Simultaneous reading of N pipes

let read_descriptors l =
  let buffer_map =
    List.map
      (fun fd ->
    Unix.set_nonblock fd;
    fd,Buffer.create 32) l in
  let data = String.create 1024 in
  let append fd len =
    let chunk =
      String.sub data 0 len in
    let buf =
      List.assoc fd buffer_map in
    if Buffer.length buf + len < Sys.max_string_length then
      Buffer.add_string buf chunk
  in
  let rec read l =
    if l <> [] then
      let (rl,_,_) = Unix.select l [] [] 0.1 in
      let ready =
    List.fold_left
      (fun acc fd ->
        let n = Unix.read fd data 0 1024 in
        if n = 0 then
          begin
        Unix.close fd;
        fd::acc
          end
        else
          begin
        append fd n;
        acc
          end) [] rl
      in
      read (List.filter (fun fd -> not (List.mem fd ready)) l)
    else ()
  in read l;
  List.map
    (fun (_,b) -> Buffer.contents b) buffer_map