I want to flush output channels periodically.

If I use flush stdout in both the signal handler and the regular code, the application hangs.

Here a minimal reproducible example (which I tried on Linux and MacOS using OCaml 4.08 and 4.10):

let n_flush = ref 0;;

let main () =
  Sys.set_signal Sys.sigalrm (Signal_handle (fun _ -> incr n_flush; flush stdout));
  ignore(Unix.setitimer Unix.ITIMER_REAL { it_value = 0.001; it_interval = 0.001 });

  for i = 1 to 100 do
    let s = string_of_int i in
    print_string s;
    flush stdout;
    Unix.sleepf 0.02;
    for j = 1 to String.length s do print_char '\b' done
  done;
  print_string ("n_flush = " ^ string_of_int !n_flush);
  print_endline " ... DONE!";;

main ();;

Note that above time intervals are a lot higher in practise (~1/10s), I pumped them up so that the issue shows more easily.