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.

By default, signals are handled asynchronously (i.e., at the time they arrive). It is inherently dangerous to call extensive code fragments, and especially exception related code from asynchronous handlers. The interface allows for synchronous handling of signals. In this case the native OS handler just schedules the signal using PL_raise(), which is checked by PL_handle_signals() at the call- and redo-port. This behaviour is realised by or-ing sig with the constant PL_SIGSYNC.^{173A better default would be to use synchronous handling, but this interface preserves backward compatibility.}

Signal handling routines may raise exceptions using PL_raise_exception(). The use of PL_throw() is not safe. If a synchronous handler raises an exception, the exception is delayed to the next call to PL_handle_signals();