9.3.2 Signalling threads

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.

thread_signal(+ThreadId, :Goal)

Make thread ThreadId execute Goal at the first opportunity. In the current implementation, this implies at the first pass through the Call port. The predicate thread_signal/2 itself places Goal into the signalled thread's signal queue and returns immediately.

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.