Here's some example code once posted on the list to have a timeout.

let set_timer tsecs =
  ignore (Unix.setitimer Unix.ITIMER_REAL
                         { Unix.it_interval = 0.0; Unix.it_value = tsecs })

exception Timeout

let handle_sigalrm signo =
  print_string "Continue for how long? "; flush stdout;
  let f = read_float() in
  if f <= 0.0
  then raise Timeout
  else set_timer f

let continueq f arg tsecs defaultval =
  let oldsig = Sys.signal Sys.sigalrm (Sys.Signal_handle handle_sigalrm) in
  try
    set_timer tsecs;
    let res = f arg in
    set_timer 0.0;
    Sys.set_signal Sys.sigalrm oldsig;
    res
  with Timeout ->
    Sys.set_signal Sys.sigalrm oldsig;
    defaultval

Now, my question is: what happens with multiple threads? The exception
is raised in whatever thread is currently running? Is there also no
control over such behaviour?

Jonathan