I don't know if POSIX has a position on signal order. But I'm pretty sure it allows signals to be coallesced... if a process is sent the same signal several times before the handler is invoked, it's in spec to only invoke it once.

Real-time signals have guaranteed order: first by number (lowest first, i.e. `SIGRTMIN`), then by the order in which they are sent.

Signals are generally the slowest IPC method, unless you're doing something stupid with a different method.

Answering both of my questions, from the post:

  sleep 0.001 # Delay to allow the receiver to process the signal

For standard signals--no, but for real-time signals, yes. The latter are still a portability issue, though.