## 4.12 Handling signals

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, -Old, :New)
  Determines the reaction on Signal. Old is unified with the old behaviour, while the behaviour is switched to New. As with similar environment control predicates, the current value is retrieved using `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. `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`.

current_signal(?Name, ?Id, ?Handler)
  Enumerate the currently defined signal handling. Name is the signal name, Id is the numerical identifier and Handler is the currently defined handler (see on_signal/3).
prolog_alert_signal(?Old, +New)
  Query or set the signal used to unblock blocking system calls on Unix systems and process pending Prolog signals. The default is `SIGUSR2`. See also --sigalert.

### 4.12.1 Notes on signal handling

Before deciding to deal with signals in your application, please consider the following:

- Portability
  On MS-Windows, the signal interface is severely limited. Different Unix brands support different sets of signals, and the relation between signal name and number may vary. Currently, the system only supports signals numbered 1 to 3269TBD: the system should support the Unix realtime signals. Installing a signal outside the limited set of supported signals in MS-Windows crashes the application.

- Safety
  Immediately delivered signals (see below) are unsafe. This implies that foreign functions called from a handler cannot safely use the SWI-Prolog API and cannot use C longjmp(). Handlers defined as `throw` are 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 asynchronous delivery.

- Time of delivery
  Using `throw` or 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.