The action description is an atom denoting the name of the predicate that will be called if Signal arrives. on_signal/3 is a meta-predicate, which implies that <Module>:<Name> refers to <Name>/1 in module <Module>. The handler is called with a single argument: the name of the signal as an atom. The Prolog names for signals are explained below.

Two predicate names have special meaning. throw implies Prolog will map the signal onto a Prolog exception as described in section 4.11. default resets the handler to the settings active before SWI-Prolog manipulated the handler.

Signals bound to a foreign function through PL_signal() are reported using the term $foreign_function(Address).

After receiving a signal mapped to throw, the exception raised has the following structure:

error(signal(<SigName>, <SigNum>), <Context>)

The signal names are defined by the POSIX standard as symbols of the form SIG<SIGNAME>. The Prolog name for a signal is the lowercase version of <SIGNAME>. The predicate current_signal/3 may be used to map between names and signals.

Initially, some signals are mapped to throw, while all other signals are default. The following signals throw an exception: fpe, alrm, xcpu, xfsz and vtalrm.