)
 
-val read : Unix.file_descr -> bytes -> int -> int -> int
-val connect : Unix.file_descr -> Unix.sockaddr -> unit
-val write : Unix.file_descr -> bytes -> int -> int -> int
-val accept : Unix.file_descr -> Unix.file_descr * Unix.sockaddr
+val get_fd : Unix.file_descr -> FD.t
+val read : FD.t -> bytes -> int -> int -> int
+val connect : FD.t -> Unix.sockaddr -> unit
+val write : FD.t -> bytes -> int -> int -> int
+val accept : FD.t -> FD.t * Unix.sockaddr
 val sleep : float -> unit
diff --git a/src/unix/nanoev_unix.ml b/src/unix/nanoev_unix.ml
index 769c590..3be11a2 100644
--- a/src/unix/nanoev_unix.ml
+++ b/src/unix/nanoev_unix.ml
@@ -1,5 +1,6 @@
 open struct
   module Trace_ = Nanoev.Trace_
+  module FD = Nanoev.FD
 
   let ( let@ ) = ( @@ )
   let now_ : unit -> float = Unix.gettimeofday
@@ -89,7 +90,7 @@ let wakeup_from_outside (self : st) : unit =
     let b = Bytes.make 1 '!' in
     ignore (Unix.write self.wakeup_wr b 0 1 : int)
 
-let get_fd_ (self : st) fd : per_fd =
+let get_per_fd_ (self : st) fd : per_fd =
   match Hashtbl.find self.fds fd with
   | per_fd -> per_fd
   | exception Not_found ->
@@ -97,10 +98,10 @@ let get_fd_ (self : st) fd : per_fd =
     Hashtbl.add self.fds fd per_fd;
     per_fd
 
-let on_readable self fd x y f : unit =
+let on_readable self (fd : FD.t) x y f : unit =
   let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.on-readable" in
   let@ self = with_lock_ self in
-  let per_fd = get_fd_ self fd in
+  let per_fd = get_per_fd_ self fd in
   per_fd.r <- Sub (x, y, f, per_fd.r);
   self.sub_up_to_date <- false;
   if Atomic.get self.in_select then wakeup_from_outside self
@@ -108,7 +109,7 @@ let on_readable self fd x y f : unit =
 let on_writable self fd x y f : unit =
   let@ _sp = Trace_.with_span ~__FILE__ ~__LINE__ "nanoev.on-writable" in
   let@ self = with_lock_ self in
-  let per_fd = get_fd_ self fd in
+  let per_fd = get_per_fd_ self fd in
   per_fd.w <- Sub (x, y, f, per_fd.w);
   self.sub_up_to_date <- false;
   if Atomic.get self.in_select then wakeup_from_outside self