let mb x = x * 1024 * 1024

open Unix

let _ =
  let filename = Sys.argv.(1) in
  let filesize = (stat filename).st_size in
  Printf.printf "%d\n" filesize;

  let restbytes = ref filesize in

  let strlen = mb 100 in
  let string = String.make strlen ' ' in
  let writesize = ref strlen in

  let fd = openfile filename [O_RDWR] 0o640 in
  while !restbytes > 0
  do
    writesize := (min !restbytes strlen);

Printf.printf "restbytes: %d, writesize: %d\n" !restbytes !writesize;
flush Pervasives.stdout;

    let num = write fd string 0 !writesize in
    restbytes := max 0 (!restbytes - num );

Printf.printf "(just written: %d)\n" num;
Printf.printf "restbytes: %d\n-----------\n" (* HERE I STHE ERROR! *)

  done;
  close fd