diff --git a/README.md b/README.md
index 2888173c..03aba97b 100644
--- a/README.md
+++ b/README.md
@@ -59,13 +59,16 @@ We can now ask Linux to suspend the process until a result (Completion Queue Ent
 let rec wait_with_retry uring =
   match Uring.wait uring with
   | None -> wait_with_retry uring        (* Interrupted *)
-  | Some { result; data } -> result, data;;
+  | Unit { result = (); data } -> Either.Left 0, data
+  | Int { result; data } -> Either.Left result, data
+  | FD { result; data } -> Either.Right result, data
+  | Error { result; data } -> failwith ("Unexpected error: " ^ Unix.error_message result);;
 ```
 
 <!-- $MDX non-deterministic=output -->
 ```ocaml
 # let result, data = wait_with_retry uring;;
-val result : int = 8
+val result : (int, Unix.file_descr) Either.t = Right <abstr>
 val data : _[> `Open_log ] = `Open_log
 ```
 
@@ -76,9 +79,7 @@ The `result` field is the return code,
 with the same meaning as the return code from the corresponding system call (`openat2` in this case).
 
 ```ocaml
-# let fd =
-    if result < 0 then failwith ("Error: " ^ string_of_int result);
-    (Obj.magic result : Unix.file_descr);;
+# let fd = Either.fold ~left:(fun _ -> assert false) ~right:Fun.id result
 val fd : Unix.file_descr = <abstr>
 ```
 
@@ -97,11 +98,12 @@ let rec write_all fd = function
       |> Option.get               (* We know we have enough space here *)
     in
     assert (Uring.submit uring = 1);
-    let result, data = wait_with_retry uring in
-    assert (data = `Write_all);  (* There aren't any other requests pending *)
-    assert (result > 0);         (* Check for error return *)
-    let bufs = Cstruct.shiftv bufs result in
-    write_all fd bufs
+    match wait_with_retry uring with
+    | Either.Left result, `Write_all ->
+        let bufs = Cstruct.shiftv bufs result in
+        write_all fd bufs
+    | _ ->
+        assert false (* There aren't any other requests pending *)
 ```
 
 ```ocaml
@@ -124,8 +126,9 @@ Some <abstr>
 - : int = 1
 
 # wait_with_retry uring;;
-- : int * ([> `Close_log | `Open_log | `Write_all ] as '_weak3) =
-(0, `Close_log)
+- : (int, Unix.file_descr) Either.t *
+    ([> `Close_log | `Open_log | `Write_all ] as '_weak3)
+= (Either.Left 0, `Close_log)
 ```
 
 The file has now been written:
diff --git a/bench/readv.ml b/bench/readv.ml
index 8f977c7d..7a493ddc 100644
--- a/bench/readv.ml
+++ b/bench/readv.ml
@@ -5,13 +5,14 @@ let buffer_size = 100   (* Use a small buffer to stress the system more *)
 let n_concurrent = 16   (* How many requests to have active at once *)
 let n_iters = 1_000_000 (* How many times to accept and resubmit *)
 
-let rec wait t handle =
-  match Uring.get_cqe_nonblocking t with
-  | Some { result; data = buf } -> handle result buf
-  | None ->
-    match Uring.wait t with
-    | None -> wait t handle
-    | Some { result; data = buf } -> handle result buf
+let rec process wait t handle = function
+  | Uring.Int { result; data = buf } -> handle result buf
+  | Error { result; data = _ } ->
+      raise (Unix.Unix_error (result, "readv", ""))
+  | FD _ | Unit _ -> failwith "Unexpected return from readv"
+  | None -> process wait t handle (wait t)
+
+let wait t handle = process Uring.wait t handle (Uring.get_cqe_nonblocking t)
 
 let run_bechmark ~polling_timeout fd =
   let got = ref 0 in
@@ -29,13 +30,9 @@ let run_bechmark ~polling_timeout fd =
   let t0 = Unix.gettimeofday () in
   for _ = 1 to n_iters do
     wait t (fun result bufs ->
-        if result < 0 then (
-          raise (Unix.Unix_error (Uring.error_of_errno result, "readv", ""))
-        ) else (
-          got := !got + result;
-          let _job : _ Uring.job = Uring.readv t fd bufs ~file_offset:Optint.Int63.zero bufs |> Option.get in
-          ()
-        )
+        got := !got + result;
+        let _job : _ Uring.job = Uring.readv t fd bufs ~file_offset:Optint.Int63.zero bufs |> Option.get in
+        ()
       )
   done;
   (* Get a snapshot of the stats before letting things finish. *)
diff --git a/lib/uring/uring.ml b/lib/uring/uring.ml
index ae5124dc..761d58a9 100644
--- a/lib/uring/uring.ml
+++ b/lib/uring/uring.ml
@@ -295,8 +295,6 @@ module Msghdr = struct
     create_with_addr ~n_fds ~fds:[] ?addr buffs
 end
 
-type 'a job = 'a Heap.entry
-
 type clock = Boottime | Realtime
 
 type probe
@@ -316,38 +314,41 @@ module Uring = struct
   external opcode_supported : probe -> Op.t -> bool = "ocaml_uring_opcode_supported" [@@noalloc]
 
   type id = Heap.ptr
+  type job = private int
 
   type offset = Optint.Int63.t
-  external submit_nop : t -> id -> bool = "ocaml_uring_submit_nop" [@@noalloc]
-  external submit_timeout : t -> id -> Sketch.ptr -> clock -> bool -> bool = "ocaml_uring_submit_timeout" [@@noalloc]
-  external submit_poll_add : t -> Unix.file_descr -> id -> Poll_mask.t -> bool = "ocaml_uring_submit_poll_add" [@@noalloc]
-  external submit_read : t -> Unix.file_descr -> id -> Cstruct.t -> offset -> bool = "ocaml_uring_submit_read" [@@noalloc]
-  external submit_write : t -> Unix.file_descr -> id -> Cstruct.t -> offset -> bool = "ocaml_uring_submit_write" [@@noalloc]
-  external submit_readv : t -> Unix.file_descr -> id -> Sketch.ptr -> offset -> bool = "ocaml_uring_submit_readv" [@@noalloc]
-  external submit_writev : t -> Unix.file_descr -> id -> Sketch.ptr -> offset -> bool = "ocaml_uring_submit_writev" [@@noalloc]
-  external submit_readv_fixed : t -> Unix.file_descr -> id -> Cstruct.buffer -> int -> int -> offset -> bool = "ocaml_uring_submit_readv_fixed_byte" "ocaml_uring_submit_readv_fixed_native" [@@noalloc]
-  external submit_writev_fixed : t -> Unix.file_descr -> id -> Cstruct.buffer -> int -> int -> offset -> bool = "ocaml_uring_submit_writev_fixed_byte" "ocaml_uring_submit_writev_fixed_native" [@@noalloc]
-  external submit_close : t -> Unix.file_descr -> id -> bool = "ocaml_uring_submit_close" [@@noalloc]
-  external submit_statx : t -> id -> Unix.file_descr -> Statx.t -> Sketch.ptr -> int -> int -> bool = "ocaml_uring_submit_statx_byte" "ocaml_uring_submit_statx_native" [@@noalloc]
-  external submit_splice : t -> id -> Unix.file_descr -> Unix.file_descr -> int -> bool = "ocaml_uring_submit_splice" [@@noalloc]
-  external submit_bind : t -> id -> Unix.file_descr -> Sockaddr.t -> bool = "ocaml_uring_submit_bind" [@@noalloc]
-  external submit_listen : t -> id -> Unix.file_descr -> int -> bool = "ocaml_uring_submit_listen" [@@noalloc]
-  external submit_connect : t -> id -> Unix.file_descr -> Sockaddr.t -> bool = "ocaml_uring_submit_connect" [@@noalloc]
-  external submit_accept : t -> id -> Unix.file_descr -> Sockaddr.t -> bool = "ocaml_uring_submit_accept" [@@noalloc]
-  external submit_cancel : t -> id -> id -> bool = "ocaml_uring_submit_cancel" [@@noalloc]
-  external submit_openat2 : t -> id -> Unix.file_descr -> Open_how.t -> bool = "ocaml_uring_submit_openat2" [@@noalloc]
-  external submit_linkat : t -> id -> Unix.file_descr -> Sketch.ptr -> Unix.file_descr -> Sketch.ptr -> int -> bool = "ocaml_uring_submit_linkat_byte" "ocaml_uring_submit_linkat_native" [@@noalloc]
-  external submit_unlinkat : t -> id -> Unix.file_descr -> Sketch.ptr -> bool -> bool = "ocaml_uring_submit_unlinkat" [@@noalloc]
-  external submit_mkdirat : t -> id -> Unix.file_descr -> Sketch.ptr -> int -> bool = "ocaml_uring_submit_mkdirat" [@@noalloc]
-  external submit_send_msg : t -> id -> Unix.file_descr -> Msghdr.t -> Sketch.ptr -> bool = "ocaml_uring_submit_send_msg" [@@noalloc]
-  external submit_recv_msg : t -> id -> Unix.file_descr -> Msghdr.t -> Sketch.ptr -> bool = "ocaml_uring_submit_recv_msg" [@@noalloc]
-  external submit_fsync : t -> id -> Unix.file_descr -> int64 -> int -> bool = "ocaml_uring_submit_fsync" [@@noalloc]
-  external submit_fdatasync : t -> id -> Unix.file_descr -> int64 -> int -> bool = "ocaml_uring_submit_fdatasync" [@@noalloc]
+  external submit_nop : t -> id -> job = "ocaml_uring_submit_nop" [@@noalloc]
+  external submit_timeout : t -> id -> Sketch.ptr -> clock -> bool -> job = "ocaml_uring_submit_timeout" [@@noalloc]
+  external submit_poll_add : t -> Unix.file_descr -> id -> Poll_mask.t -> job = "ocaml_uring_submit_poll_add" [@@noalloc]
+  external submit_read : t -> Unix.file_descr -> id -> Cstruct.t -> offset -> job = "ocaml_uring_submit_read" [@@noalloc]
+  external submit_write : t -> Unix.file_descr -> id -> Cstruct.t -> offset -> job = "ocaml_uring_submit_write" [@@noalloc]
+  external submit_readv : t -> Unix.file_descr -> id -> Sketch.ptr -> offset -> job = "ocaml_uring_submit_readv" [@@noalloc]
+  external submit_writev : t -> Unix.file_descr -> id -> Sketch.ptr -> offset -> job = "ocaml_uring_submit_writev" [@@noalloc]
+  external submit_readv_fixed : t -> Unix.file_descr -> id -> Cstruct.buffer -> int -> int -> offset -> job = "ocaml_uring_submit_readv_fixed_byte" "ocaml_uring_submit_readv_fixed_native" [@@noalloc]
+  external submit_writev_fixed : t -> Unix.file_descr -> id -> Cstruct.buffer -> int -> int -> offset -> job = "ocaml_uring_submit_writev_fixed_byte" "ocaml_uring_submit_writev_fixed_native" [@@noalloc]
+  external submit_close : t -> Unix.file_descr -> id -> job = "ocaml_uring_submit_close" [@@noalloc]
+  external submit_statx : t -> id -> Unix.file_descr option -> Statx.t -> Sketch.ptr -> int -> int -> job = "ocaml_uring_submit_statx_byte" "ocaml_uring_submit_statx_native" [@@noalloc]
+  external submit_splice : t -> id -> Unix.file_descr -> Unix.file_descr -> int -> job = "ocaml_uring_submit_splice" [@@noalloc]
+  external submit_bind : t -> id -> Unix.file_descr -> Sockaddr.t -> job = "ocaml_uring_submit_bind" [@@noalloc]
+  external submit_listen : t -> id -> Unix.file_descr -> int -> job = "ocaml_uring_submit_listen" [@@noalloc]
+  external submit_connect : t -> id -> Unix.file_descr -> Sockaddr.t -> job = "ocaml_uring_submit_connect" [@@noalloc]
+  external submit_accept : t -> id -> Unix.file_descr -> Sockaddr.t -> job = "ocaml_uring_submit_accept" [@@noalloc]
+  external submit_cancel : t -> id -> job -> job = "ocaml_uring_submit_cancel" [@@noalloc]
+  external submit_openat2 : t -> id -> Unix.file_descr option -> Open_how.t -> job = "ocaml_uring_submit_openat2" [@@noalloc]
+  external submit_linkat : t -> id -> Unix.file_descr option -> Sketch.ptr -> Unix.file_descr option -> Sketch.ptr -> int -> job = "ocaml_uring_submit_linkat_byte" "ocaml_uring_submit_linkat_native" [@@noalloc]
+  external submit_unlinkat : t -> id -> Unix.file_descr option -> Sketch.ptr -> bool -> job = "ocaml_uring_submit_unlinkat" [@@noalloc]
+  external submit_mkdirat : t -> id -> Unix.file_descr option -> Sketch.ptr -> int -> job = "ocaml_uring_submit_mkdirat" [@@noalloc]
+  external submit_send_msg : t -> id -> Unix.file_descr -> Msghdr.t -> Sketch.ptr -> job = "ocaml_uring_submit_send_msg" [@@noalloc]
+  external submit_recv_msg : t -> id -> Unix.file_descr -> Msghdr.t -> Sketch.ptr -> job = "ocaml_uring_submit_recv_msg" [@@noalloc]
+  external submit_fsync : t -> id -> Unix.file_descr -> int64 -> int -> job = "ocaml_uring_submit_fsync" [@@noalloc]
+  external submit_fdatasync : t -> id -> Unix.file_descr -> int64 -> int -> job = "ocaml_uring_submit_fdatasync" [@@noalloc]
 
   type cqe_option = private
     | Cqe_none
-    | Cqe_some of { user_data_id : id; res: int }
-  [@@ocaml.warning "-37" (* Avoids "Unused constructor" warning on OCaml <= 4.09. *)]
+    | Cqe_unit of { user_data_id : id; result: unit }
+    | Cqe_int of { user_data_id : id; result: int }
+    | Cqe_fd of { user_data_id : id; result: Unix.file_descr }
+    | Cqe_error of { user_data_id : id; result: Unix.error }
 
   external wait_cqe : t -> cqe_option = "ocaml_uring_wait_cqe"
   external wait_cqe_timeout : float -> t -> cqe_option = "ocaml_uring_wait_cqe_timeout"
@@ -357,6 +358,8 @@ module Uring = struct
   external register_eventfd : t -> Unix.file_descr -> unit = "ocaml_uring_register_eventfd"
 end
 
+type 'a job = 'a Heap.entry * Uring.job
+
 type 'a t = {
   id : < >;
   uring: Uring.t;
@@ -439,19 +442,19 @@ let exit t =
   Uring.exit t.uring;
   unregister_gc_root t
 
-let with_id_full : type a. a t -> (Heap.ptr -> bool) -> a -> extra_data:'b -> a job option =
+let with_id_full : type a. a t -> (Heap.ptr -> Uring.job) -> a -> extra_data:'b -> a job option =
  fun t fn datum ~extra_data ->
    match Heap.alloc t.data datum ~extra_data with
-  | exception (Invalid_argument _ as ex) -> check t; raise ex
-  | entry ->
- let ptr = Heap.ptr entry in
- let has_space = fn ptr in
- if has_space then
-   Some entry
- else (
-   ignore (Heap.free t.data ptr : a);
-   None
- )
+   | exception (Invalid_argument _ as ex) -> check t; raise ex
+   | entry ->
+     let ptr = Heap.ptr entry in
+     let job = fn ptr in
+     if (job :> int) >= 0 then
+       Some (entry, job)
+     else (
+       ignore (Heap.free t.data ptr : a);
+       None
+     )
 
 let with_id t fn a = with_id_full t fn a ~extra_data:()
 
@@ -465,9 +468,7 @@ let timeout ?(absolute = false) t clock timeout_ns user_data =
   set_timespec timespec_ptr timeout_ns;
   with_id t (fun id -> Uring.submit_timeout t.uring id timespec_ptr clock absolute) user_data
 
-let at_fdcwd : Unix.file_descr = Obj.magic Config.at_fdcwd
-
-let openat2 t ~access ~flags ~perm ~resolve ?(fd=at_fdcwd) path user_data =
+let openat2 t ~access ~flags ~perm ~resolve ?fd path user_data =
   let open_flags = flags lor match access with
     | `R  -> Open_flags.rdonly
     | `W  -> Open_flags.wronly
@@ -482,20 +483,20 @@ module Linkat_flags = struct
   let symlink_follow  = Config.At.symlink_follow
 end
 
-let linkat t ?(old_dir_fd=at_fdcwd) ?(new_dir_fd=at_fdcwd) ~flags ~old_path ~new_path user_data =
+let linkat t ?old_dir_fd ?new_dir_fd ~flags ~old_path ~new_path user_data =
   with_id t (fun id ->
     let old_path_buf = Sketch.String.alloc t.sketch old_path in
     let new_path_buf = Sketch.String.alloc t.sketch new_path in
     Uring.submit_linkat t.uring id old_dir_fd old_path_buf new_dir_fd new_path_buf flags
   ) user_data
 
-let unlink t ~dir ?(fd=at_fdcwd) path user_data =
+let unlink t ~dir ?fd path user_data =
   with_id t (fun id ->
       let buf = Sketch.String.alloc t.sketch path in
       Uring.submit_unlinkat t.uring id fd buf dir
     ) user_data
 
-let mkdirat t ~mode ?(fd=at_fdcwd) path user_data =
+let mkdirat t ~mode ?fd path user_data =
   with_id t (fun id ->
       let buf = Sketch.String.alloc t.sketch path in
       Uring.submit_mkdirat t.uring id fd buf mode
@@ -541,7 +542,7 @@ let poll_add t fd poll_mask user_data =
 let close t fd user_data =
   with_id t (fun id -> Uring.submit_close t.uring fd id) user_data
 
-let statx t ?(fd=at_fdcwd) ~mask path statx flags user_data =
+let statx t ?fd ~mask path statx flags user_data =
   let spath = Sketch.String.alloc t.sketch path in
   with_id_full t (fun id -> Uring.submit_statx t.uring id fd statx spath flags mask) user_data ~extra_data:statx
 
@@ -584,9 +585,9 @@ let fsync t ?(off=0L) ?(len=0) fd user_data =
 let fdatasync t ?(off=0L) ?(len=0) fd user_data =
   with_id t (fun id -> Uring.submit_fdatasync t.uring id fd off len) user_data
 
-let cancel t job user_data =
-  ignore (Heap.ptr job : Uring.id);  (* Check it's still valid *)
-  with_id t (fun id -> Uring.submit_cancel t.uring id (Heap.ptr job)) user_data
+let cancel t (entry, job) user_data =
+  ignore (Heap.ptr entry : Uring.id);  (* Check it's still valid *)
+  with_id t (fun id -> Uring.submit_cancel t.uring id job) user_data
 
 let sqe_ready t = Uring.sq_ready t.uring
 
@@ -613,14 +614,26 @@ let submit t =
 
 type 'a completion_option =
   | None
-  | Some of { result: int; data: 'a }
+  | Unit of { result: unit; data: 'a }
+  | Int of { result: int; data: 'a }
+  | FD of { result: Unix.file_descr; data: 'a }
+  | Error of { result: Unix.error; data: 'a }
 
 let fn_on_ring fn t =
   match fn t.uring with
   | Uring.Cqe_none -> None
-  | Uring.Cqe_some { user_data_id; res } ->
+  | Uring.Cqe_unit { user_data_id; result } ->
+    let data = Heap.free t.data user_data_id in
+    Unit { result; data }
+  | Uring.Cqe_int { user_data_id; result } ->
+    let data = Heap.free t.data user_data_id in
+    Int { result; data }
+  | Uring.Cqe_fd { user_data_id; result } ->
+    let data = Heap.free t.data user_data_id in
+    FD { result; data }
+  | Uring.Cqe_error { user_data_id; result } ->
     let data = Heap.free t.data user_data_id in
-    Some { result = res; data }
+    Error { result; data }
 
 let get_cqe_nonblocking t =
   check t;
diff --git a/lib/uring/uring.mli b/lib/uring/uring.mli
index 55d431bc..8a963f1a 100644
--- a/lib/uring/uring.mli
+++ b/lib/uring/uring.mli
@@ -842,11 +842,14 @@ val submit : 'a t -> int
 
 type 'a completion_option =
   | None
-  | Some of { result: int; data: 'a } (**)
+  | Unit of { result: unit; data: 'a }
+  | Int of { result: int; data: 'a }
+  | FD of { result: Unix.file_descr; data: 'a }
+  | Error of { result: Unix.error; data: 'a } (**)
 (** The type of results of calling {!wait} and {!peek}. [None] denotes that
     either there were no completions in the queue or an interrupt / timeout
-    occurred. [Some] contains both the user data attached to the completed
-    request and the integer syscall result. *)
+    occurred. The other constructors contain both the user data attached to the
+    completed request and syscall result. *)
 
 val wait : ?timeout:float -> 'a t -> 'a completion_option
 (** [wait ?timeout t] will block indefinitely (the default) or for [timeout]
diff --git a/lib/uring/uring_stubs.c b/lib/uring/uring_stubs.c
index 3ae351aa..baa7d7f9 100644
--- a/lib/uring/uring_stubs.c
+++ b/lib/uring/uring_stubs.c
@@ -145,14 +145,21 @@ value ocaml_uring_exit(value v_uring) {
   CAMLreturn(Val_unit);
 }
 
+#define Data_val(v_id, kind) ((void *)(Long_val(v_id) | ((intnat)kind << (8 * sizeof(value) - 4))))
+#define DATA_MASK (~((intnat)((intnat)3 << (8 * sizeof(value) - 4))))
+#define RES_UNIT 0
+#define RES_INT 1
+#define RES_FD 2
+
 value /* noalloc */
 ocaml_uring_submit_nop(value v_uring, value v_id) {
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   io_uring_prep_nop(sqe);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
@@ -193,10 +200,11 @@ ocaml_uring_submit_timeout(value v_uring, value v_id, value v_sketch_ptr, value
     flags |= IORING_TIMEOUT_ABS;
 
   sqe = io_uring_get_sqe(ring);
-  if (!sqe) return Val_false;
+  if (!sqe) return Val_long(-1);
   io_uring_prep_timeout(sqe, t, 0, flags);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return Val_true;
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return Val_long((intnat)id);
 }
 
 struct open_how_data {
@@ -242,27 +250,37 @@ ocaml_uring_make_open_how(value v_flags, value v_mode, value v_resolve, value v_
   CAMLreturn(v);
 }
 
+static int with_at_fdcwd(value v_fd)
+{
+  if (Is_none(v_fd))
+    return AT_FDCWD;
+  else
+    return Int_val(Some_val(v_fd));
+}
+
 // Caller must ensure v_open_how is not GC'd until the job is finished.
 value /* noalloc */
 ocaml_uring_submit_openat2(value v_uring, value v_id, value v_fd, value v_open_how) {
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   struct open_how_data *data = Open_how_val(v_open_how);
-  io_uring_prep_openat2(sqe, Int_val(v_fd), data->path, &data->how);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  io_uring_prep_openat2(sqe, with_at_fdcwd(v_fd), data->path, &data->how);
+  void *id = Data_val(v_id, RES_FD);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
 ocaml_uring_submit_close(value v_uring, value v_fd, value v_id) {
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   dprintf("submit_close: fd:%d\n", Int_val(v_fd));
   io_uring_prep_close(sqe, Int_val(v_fd));
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
@@ -270,11 +288,12 @@ ocaml_uring_submit_poll_add(value v_uring, value v_fd, value v_id, value v_poll_
   int poll_mask = Int_val(v_poll_mask);
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   dprintf("submit_poll_add: fd:%d mask:%x\n", Int_val(v_fd), poll_mask);
   io_uring_prep_poll_add(sqe, Int_val(v_fd), poll_mask);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_INT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
@@ -306,11 +325,12 @@ ocaml_uring_submit_readv(value v_uring, value v_fd, value v_id, value v_sketch_p
   size_t len = Sketch_ptr_len_val(v_sketch_ptr) / sizeof(*iovs);
 
   if (sqe == NULL)
-    return (Val_false);
+    return (Val_long(-1));
   dprintf("submit_readv: %d ents len[0] %lu off %d\n", len, iovs[0].iov_len, Int63_val(v_fileoff));
   io_uring_prep_readv(sqe, Int_val(v_fd), iovs, len, Int63_val(v_fileoff));
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_INT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 // Caller must ensure the buffers pointed to by v_sketch_ptr are not GC'd until the job is finished.
@@ -322,11 +342,12 @@ ocaml_uring_submit_writev(value v_uring, value v_fd, value v_id, value v_sketch_
   size_t len = Sketch_ptr_len_val(v_sketch_ptr) / sizeof(*iovs);
 
   if (sqe == NULL)
-    return (Val_false);
+    return (Val_long(-1));
   dprintf("submit_writev: %d ents len[0] %lu off %d\n", len, iovs[0].iov_len, Int63_val(v_fileoff));
   io_uring_prep_writev(sqe, Int_val(v_fd), iovs, len, Int63_val(v_fileoff));
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_INT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 // Caller must ensure the buffers are not released until this job completes.
@@ -335,11 +356,12 @@ ocaml_uring_submit_readv_fixed_native(value v_uring, value v_fd, value v_id, val
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
   void *buf = Caml_ba_data_val(v_ba) + Long_val(v_off);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   dprintf("submit_readv_fixed: buf %p off %d len %d fileoff %d", buf, Int_val(v_off), Int_val(v_len), Int63_val(v_fileoff));
   io_uring_prep_read_fixed(sqe, Int_val(v_fd), buf, Int_val(v_len), Int63_val(v_fileoff), 0);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_INT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value
@@ -361,11 +383,12 @@ ocaml_uring_submit_writev_fixed_native(value v_uring, value v_fd, value v_id, va
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
   void *buf = Caml_ba_data_val(v_ba) + Long_val(v_off);
   if (!sqe)
-    return (Val_false);
+    return (Val_long(-1));
   dprintf("submit_writev_fixed: buf %p off %d len %d fileoff %d", buf, Int_val(v_off), Int_val(v_len), Int63_val(v_fileoff));
   io_uring_prep_write_fixed(sqe, Int_val(v_fd), buf, Int_val(v_len), Int63_val(v_fileoff), 0);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_INT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value
@@ -388,12 +411,13 @@ ocaml_uring_submit_read(value v_uring, value v_fd, value v_id, value v_cstruct,
   value v_off = Field(v_cstruct, 1);
   value v_len = Field(v_cstruct, 2);
   void *buf = Caml_ba_data_val(v_ba) + Long_val(v_off);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   dprintf("submit_read: fd %d buff %p len %zd fileoff %d\n",
 	  Int_val(v_fd), buf, Long_val(v_len), Int63_val(v_fileoff));
   io_uring_prep_read(sqe, Int_val(v_fd), buf, Long_val(v_len), Int63_val(v_fileoff));
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_INT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
@@ -404,25 +428,27 @@ ocaml_uring_submit_write(value v_uring, value v_fd, value v_id, value v_cstruct,
   value v_off = Field(v_cstruct, 1);
   value v_len = Field(v_cstruct, 2);
   void *buf = Caml_ba_data_val(v_ba) + Long_val(v_off);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   dprintf("submit_write: fd %d buff %p len %zd fileoff %d\n",
 	  Int_val(v_fd), buf, Long_val(v_len), Int63_val(v_fileoff));
   io_uring_prep_write(sqe, Int_val(v_fd), buf, Long_val(v_len), Int63_val(v_fileoff));
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_INT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
 ocaml_uring_submit_splice(value v_uring, value v_id, value v_fd_in, value v_fd_out, value v_nbytes) {
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   io_uring_prep_splice(sqe,
 		       Int_val(v_fd_in), (int64_t) -1,
 		       Int_val(v_fd_out), (int64_t) -1,
 		       Int_val(v_nbytes), 0);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_INT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 #define Statx_val(v) (*((struct statx **) Data_custom_val(v)))
@@ -481,11 +507,12 @@ value
 ocaml_uring_submit_statx_native(value v_uring, value v_id, value v_fd, value v_statx, value v_sketch_ptr, value v_flags, value v_mask) {
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   char *path = Sketch_ptr_val(v_sketch_ptr);
-  io_uring_prep_statx(sqe, Int_val(v_fd), path, Int_val(v_flags), Int_val(v_mask), Statx_val(v_statx));
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  io_uring_prep_statx(sqe, with_at_fdcwd(v_fd), path, Int_val(v_flags), Int_val(v_mask), Statx_val(v_statx));
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value
@@ -783,10 +810,11 @@ ocaml_uring_submit_bind(value v_uring, value v_id, value v_fd, value v_sockaddr)
   struct io_uring_sqe *sqe;
   struct sock_addr_data *addr = Sock_addr_val(v_sockaddr);
   sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   io_uring_prep_bind(sqe, Int_val(v_fd), &(addr->sock_addr_addr.s_gen), addr->sock_addr_len);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
@@ -794,10 +822,11 @@ ocaml_uring_submit_listen(value v_uring, value v_id, value v_fd, value v_backlog
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe;
   sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   io_uring_prep_listen(sqe, Int_val(v_fd), Int_val(v_backlog));
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 // v_sockaddr must not be GC'd while the call is in progress
@@ -807,10 +836,11 @@ ocaml_uring_submit_connect(value v_uring, value v_id, value v_fd, value v_sockad
   struct io_uring_sqe *sqe;
   struct sock_addr_data *addr = Sock_addr_val(v_sockaddr);
   sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   io_uring_prep_connect(sqe, Int_val(v_fd), &(addr->sock_addr_addr.s_gen), addr->sock_addr_len);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 // v_msghdr must not be GC'd while the call is in progress
@@ -819,13 +849,14 @@ ocaml_uring_submit_send_msg(value v_uring, value v_id, value v_fd, value v_msghd
   struct io_uring *ring = Ring_val(v_uring);
   struct msghdr *msg = Msghdr_val(Field(v_msghdr, 0));
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
-  if (sqe == NULL) return (Val_false);
+  if (sqe == NULL) return (Val_long(-1));
   msg->msg_iov = Sketch_ptr_val(v_sketch_ptr);
   msg->msg_iovlen = Sketch_ptr_len_val(v_sketch_ptr) / sizeof(struct iovec);
   dprintf("submit_sendmsg\n");
   io_uring_prep_sendmsg(sqe, Int_val(v_fd), msg, 0);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_INT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 // v_msghdr must not be GC'd while the call is in progress
@@ -834,13 +865,14 @@ ocaml_uring_submit_recv_msg(value v_uring, value v_id, value v_fd, value v_msghd
   struct io_uring *ring = Ring_val(v_uring);
   struct msghdr *msg = Msghdr_val(Field(v_msghdr, 0));
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
-  if (sqe == NULL) return (Val_false);
+  if (sqe == NULL) return (Val_long(-1));
   msg->msg_iov = Sketch_ptr_val(v_sketch_ptr);
   msg->msg_iovlen = Sketch_ptr_len_val(v_sketch_ptr) / sizeof(struct iovec);
   dprintf("submit_recvmsg:msghdr %p: registering iobuf base %p len %lu\n", msg, msg->msg_iov[0].iov_base, msg->msg_iov[0].iov_len);
   io_uring_prep_recvmsg(sqe, Int_val(v_fd), msg, 0);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_INT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 // v_sockaddr must not be GC'd while the call is in progress
@@ -851,10 +883,11 @@ ocaml_uring_submit_accept(value v_uring, value v_id, value v_fd, value v_sockadd
   struct sock_addr_data *addr = Sock_addr_val(v_sockaddr);
   addr->sock_addr_len = sizeof(union sock_addr_union);
   sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   io_uring_prep_accept(sqe, Int_val(v_fd), &(addr->sock_addr_addr.s_gen), &addr->sock_addr_len, SOCK_CLOEXEC);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_FD);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
@@ -872,10 +905,11 @@ ocaml_uring_submit_unlinkat(value v_uring, value v_id, value v_fd, value v_sketc
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
   int flags = Bool_val(v_rmdir) ? AT_REMOVEDIR : 0;
   char *path = Sketch_ptr_val(v_sketch_ptr);
-  if (!sqe) return (Val_false);
-  io_uring_prep_unlinkat(sqe, Int_val(v_fd), path, flags);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  if (!sqe) return (Val_long(-1));
+  io_uring_prep_unlinkat(sqe, with_at_fdcwd(v_fd), path, flags);
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
@@ -883,10 +917,11 @@ ocaml_uring_submit_mkdirat(value v_uring, value v_id, value v_fd, value v_sketch
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
   char *path = Sketch_ptr_val(v_sketch_ptr);
-  if (!sqe) return (Val_false);
-  io_uring_prep_mkdirat(sqe, Int_val(v_fd), path, Int_val(v_mode));
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  if (!sqe) return (Val_long(-1));
+  io_uring_prep_mkdirat(sqe, with_at_fdcwd(v_fd), path, Int_val(v_mode));
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
@@ -894,12 +929,13 @@ ocaml_uring_submit_fsync(value v_uring, value v_id, value v_fd, value v_off, val
 {
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   io_uring_prep_fsync(sqe, Int_val(v_fd), 0);
   sqe->off = Int64_val(v_off);
   sqe->len = Int_val(v_len);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
@@ -907,12 +943,13 @@ ocaml_uring_submit_fdatasync(value v_uring, value v_id, value v_fd, value v_off,
 {
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   io_uring_prep_fsync(sqe, Int_val(v_fd), IORING_FSYNC_DATASYNC);
   sqe->off = Int64_val(v_off);
   sqe->len = Int_val(v_len);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value /* noalloc */
@@ -920,10 +957,12 @@ ocaml_uring_submit_cancel(value v_uring, value v_id, value v_target) {
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_sqe *sqe;
   sqe = io_uring_get_sqe(ring);
-  if (!sqe) return (Val_false);
+  if (!sqe) return (Val_long(-1));
   io_uring_prep_cancel(sqe, (void *)Long_val(v_target), 0);
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
-  return (Val_true);
+  /* RES_UNIT, since flags == 0 */
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
+  return (Val_long((intnat)id));
 }
 
 value ocaml_uring_submit(value v_uring)
@@ -938,21 +977,34 @@ value ocaml_uring_submit(value v_uring)
 
 #define Val_cqe_none Val_int(0)
 
-static value Val_cqe_some(value id, value res) {
-  CAMLparam2(id, res);
-  CAMLlocal1(some);
-  some = caml_alloc(2, 0);
-  Store_field(some, 0, id);
-  Store_field(some, 1, res);
+static value cqe_error(intnat id, int res) {
+  CAMLparam0();
+  CAMLlocal1(err);
+  err = unix_error_of_code(-res);
+  value some = caml_alloc_small(2, 3);
+  Field(some, 0) = Val_long(id & DATA_MASK);
+  Field(some, 1) = err;
   CAMLreturn(some);
 }
 
+static value Val_cqe_some(intnat id, int res) {
+  value some;
+  if (res < 0) {
+    some = cqe_error(id, res);
+  } else {
+    some = caml_alloc_small(2, (id >> (8 * sizeof(value) - 4)));
+    Field(some, 1) = Val_int(res);
+  }
+  Field(some, 0) = Val_long(id & DATA_MASK);
+  return some;
+}
+
 value ocaml_uring_wait_cqe_timeout(value v_timeout, value v_uring)
 {
   CAMLparam2(v_uring, v_timeout);
   double timeout = Double_val(v_timeout);
   struct __kernel_timespec t;
-  long id;
+  intnat id;
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_cqe *cqe;
   int res;
@@ -971,17 +1023,17 @@ value ocaml_uring_wait_cqe_timeout(value v_timeout, value v_uring)
   } else {
     if (!cqe)
       CAMLreturn(Val_cqe_none);
-    id = (long)io_uring_cqe_get_data(cqe);
+    id = (intnat)io_uring_cqe_get_data(cqe);
     int cqe_res = cqe->res;
     io_uring_cqe_seen(ring, cqe);
-    CAMLreturn(Val_cqe_some(Val_int(id), Val_int(cqe_res)));
+    CAMLreturn(Val_cqe_some(id, cqe_res));
   }
 }
 
 value ocaml_uring_wait_cqe(value v_uring)
 {
   CAMLparam1(v_uring);
-  long id;
+  intnat id;
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_cqe *cqe;
   int res;
@@ -996,17 +1048,17 @@ value ocaml_uring_wait_cqe(value v_uring)
       unix_error(-res, "io_uring_wait_cqe", Nothing);
     }
   } else {
-    id = (long)io_uring_cqe_get_data(cqe);
+    id = (intnat)io_uring_cqe_get_data(cqe);
     int cqe_res = cqe->res;
     io_uring_cqe_seen(ring, cqe);
-    CAMLreturn(Val_cqe_some(Val_int(id), Val_int(cqe_res)));
+    CAMLreturn(Val_cqe_some(id, cqe_res));
   }
 }
 
 value ocaml_uring_peek_cqe(value v_uring)
 {
   CAMLparam1(v_uring);
-  long id;
+  intnat id;
   struct io_uring *ring = Ring_val(v_uring);
   struct io_uring_cqe *cqe;
   int res;
@@ -1019,10 +1071,10 @@ value ocaml_uring_peek_cqe(value v_uring)
       unix_error(-res, "io_uring_peek_cqe", Nothing);
     }
   } else {
-    id = (long)io_uring_cqe_get_data(cqe);
+    id = (intnat)io_uring_cqe_get_data(cqe);
     int cqe_res = cqe->res;
     io_uring_cqe_seen(ring, cqe);
-    CAMLreturn(Val_cqe_some(Val_int(id), Val_int(cqe_res)));
+    CAMLreturn(Val_cqe_some(id, cqe_res));
   }
 }
 
@@ -1095,12 +1147,13 @@ ocaml_uring_submit_linkat_native(value v_uring, value v_id,
   struct io_uring_sqe *sqe = io_uring_get_sqe(ring);
 
   if (!sqe)
-    return Val_false;
+    return Val_long(-1);
 
-  io_uring_prep_linkat(sqe, Int_val(v_old_dir), old_path, Int_val(v_new_dir), new_path, Int_val(v_flags));
-  io_uring_sqe_set_data(sqe, (void *)Long_val(v_id));
+  io_uring_prep_linkat(sqe, with_at_fdcwd(v_old_dir), old_path, with_at_fdcwd(v_new_dir), new_path, Int_val(v_flags));
+  void *id = Data_val(v_id, RES_UNIT);
+  io_uring_sqe_set_data(sqe, id);
 
-  return Val_true;
+  return Val_long((intnat)id);
 }
 
 value
diff --git a/tests/main.md b/tests/main.md
index 51da6ac5..c165e019 100644
--- a/tests/main.md
+++ b/tests/main.md
@@ -18,9 +18,26 @@ end
 
 let rec consume t =
   match Uring.wait ~timeout:1. t with
-  | Some { data; result } -> (data, result)
+  | Int { data; result } -> (data, result)
+  | Unit { data; result = _ } -> (data, 0)
+  | FD _ | Error _ -> failwith "Unexpected return from syscall"
   | None -> consume t
 
+let rec consume_result t =
+  match Uring.wait ~timeout:1. t with
+  | Error { data; result } -> (data, Error result)
+  | Unit { data; result = () } -> (data, Ok 0)
+  | Int { data; result } -> (data, Ok result)
+  | FD _ -> failwith "Unexpected return from syscall"
+  | None -> consume_result t
+
+let rec consume_fd path t =
+  match Uring.wait ~timeout:1. t with
+  | FD { data; result } -> data, result
+  | Error { data = _; result } -> raise (Unix.Unix_error (result, "openat2", path))
+  | Int _ | Unit _ -> failwith "Unexpected return from syscall"
+  | None -> consume_fd path t
+
 let traceln fmt =
   Format.printf (fmt ^^ "@.")
 ```
@@ -134,10 +151,7 @@ val t : [ `Open ] Uring.t = <abstr>
 # Uring.submit t;;;
 - : int = 1
 
-# let token, fd =
-    let token, fd = consume t in
-    assert (fd >= 0);
-    token, (Obj.magic fd : Unix.file_descr);;
+# let token, fd = consume_fd "/dev/null" t;;
 val token : [ `Open ] = `Open
 val fd : Unix.file_descr = <abstr>
 
@@ -170,10 +184,7 @@ val t : [ `Create ] Uring.t = <abstr>
 # Uring.submit t;;
 - : int = 1
 
-# let token, fd =
-    let token, fd = consume t in
-    assert (fd >= 0);
-    token, (Obj.magic fd : Unix.file_descr);;
+# let token, fd = consume_fd "test-openat" t;;
 val token : [ `Create ] = `Create
 val fd : Unix.file_descr = <abstr>
 
@@ -252,10 +263,7 @@ Now using `~fd`:
 # Uring.submit t;;
 - : int = 1
 
-# let token, fd =
-    let token, fd = consume t in
-    assert (fd >= 0);
-    token, (Obj.magic fd : Unix.file_descr);;
+# let token, fd = consume_fd "test-openat" t;;
 val token : [ `Open_path | `Statx ] = `Open_path
 val fd : Unix.file_descr = <abstr>
 
@@ -301,14 +309,9 @@ val t : [ `Get_path ] Uring.t = <abstr>
                             path
                             `Get_path));
     traceln "Submitted %d" (Uring.submit t);
-    let `Get_path, fd = consume t in
-    if fd >= 0 then (
-      let fd : Unix.file_descr = Obj.magic fd in
-      Unix.close fd;
-      traceln "Opened %S OK" path
-    ) else (
-      raise (Unix.Unix_error (Uring.error_of_errno fd, "openat2", path))
-    );;
+    let `Get_path, fd = consume_fd path t in
+    Unix.close fd;
+    traceln "Opened %S OK" path;;
 val get : resolve:Uring.Resolve.t -> string -> unit = <fun>
 
 # get ~resolve:Uring.Resolve.empty ".";;
@@ -552,8 +555,8 @@ val fd : unit = ()
 Ask to read from a pipe (with no data available), then cancel it.
 
 ```ocaml
-# exception Multiple of Unix.error list;;
-exception Multiple of Unix.error list
+# exception Multiple of (int, Unix.error) result list;;
+exception Multiple of (int, Unix.error) result list
 
 # let t : [ `Cancel | `Read ] Uring.t = Uring.create ~queue_depth:5 ();;
 val t : [ `Cancel | `Read ] Uring.t = <abstr>
@@ -571,19 +574,19 @@ val read : [ `Cancel | `Read ] Uring.job = <abstr>
 - : [ `Cancel | `Read ] Uring.job option = Some <abstr>
 # Uring.submit t;;
 - : int = 2
-# let t1, r1 = consume t in
-  let t2, r2 = consume t in
+# let t1, r1 = consume_result t in
+  let t2, r2 = consume_result t in
   let r_read, r_cancel =
     match t1, t2 with
     | `Read, `Cancel -> r1, r2
     | `Cancel, `Read -> r2, r1
     | _ -> assert false
   in
-  begin match Uring.error_of_errno r_read, Uring.error_of_errno r_cancel with
-    | EINTR, EALREADY
+  begin match r_read, r_cancel with
+    | Error EINTR, Error EALREADY
       (* Occasionally, the read is actually busy just as we try to cancel.
          In that case it gets interrupted and the cancel returns EALREADY. *)
-    | EUNKNOWNERR 125 (* ECANCELLED *), EUNKNOWNERR 0 ->
+    | Error (EUNKNOWNERR 125) (* ECANCELLED *), Ok 0 ->
       (* This is the common case. The read is blocked and can just be removed. *)
       ()
     | e1, e2 -> raise (Multiple [e1; e2])
@@ -617,21 +620,22 @@ val read : [ `Cancel | `Read ] Uring.job = <abstr>
 - : [ `Cancel | `Read ] Uring.job option = Some <abstr>
 # Uring.submit t;;
 - : int = 1
-# let t1, r1 = consume t in
-  let t2, r2 = consume t in
+# let t1, r1 = consume_result t in
+  let t2, r2 = consume_result t in
   let r_read, r_cancel =
     match t1, t2 with
     | `Read, `Cancel -> r1, r2
     | `Cancel, `Read -> r2, r1
     | _ -> assert false
   in
-  if r_read = 1 then (
-    match Uring.error_of_errno r_cancel with
-    | ENOENT -> ()
-    | e -> raise (Unix.Unix_error (e, "cancel", ""))
+  if r_read = Ok 1 then (
+    match r_cancel with
+    | Ok _
+    | Error ENOENT -> ()
+    | Error e -> raise (Unix.Unix_error (e, "cancel", ""))
   ) else (
-    match Uring.error_of_errno r_read, Uring.error_of_errno r_cancel with
-    | EUNKNOWNERR 125 (* ECANCELLED *), EUNKNOWNERR 0 ->
+    match r_read, r_cancel with
+    | Error (EUNKNOWNERR 125) (* ECANCELLED *), Ok 0 ->
       (* This isn't the case we want to test, but it can happen sometimes. *)
       ()
     | e1, e2 -> raise (Multiple [e1; e2])
@@ -814,10 +818,10 @@ val check : unit -> bool * bool = <fun>
 - : unit Uring.job option = Some <abstr>
 
 # Uring.wait t;;
-- : unit Uring.completion_option = Uring.Some {Uring.result = 0; data = ()}
+- : unit Uring.completion_option = Uring.Unit {Uring.result = (); data = ()}
 
 # Uring.wait t;;
-- : unit Uring.completion_option = Uring.Some {Uring.result = 0; data = ()}
+- : unit Uring.completion_option = Uring.Unit {Uring.result = (); data = ()}
 
 # check ();;
 - : bool * bool = (false, false)
@@ -840,7 +844,7 @@ val t : unit Uring.t = <abstr>
 # Uring.submit t;;
 - : int = 1
 # Uring.wait t;;
-- : unit Uring.completion_option = Uring.Some {Uring.result = 0; data = ()}
+- : unit Uring.completion_option = Uring.Unit {Uring.result = (); data = ()}
 # (Unix.lstat "new-symlink").st_kind;;
 - : Unix.file_kind = Unix.S_LNK
 ```
@@ -852,7 +856,7 @@ This currently doesn't work due to https://github.com/axboe/liburing/issues/955:
     # Uring.submit t;;
     - : int = 1
     # Uring.wait t;;
-    - : unit Uring.completion_option = Uring.Some {Uring.result = 0; data = ()}
+    - : unit Uring.completion_option = Uring.Unit {Uring.result = (); data = ()}
     # (Unix.lstat "new-file").st_kind;;
     - : Unix.file_kind = Unix.S_REG
 
@@ -876,7 +880,7 @@ val t : [ `Mkdir of int ] Uring.t = <abstr>
 - : int = 1
 # Uring.wait t;;
 - : [ `Mkdir of int ] Uring.completion_option =
-Uring.Some {Uring.result = 0; data = `Mkdir 0}
+Uring.Unit {Uring.result = (); data = `Mkdir 0}
 # Printf.sprintf "0o%o" ((Unix.stat "mkdir").st_perm land 0o777);;
 - : string = "0o700"
 # let v = Uring.mkdirat t ~mode:0o755 "mkdir" (`Mkdir 1);;
@@ -885,7 +889,7 @@ val v : [ `Mkdir of int ] Uring.job option = Some <abstr>
 - : int = 1
 # Uring.wait t;;
 - : [ `Mkdir of int ] Uring.completion_option =
-Uring.Some {Uring.result = -17; data = `Mkdir 1}
+Uring.Error {Uring.result = Unix.EEXIST; data = `Mkdir 1}
 # Uring.exit t;;
 - : unit = ()
 ```
@@ -905,8 +909,8 @@ val t : [ `Timeout ] Uring.t = <abstr>
 # Uring.submit t;;
 - : int = 1
 
-# let `Timeout, timeout = consume t;;
-val timeout : int = -62
+# let `Timeout, timeout = consume_result t;;
+val timeout : (int, Unix.error) result = Error (Unix.EUNKNOWNERR 62)
 
 # let ns = 
     ((Unix.gettimeofday () +. 0.01) *. 1e9)
@@ -915,15 +919,15 @@ val timeout : int = -62
   Uring.(timeout ~absolute:true t Realtime ns `Timeout);;
 - : [ `Timeout ] Uring.job option = Some <abstr>
 
-# let `Timeout, timeout = consume t;;
-val timeout : int = -62
+# let `Timeout, timeout = consume_result t;;
+val timeout : (int, Unix.error) result = Error (Unix.EUNKNOWNERR 62)
 
 # let ns1 = Int64.(mul 10L 1_000_000L) in
   Uring.(timeout ~absolute:true t Boottime ns1 `Timeout);;
 - : [ `Timeout ] Uring.job option = Some <abstr>
 
-# let `Timeout, timeout = consume t;;
-val timeout : int = -62
+# let `Timeout, timeout = consume_result t;;
+val timeout : (int, Unix.error) result = Error (Unix.EUNKNOWNERR 62)
 
 # Uring.exit t;;
 - : unit = ()
diff --git a/tests/poll_add.ml b/tests/poll_add.ml
index 5bceb1c3..b589c880 100644
--- a/tests/poll_add.ml
+++ b/tests/poll_add.ml
@@ -13,7 +13,8 @@ let () =
   let rec retry () =
     match Uring.wait t with
     | None -> retry ()
-    | Some { result; _ } -> result
+    | Int { result; _ } -> result
+    | FD _ | Error _ | Unit _ -> failwith "Unexpected return from poll"
   in
   let res = retry () in
   Printf.eprintf "poll_add: %x\n%!" res;
diff --git a/tests/sketch.md b/tests/sketch.md
index 8fcc5a9f..36312653 100644
--- a/tests/sketch.md
+++ b/tests/sketch.md
@@ -11,7 +11,8 @@ let ldup n x = List.init n (Fun.const x)
 
 let rec consume t =
   match Uring.wait t with
-  | Some { data; result } -> (data, result)
+  | Int { data; result } -> (data, result)
+  | FD _ | Unit _ | Error _ -> failwith "Unexpected return from readv"
   | None -> consume t
 ```
 
diff --git a/tests/socket_ops.ml b/tests/socket_ops.ml
index 36b5c2be..a9541aef 100644
--- a/tests/socket_ops.ml
+++ b/tests/socket_ops.ml
@@ -13,45 +13,43 @@ let () =
   let addr = Unix.ADDR_INET (Unix.inet_addr_loopback, 0) in
 
   (* Use io_uring for bind operation *)
-  let bind_result =
+  let () =
     match Uring.bind t server_sock addr () with
     | None -> failwith "Failed to submit bind operation"
     | Some _job ->
         let _submitted = Uring.submit t in
         match Uring.wait t with
         | Uring.None -> failwith "No completion for bind"
-        | Uring.Some { result; data = _ } ->
-            if result < 0 then begin
-              Uring.close t server_sock () |> ignore;
-              Uring.submit t |> ignore;
-              Uring.exit t;
-              let err = Uring.error_of_errno (-result) in
-              failwith (sprintf "Bind failed: %s" (Unix.error_message err))
-            end else
-              result
+        | Uring.Unit _ ->
+            print_endline "Bind completed successfully:"
+        | Uring.Error { result; data = _ } ->
+            Uring.close t server_sock () |> ignore;
+            Uring.submit t |> ignore;
+            Uring.exit t;
+            failwith (sprintf "Bind failed: %s" (Unix.error_message result))
+        | Uring.Int _ | Uring.FD _ ->
+            failwith "Unexpected return from bind operation"
   in
-  printf "Bind completed with result: %d\n" bind_result;
 
   (* Use io_uring for listen operation *)
   let backlog = 10 in
-  let listen_result =
+  let () =
     match Uring.listen t server_sock backlog () with
     | None -> failwith "Failed to submit listen operation"
     | Some _job ->
         let _submitted = Uring.submit t in
         match Uring.wait t with
         | Uring.None -> failwith "No completion for listen"
-        | Uring.Some { result; data = _ } ->
-            if result < 0 then begin
-              Uring.close t server_sock () |> ignore;
-              Uring.submit t |> ignore;
-              Uring.exit t;
-              let err = Uring.error_of_errno (-result) in
-              failwith (sprintf "Listen failed: %s" (Unix.error_message err))
-            end else
-              result
+        | Uring.Unit _ ->
+            print_endline "Listen completed successfully"
+        | Uring.Error { result; data = _ } ->
+            Uring.close t server_sock () |> ignore;
+            Uring.submit t |> ignore;
+            Uring.exit t;
+            failwith (sprintf "Listen failed: %s" (Unix.error_message result))
+        | Uring.Int _ | Uring.FD _ ->
+            failwith "Unexpected return from listen operation"
   in
-  printf "Listen completed with result: %d\n" listen_result;
 
   (* Get the actual bound port - Unix.getsockname is necessary for socket introspection *)
   let actual_addr = Unix.getsockname server_sock in
@@ -71,30 +69,28 @@ let () =
 
   (* Use io_uring for connect operation *)
   let connect_addr = Unix.ADDR_INET (Unix.inet_addr_loopback, port) in
-  let connect_result =
+  let () =
     match Uring.connect t client_sock connect_addr () with
     | None -> failwith "Failed to submit connect operation"
     | Some _job ->
         let _submitted = Uring.submit t in
         match Uring.wait t with
         | Uring.None -> failwith "No completion for connect"
-        | Uring.Some { result; data = _ } ->
+        | Uring.Unit _ ->
+            print_endline "Connect initiated successfully\n"
+        | Uring.Error { result = Unix.EINPROGRESS; data = _ } ->
             (* Connect may return -EINPROGRESS for non-blocking sockets, which is normal *)
-            if result < 0 && result <> (-115) (* -EINPROGRESS *) then begin
-              Uring.close t client_sock () |> ignore;
-              Uring.close t server_sock () |> ignore;
-              Uring.submit t |> ignore;
-              Uring.exit t;
-              let err = Uring.error_of_errno (-result) in
-              failwith (sprintf "Connect failed: %s (errno: %d)" (Unix.error_message err) (-result))
-            end else
-              result
+            print_endline "Connect initiated successfully (result: EINPROGRESS)"
+        | Uring.Error { result; data = _ } ->
+            Uring.close t client_sock () |> ignore;
+            Uring.close t server_sock () |> ignore;
+            Uring.submit t |> ignore;
+            Uring.exit t;
+            failwith (sprintf "Connect failed: %s" (Unix.error_message result))
+        | Uring.Int _ | Uring.FD _ ->
+            failwith "Unexpected return from connect operation"
   in
 
-  if connect_result = 0 || connect_result = (-115) then
-    printf "Connect initiated successfully (result: %d)\n" connect_result
-  else
-    printf "Connect completed with result: %d\n" connect_result;
 
   (* Get the client socket's local port - Unix.getsockname is necessary for socket introspection *)
   let client_addr = Unix.getsockname client_sock in
@@ -122,12 +118,15 @@ let () =
     if pending > 0 then
       match Uring.wait t with
       | Uring.None -> failwith "No completion for close"
-      | Uring.Some { result; data = _ } ->
-          if result < 0 then
-            printf "Close warning: %s\n" (Unix.error_message (Uring.error_of_errno (-result)));
+      | Uring.Unit _ ->
           wait_closes (pending - 1)
+      | Uring.Error { result; data = _ } ->
+          printf "Close warning: %s\n" (Unix.error_message result);
+          wait_closes (pending - 1)
+      | Uring.Int _ | Uring.FD _ ->
+          failwith "Unexpected return from close operation"
   in
   wait_closes 2;
 
   Uring.exit t;
-  printf "Test completed successfully!\n"
\ No newline at end of file
+  printf "Test completed successfully!\n"
diff --git a/tests/urcat.ml b/tests/urcat.ml
index b114f5ca..1f5e4328 100644
--- a/tests/urcat.ml
+++ b/tests/urcat.ml
@@ -11,7 +11,8 @@ let get_file_size fd =
 let get_completion_and_print uring =
   let iov, len =
     match Uring.wait uring with
-    | Some { data; result } -> (data, result)
+    | Int { data; result } -> (data, result)
+    | FD _ | Error _ | Unit _ -> failwith "Unexpected return from read"
     | None -> failwith "retry"
   in
   let remaining = ref len in
diff --git a/tests/urcp_fixed_lib.ml b/tests/urcp_fixed_lib.ml
index 83d93c6f..a5379504 100644
--- a/tests/urcp_fixed_lib.ml
+++ b/tests/urcp_fixed_lib.ml
@@ -49,10 +49,6 @@ let queue_read uring t len =
   t.read_left <- t.read_left - len;
   t.reads <- t.reads + 1
 
-(* TODO compile time check *)
-let eagain = -11
-let eintr = -4
-
 (* Check that a read has completely finished, and if not
  * queue it up for completing the remaining amount *)
 let handle_read_completion uring req res =
@@ -61,13 +57,6 @@ let handle_read_completion uring req res =
   match res with
   | 0 ->
     Logs.debug (fun l -> l "eof %a" pp_req req);
-  | n when n = eagain || n = eintr ->
-    (* requeue the request *)
-    let r = Uring.read_fixed ~file_offset:req.fileoff uring req.t.infd ~off:req.fixed_off ~len:req.len req in
-    assert(r <> None);
-    Logs.debug (fun l -> l "requeued eintr read: %a" pp_req req);
-  | n when n < 0 ->
-    raise (Failure ("unix errorno " ^ (string_of_int n)))
   | n when n < bytes_to_read ->
     (* handle short read *)
     req.off <- req.off + n;
@@ -90,12 +79,6 @@ let handle_write_completion uring req res =
   let bytes_to_write = req.len - req.off in
   match res with
   | 0 -> raise End_of_file
-  | n when n = eagain || n = eintr ->
-    (* requeue the request *)
-    let r = Uring.write_fixed ~file_offset:req.fileoff uring req.t.outfd ~off:req.fixed_off ~len:req.len req in
-    assert(r <> None);
-    Logs.debug (fun l -> l "requeued eintr read: %a" pp_req req);
-  | n when n < 0 -> failwith (Fmt.str "unix error %d" (-n))
   | n when n < bytes_to_write ->
     (* handle short write  *)
     req.off <- req.off + n;
@@ -115,6 +98,19 @@ let handle_completion uring req res =
   |`R -> handle_read_completion uring req res
   |`W -> handle_write_completion uring req res
 
+let handle_retry uring req =
+  match req.op with
+  |`R ->
+    (* requeue the request *)
+    let r = Uring.read_fixed ~file_offset:req.fileoff uring req.t.infd ~off:req.fixed_off ~len:req.len req in
+    assert(r <> None);
+    Logs.debug (fun l -> l "requeued eintr read: %a" pp_req req)
+  |`W ->
+    (* requeue the request *)
+    let r = Uring.write_fixed ~file_offset:req.fileoff uring req.t.outfd ~off:req.fixed_off ~len:req.len req in
+    assert(r <> None);
+    Logs.debug (fun l -> l "requeued eintr read: %a" pp_req req)
+
 let copy_file uring t =
   (* Create a set of read requests that we will turn into write requests
    * up until the queue depth *)
@@ -137,11 +133,16 @@ let copy_file uring t =
       if t.write_left > 0 then begin
         let check_q = if !got_completion then Uring.get_cqe_nonblocking uring else Uring.wait uring  in
         match check_q with
-        |None -> Logs.debug (fun l -> l "completions: retry so finishing loop")
-        |Some { data; result } ->
+        | None -> Logs.debug (fun l -> l "completions: retry so finishing loop")
+        | Int { data; result } ->
           handle_completion uring data result;
           got_completion := true;
-          handle_completions ();
+          handle_completions ()
+        | Error { data; result = (Unix.EAGAIN | Unix.EINTR) } ->
+            handle_retry uring data
+        | Error { data = _; result } ->
+            failwith ("Unexpected error: " ^ Unix.error_message result)
+        | FD _ | Unit _ -> failwith "Unexpected return from syscall"
       end
     in
     handle_completions ();
diff --git a/tests/urcp_lib.ml b/tests/urcp_lib.ml
index b1d194fe..2b5f2118 100644
--- a/tests/urcp_lib.ml
+++ b/tests/urcp_lib.ml
@@ -54,10 +54,6 @@ let queue_read uring t len =
   t.read_left <- t.read_left - len;
   t.reads <- t.reads + 1
 
-(* TODO compile time check *)
-let eagain = -11
-let eintr = -4
-
 (* Check that a read has completely finished, and if not
  * queue it up for completing the remaining amount *)
 let handle_read_completion uring req res =
@@ -66,13 +62,6 @@ let handle_read_completion uring req res =
   match res with
   | 0 ->
     Logs.debug (fun l -> l "eof %a" pp_req req);
-  | n when n = eagain || n = eintr ->
-    (* requeue the request *)
-    let r = Uring.readv ~file_offset:req.fileoff uring req.t.infd req.iov.next req in
-    assert(r <> None);
-    Logs.debug (fun l -> l "requeued eintr read: %a" pp_req req);
-  | n when n < 0 ->
-    raise (Failure ("unix errorno " ^ (string_of_int n)))
   | n when n < bytes_to_read ->
     (* handle short read so new iovec and resubmit *)
     req.iov.next <- Cstruct.shiftv req.iov.next n;
@@ -97,11 +86,6 @@ let handle_write_completion uring req res =
   let bytes_to_write = req.len - req.off in
   match res with
   | 0 -> raise End_of_file
-  | n when n = eagain || n = eintr ->
-    (* requeue the request *)
-    let r = Uring.writev ~file_offset:req.fileoff uring req.t.infd req.iov.next req in
-    assert(r <> None);
-    Logs.debug (fun l -> l "requeued eintr read: %a" pp_req req);
   | n when n < bytes_to_write ->
     (* handle short write so new iovec and resubmit *)
     req.iov.next <- Cstruct.shiftv req.iov.next n;
@@ -120,6 +104,19 @@ let handle_completion uring req res =
   |`R -> handle_read_completion uring req res
   |`W -> handle_write_completion uring req res
 
+let handle_retry uring req =
+  match req.op with
+  |`R ->
+    (* requeue the request *)
+    let r = Uring.writev ~file_offset:req.fileoff uring req.t.infd req.iov.next req in
+    assert(r <> None);
+    Logs.debug (fun l -> l "requeued eintr read: %a" pp_req req)
+  |`W ->
+    (* requeue the request *)
+    let r = Uring.readv ~file_offset:req.fileoff uring req.t.infd req.iov.next req in
+    assert(r <> None);
+    Logs.debug (fun l -> l "requeued eintr read: %a" pp_req req)
+
 let copy_file uring t =
   (* Create a set of read requests that we will turn into write requests
    * up until the queue depth *)
@@ -142,11 +139,16 @@ let copy_file uring t =
       if t.write_left > 0 then begin
         let check_q = if !got_completion then Uring.get_cqe_nonblocking uring else Uring.wait uring  in
         match check_q with
-        |None -> Logs.debug (fun l -> l "completions: retry so finishing loop")
-        |Some { data; result } ->
+        | None -> Logs.debug (fun l -> l "completions: retry so finishing loop")
+        | Int { data; result } ->
           handle_completion uring data result;
           got_completion := true;
-          handle_completions ();
+          handle_completions ()
+        | Error { data; result = (Unix.EAGAIN | Unix.EINTR) } ->
+            handle_retry uring data
+        | Error { data = _; result } ->
+            failwith ("Unexpected error: " ^ Unix.error_message result)
+        | FD _ | Unit _ -> failwith "Unexpected return from syscall"
       end
     in
     handle_completions ();
diff --git a/tests/urstat.ml b/tests/urstat.ml
index 702c29ec..bff67fff 100644
--- a/tests/urstat.ml
+++ b/tests/urstat.ml
@@ -13,7 +13,8 @@ let pp_time f (sec, nsec) =
 let get_completion_and_print uring =
   let (fname, buf), _ =
     match Uring.wait uring with
-    | Some { data; result } -> (data, result)
+    | Unit { data; result } -> (data, result)
+    | Int _ | FD _ | Error _ -> failwith "Unexpected return from statx"
     | None -> failwith "retry"
   in
   let kind = S.kind buf in