Re: Looking for stubs for sendmsg/recvmsg

[Sylvain Le Gall <sylvain@le-gall.net>]

Hello,

On 19-11-2010, Goswin von Brederlow <goswin-v-b@web.de> wrote:
> Sylvain Le Gall <sylvain@le-gall.net> writes:
>
>> On 18-11-2010, Goswin von Brederlow <goswin-v-b@web.de> wrote:
>>> Hi,
>>>
>>> I'm looking for stubs for
>>>
>>>        ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);
>>>        ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);
>>>
>>> Specifically I need those to send (among normal messages) an
>>> Unix.file_descr over a Unix Domain Socket.
>>>
>>> Does anyone know of a module that has them?
>>>
>>
>> If you don't find one and plan to write it yourself, this would be a
>> good addition to extunix:
>> http://extunix.forge.ocamlcore.org
>>
>> Regards,
>> Sylvain Le Gall
>
> I'm thinking of changing Unix.file_descr from int to a custom block
> (containing the FD) with finalizer. Unix.close would set the FD to -1
> and the finalizer gives an error if FD != -1 and closes it.
>
> Actually I want that tunable with 3 possible behaviours:
>
> type fd_leak_mode = Silent | Complain | Fail
> val set_leak_mode : fd_leak_mode -> unit = <fun>
>
> Silent just closes the FD if it is still open, Complain (default)
> outputs to stderr and closes it and Fail aborts.
>
> That would change most of the Unix module and mean a complete fork of it.

Not that much, if you proceed in another way. I think what you are
looking for is a fd leak detector?

Here is a small modules that I have written for this purpose:

File UnixExt.ml:
(** Count open/close call *)
IFNDEF NDBUG THEN
let fd_opened =
  Hashtbl.create 13
;;

let fd_once_opened =
  Hashtbl.create 13
;;

let fd_open fd fn out =
  dbug_print
    (fun () -> 
       Printf.sprintf "%s '%s'" 
         (if out then "open-out" else "open-in")
         fn);
  Hashtbl.add fd_opened fd (fn, out)
;;

let fd_close fd =
  try 
    let (fn, out) as data = 
      Hashtbl.find fd_opened fd 
    in
      dbug_print
        (fun () ->
           Printf.sprintf "%s '%s'" 
             (if out then "close-out" else "close-in")
             fn);
      Hashtbl.add fd_once_opened fd data;
      Hashtbl.remove fd_opened fd;
  with Not_found ->
    begin 
      dbug_print
        (fun () -> 
           let fn =
             try 
               fst (Hashtbl.find fd_once_opened fd)
             with Not_found ->
               "unknown"
           in
             Printf.sprintf "Trying to close %s again" fn)
    end
;;

let () = 
  at_exit
    (fun () ->
       let exit_error =
         ref false
       in
         Hashtbl.iter 
           (fun fd (fn, out) ->
              if fd <> Unix.stdin && fd <> Unix.stdout && fd <> Unix.stderr then
                begin
                  Printf.eprintf "Not closed '%s' (out: %b)\n" fn out;
                  exit_error := true
                end)
           fd_opened;
         Hashtbl.clear fd_opened;
         if !exit_error then 
           exit 3
    )
;;

let opened_files () = 
  let lst = 
    ref []
  in
    Hashtbl.iter 
      (fun _ e -> lst := e :: !lst)
      fd_opened;
    List.sort compare !lst;

ELSE
let fd_open _ _ _ = 
  ()
;;

let fd_close _ = 
  ()
;;

let opened_files () = 
  []
;;
ENDIF

(** See UnixExt.mli *)
let to_file_descr_in fd = 
  fd_open fd "<converted>" false;
  fd
;;

(** See UnixExt.mli *)
let to_file_descr_out fd = 
  fd_open fd "<converted>" true;
  fd
;;

(** See UnixExt.mli *)
let close_in fd =
  Unix.close fd;
  fd_close fd
;;

(** See UnixExt.mli *)
let stdout =
  fd_open Unix.stdout "<stdout>" true;
  Unix.stdout
;;

[...override other functions that open/close fd...]

Then in the modules using this features, you just have to open UnixExt
after Unix...

You can even probably design a library that will transparently hide Unix
with a custom Unix module providing this feature.

>
> Would that be something for extunix too?
>

I don't think so. At least, this is not currently the purpose of
extunix... 

Regards,
Sylvain Le Gall