diff --git a/src/Tiny_httpd_server.ml b/src/Tiny_httpd_server.ml
index 43b7cfd5..30c35d11 100644
--- a/src/Tiny_httpd_server.ml
+++ b/src/Tiny_httpd_server.ml
@@ -897,7 +897,7 @@ let handle_client_ (self : t) (client_sock : Unix.file_descr) : unit =
 
 let is_ipv6 self = String.contains self.addr ':'
 
-let run (self : t) : (unit, _) result =
+let run ?(after_init = ignore) (self : t) : (unit, _) result =
   try
     if self.masksigpipe then
       ignore (Unix.sigprocmask Unix.SIG_BLOCK [ Sys.sigpipe ] : _ list);
@@ -925,6 +925,7 @@ let run (self : t) : (unit, _) result =
       Unix.listen sock (2 * self.sem_max_connections.Sem_.n)
     );
     self.sock <- Some sock;
+    after_init ();
     while self.running do
       (* limit concurrency *)
       Sem_.acquire 1 self.sem_max_connections;
diff --git a/src/Tiny_httpd_server.mli b/src/Tiny_httpd_server.mli
index f4a41864..8f0e90a1 100644
--- a/src/Tiny_httpd_server.mli
+++ b/src/Tiny_httpd_server.mli
@@ -542,13 +542,17 @@ val stop : t -> unit
 (** Ask the server to stop. This might not have an immediate effect
     as {!run} might currently be waiting on IO. *)
 
-val run : t -> (unit, exn) result
+val run : ?after_init:(unit -> unit) -> t -> (unit, exn) result
 (** Run the main loop of the server, listening on a socket
     described at the server's creation time, using [new_thread] to
     start a thread for each new client.
 
     This returns [Ok ()] if the server exits gracefully, or [Error e] if
-    it exits with an error. *)
+    it exits with an error.
+
+    @param after_init is called after the server starts listening. @since NEXT_RELEASE .
+
+*)
 
 (**/**)
 
diff --git a/src/bin/http_of_dir.ml b/src/bin/http_of_dir.ml
index 9d192fdf..87a96811 100644
--- a/src/bin/http_of_dir.ml
+++ b/src/bin/http_of_dir.ml
@@ -5,15 +5,17 @@ module Pf = Printf
 
 let serve ~config (dir : string) addr port j : _ result =
   let server = S.create ~max_connections:j ~addr ~port () in
-  Printf.printf "serve directory %s on http://%(%s%):%d\n%!" dir
-    (if S.is_ipv6 server then
-      "[%s]"
-    else
-      "%s")
-    addr port;
+  let after_init () =
+    Printf.printf "serve directory %s on http://%(%s%):%d\n%!" dir
+      (if S.is_ipv6 server then
+        "[%s]"
+      else
+        "%s")
+      addr (S.port server)
+  in
 
   D.add_dir_path ~config ~dir ~prefix:"" server;
-  S.run server
+  S.run ~after_init server
 
 let parse_size s : int =
   try Scanf.sscanf s "%dM" (fun n -> n * 1_024 * 1_024)