An exception converter transforms the given OCaml exception into a 9P error response. 9P clients like Linux work better if they receive 9P2000.u errno values which happen to match their local errno definition.

Here is an example converter for Linux and OS X hosts:

let unix_exn_converter info exn =
  let is_unix = (info.Protocol_9p_info.version = Types.Version.unix) in
  match exn with
  | Unix.Unix_error(err, _, _) ->
    let host = match info.Protocol_9p_info.aname with
      | "linux#/" when is_unix -> Some Errno_host.Linux.v4_0_5
      | "osx#/" when is_unix -> Some Errno_host.OSX.v10_11_1
      | _ -> None
    in
    let errno = match host with
      | None -> None
      | Some host -> match Errno_unix.of_unix ~host err with
        | [] -> None
        | errno::_ -> match Errno.to_code ~host errno with
          | None -> None
          | Some i -> Some (Int32.of_int i)
    in
    Response.Err {
      Response.Err.ename = Unix.error_message err;
      errno;
    }
  | e ->
    Response.Err {
      Response.Err.ename = Printexc.to_string e;
      errno = None;
    }