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.

Four names have special meaning. throw implies Prolog will map the signal onto a Prolog exception as described in section 4.10, ignore causes Prolog to ignore the signal entirely, debug specifies the debug interrupt prompt that is initially bound to SIGINT (Control-C) and 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, the following signals are handled unless the command line option --no-signals is specified:

int

Prompts the user, allowing to inspect the current state of the process and start the tracer.

usr2

Bound to an empty signal handler used to make blocking system calls return. This allows thread_signal/2 to interrupt threads blocked in a system call. See also prolog_alert_signal/2.

pipe

Ignored.

hup, term, abrt, quit

Causes normal Prolog cleanup (e.g., at_halt/1) before terminating the process with the same signal.

segv, ill, bus, sys

Dumps the C and Prolog stacks and runs cleanup before terminating the process with the same signal.

fpe, alrm, xcpu, xfsz, vtalrm

Throw a Prolog exception (see above).