Catching Signals (Software Interrupts)

SWI-Prolog offers both a C and Prolog interface to deal with software interrupts (signals). The Prolog mapping is defined in signal. This subsection deals with handling signals from C.

If a signal is not used by Prolog and the handler does not call Prolog in any way, the native signal interface routines may be used.

Some versions of SWI-Prolog, notably running on popular Unix platforms, handle SIG_SEGV for guarding the Prolog stacks. If the application whishes to handle this signal too, it should use PL_signal to install its handler after initialisating Prolog. SWI-Prolog will pass SIG_SEGV to the user code if it detected the signal is not related to a Prolog stack overflow.

Any handler that wishes to call one of the Prolog interface functions should call PL_signal for its installation.

void (*)()PL_signalsig, func This function is equivalent to the BSD-Unix signal() function, regardless of the platform used. The signal handler is blocked while the signal routine is active, and automatically reactivated after the handler returns.

After a signal handler is registered using this function, the native signal interface redirects the signal to a generic signal handler inside SWI-Prolog. This generic handler validates the environment, creates a suitable environment for calling the interface functions described in this chapter and finally calls the registered user-handler.