As of version 3.1.0, SWI-Prolog is able to handle software interrupts (signals) in Prolog as well as in foreign (C) code (see section 11.4.13).
Signals are used to handle internal errors (execution of a non-existing CPU instruction, arithmetic domain errors, illegal memory access, resource overflow, etc.), as well as for dealing with asynchronous interprocess communication.
Signals are defined by the POSIX standard and part of all Unix machines. The MS-Windows Win32 provides a subset of the signal handling routines, lacking the vital functionality to raise a signal in another thread for achieving asynchronous interprocess (or interthread) communication (Unix kill() function).
on_signal(Signal, Current, Current).
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.
Prolog will map the signal onto a Prolog exception as described in
resets the handler to the settings active before SWI-Prolog manipulated
Signals bound to a foreign function through PL_signal()
are reported using the term
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
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
Before deciding to deal with signals in your application, please consider the following:
throware unsafe. Handlers defined to call a predicate are safe. Note that the predicate can call throw/1, but the delivery is delayed until Prolog is in a safe state.
The C-interface described in section
11.4.13 provides the option
PL_SIGSYNC to select either safe synchronous or unsafe
throwor a foreign handler, signals are delivered immediately (as defined by the OS). When using a Prolog predicate, delivery is delayed to a safe moment. Blocking system calls or foreign loops may cause long delays. Foreign code can improve on that by calling PL_handle_signals().
Signals are blocked when the garbage collector is active.