If you wish to call Prolog back from a signal handler or a thread other
than the thread that called SP_initialize(), that is, the
main thread, you cannot use SP_query() etc. directly.
The call to Prolog has to be delayed until such time that the Prolog
execution can accept an interrupt and the call has to be performed from
the main thread (the Prolog execution thread). The function
SP_event() serves this purpose, and installs the function
func to be called from Prolog (in the main thread) when the
execution can accept a callback.
A queue of functions, with corresponding arguments, is maintained;
that is, if several calls to SP_event() occur before Prolog can
accept an interrupt, the functions are queued and executed in turn at
the next possible opportunity. A func installed with
SP_event() will not be called until SICStus is actually
running. One way of ensuring that all pending functions installed with
SP_event() are run is to call, from the main thread, some dummy
goal, such as,
SP_query_cut_fail(SP_predicate("true",0,"user")).
While SP_event() is safe to call from any thread, it is not safe
to call from arbitrary signal handlers. If you want to call
SP_event() when a signal is delivered, you need to install your
signal handler with SP_signal() (see below).
Note that SP_event() is one of the very few functions in
the SICStus API that can safely be called from another thread than the
main thread.