diff --git a/example/main.ml b/example/main.ml
index 6999ece..e5645f3 100644
--- a/example/main.ml
+++ b/example/main.ml
@@ -1,8 +1,16 @@
 let ( / ) = Eio.Path.( / )
 
+module L = Eio_mem.Low_level
+
 let () =
-  Eio_main.run @@ fun _ ->
+  Eio_main.run @@ fun e ->
   Eio_mem.run @@ fun env ->
-  let cwd = Eio.Stdenv.cwd env in
-  Eio.Path.save ~create:(`Exclusive 0o666) (cwd / "test-file") "my-data";
-  Eio.traceln "Got %S" @@ Eio.Path.load (cwd / "test-file")
+  let () =
+    Eio.Path.with_open_out ~create:(`If_missing 0o644) (env#fs / "hello.txt")
+    @@ fun flow ->
+    let fd = Eio_mem.Resource.fd_opt flow |> Option.get in
+    let nfd = L.dup fd in
+    let _ : int = L.writev nfd [ Cstruct.of_string "Hello, World" ] in
+    ()
+  in
+  Eio.traceln "Got: %s" (Eio.Path.load (env#fs / "hello.txt"))
diff --git a/src/dune b/src/dune
index a2b57f8..a984522 100644
--- a/src/dune
+++ b/src/dune
@@ -1,4 +1,4 @@
 (library
  (name eio_mem)
  (public_name eio_mem)
- (libraries eio fpath))
+ (libraries eio eio.unix fpath))
diff --git a/src/eio_mem.ml b/src/eio_mem.ml
index 06f5c8a..cbc83f6 100644
--- a/src/eio_mem.ml
+++ b/src/eio_mem.ml
@@ -15,6 +15,8 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  *)
 open Eio.Std
+module Resource = Resource
+module Low_level = Low_level
 
 module Fs = struct
   module Config = struct
diff --git a/src/eio_mem.mli b/src/eio_mem.mli
index 162ba3f..1c512b1 100644
--- a/src/eio_mem.mli
+++ b/src/eio_mem.mli
@@ -22,6 +22,24 @@ module Fs : sig
   (** A [tree]-like pretty printer. *)
 end
 
+module Low_level = Low_level
+
+module Resource : sig
+  type 'a t = ([> `Mem_fd ] as 'a) Eio.Resource.t
+  (** Resources that have FDs are tagged with [`Mem_fd]. *)
+
+  type ('t, _, _) Eio.Resource.pi +=
+    | T : ('t, 't -> Low_level.Fd.t, [> `Mem_fd ]) Eio.Resource.pi
+
+  val fd : _ t -> Low_level.Fd.t
+  (** [fd t] returns the FD being wrapped by a resource. *)
+
+  val fd_opt : _ Eio.Resource.t -> Low_level.Fd.t option
+  (** [fd_opt t] returns the FD being wrapped by a generic resource, if any.
+
+      This just probes [t] using {!extension-T}. *)
+end
+
 type stdenv = < fs : Eio.Fs.dir_ty Eio.Path.t ; cwd : Eio.Fs.dir_ty Eio.Path.t >
 
 val run : ?cwd:string -> (stdenv -> 'a) -> 'a
diff --git a/src/flow.ml b/src/flow.ml
index 47aa4ce..7524ce8 100644
--- a/src/flow.ml
+++ b/src/flow.ml
@@ -48,6 +48,8 @@ end
 module type FLOW = sig
   include Eio.File.Pi.WRITE
   include Eio.Net.Pi.STREAM_SOCKET with type t := t
+
+  val fd : t -> Low_level.Fd.t
 end
 
 let flow_handler (type t tag)
@@ -57,6 +59,7 @@ let flow_handler (type t tag)
   Eio.Resource.handler
   @@ Eio.Resource.bindings (Eio.Net.Pi.stream_socket (module X))
   @ Eio.Resource.bindings (Eio.File.Pi.rw (module X))
+  @ [ Eio.Resource.H (Resource.T, X.fd) ]
 
 let handler = flow_handler (module Impl)
 let of_fd fd = Eio.Resource.T (fd, handler)
diff --git a/src/low_level.ml b/src/low_level.ml
index 8efe655..55cc050 100644
--- a/src/low_level.ml
+++ b/src/low_level.ml
@@ -53,8 +53,13 @@ module Error = struct
     let exn = Eio.Exn.create Eio.Fs.(E (Permission_denied (Eio_mem path))) in
     raise exn
 
-  let badf () =
-    let exn = Eio.Exn.create (Eio.Exn.X (Eio_mem "Bad file descriptor")) in
+  let badf ?msg () =
+    let msg =
+      match msg with
+      | None -> "Bad file descriptor"
+      | Some m -> Fmt.str "Bad file descriptor: %s" m
+    in
+    let exn = Eio.Exn.create (Eio.Exn.X (Eio_mem msg)) in
     raise exn
 
   let err ctx =
@@ -206,6 +211,33 @@ let next_fd_from_fds fds =
 
 let next_fd () = read_fd_table next_fd_from_fds
 
+let dup fd =
+  let r = read_fd_table (Fd_table.find_opt fd) in
+  match r with
+  | None -> Error.badf ()
+  | Some r ->
+      let new_fd = next_fd () in
+      let () = update_fd_table @@ fun tbl -> Fd_table.add new_fd r tbl in
+      new_fd
+
+let close fd =
+  update_fd_table @@ fun fds ->
+  match Fd_table.find_opt fd fds with
+  | None -> Error.badf ~msg:"close" ()
+  | Some _ -> Fd_table.remove fd fds
+
+let dup2 ~src ~tgt =
+  let src_r = read_fd_table (Fd_table.find_opt src) in
+  let tgt_r = read_fd_table (Fd_table.find_opt tgt) in
+  match src_r with
+  | None -> Error.badf ()
+  | Some r -> (
+      match tgt_r with
+      | None -> update_fd_table @@ fun tbl -> Fd_table.add tgt r tbl
+      | Some _ ->
+          close tgt;
+          update_fd_table @@ fun tbl -> Fd_table.add tgt r tbl)
+
 let default_stat =
   Eio.File.Stat.
     {
@@ -411,12 +443,6 @@ let entries = function
   | Resource.Dir { Dir.entries; _ } -> entries
   | r -> Fmt.invalid_arg "entries: %a" Resource.pp r
 
-let close fd =
-  update_fd_table @@ fun fds ->
-  match Fd_table.find_opt fd fds with
-  | None -> Error.badf ()
-  | Some _ -> Fd_table.remove fd fds
-
 let openat ~mode:_ ~sw dir_fd path flags =
   let path = check_path path in
   let is_create = Open_flags.(mem o_creat flags) in
diff --git a/test/dune b/test/dune
index a8b1c93..aa4c223 100644
--- a/test/dune
+++ b/test/dune
@@ -1,3 +1,3 @@
 (mdx
  (files fs.md)
- (libraries eio eio.core eio.mock optint eio_mem fmt))
+ (libraries eio eio.core cstruct eio.mock optint eio_mem fmt))
diff --git a/test/fs.md b/test/fs.md
index 92403fc..cf78672 100644
--- a/test/fs.md
+++ b/test/fs.md
@@ -10,6 +10,8 @@ open Eio.Std
 
 let ( / ) = Path.( / )
 
+module L = Eio_mem.Low_level
+
 let run ?clear:(paths = []) ?(cwd="/home/bactrian") fn =
   Eio_mock.Backend.run @@ fun () ->
   Eio_mem.run ~cwd @@ fun env ->
@@ -379,3 +381,18 @@ Confined:
 - : unit = ()
 ```
 
+# Low level API
+
+```ocaml
+# run ~clear:[ "hello.txt"; "world.txt" ] @@ fun env ->
+  let () =
+    Eio.Path.with_open_out ~create:(`If_missing 0o644) (env#fs / "hello.txt") @@ fun flow ->
+    let fd = Eio_mem.Resource.fd_opt flow |> Option.get in
+    let nfd = L.dup fd in
+    let _ : int = L.writev nfd [ Cstruct.of_string "Hello, World" ] in
+    ()
+  in
+  Eio.traceln "Got: %s" (Eio.Path.load (env#fs / "hello.txt")) 
++Got: Hello, World
+- : unit = ()
+```