Signals (interrupts) do not cooperate well with the world of multithreading, mainly because the status of mutexes cannot be guaranteed easily. At the call port, the Prolog virtual machine holds no locks and therefore the asynchronous execution is safe.

Goal can be any valid Prolog goal, including throw/1 to make the receiving thread generate an exception, and trace/0 to start tracing the receiving thread.

In the Windows version, the receiving thread immediately executes the signal if it reaches a Windows GetMessage() call, which generally happens if the thread is waiting for (user) input.