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These predicates provide a mechanism to make another thread execute some goal as an interrupt. Signalling threads is safe as these interrupts are only checked at safe points in the virtual machine. Nevertheless, signalling in multithreaded environments should be handled with care as the receiving thread may hold a mutex (see with_mutex/2). Signalling probably only makes sense to start debugging threads and to cancel no-longer-needed threads with throw/1, where the receiving thread should be designed carefully to handle exceptions at any point.
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.
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.