]> > > is a "leave_blocking_section()" call enough to guarentee that? > > If the callback is invoked in a thread, yes. But then you are back > to square 1: leave_blocking_section will wait for the master lock, > and your program is not real-time. So what about the alternative option, as Xavier suggested earlier, to enter an event loop that that waits in an "enter_blocking_section" call with threads but no other running threads, and then callbacks are made during the loop one at a time? I think at the end, more speculation probably won't help much. I think I should just try different ways and do ad-hoc experiments to see which ones actually work acceptably, in the presence of garbage collection, interrupting signals, other threads, etc. If none of them work, at least there is a blocking I/O interface left for O'Caml. Speak of which, if I write a C function that blocks, does the bytecode thread scheduler know how to schedule other threads during the mean time? And native threads? liulk