- Reference manual
- Built-in Predicates
- Notation of Predicate Descriptions
- Character representation
- Loading Prolog source files
- Editor Interface
- Verify Type of a Term
- Comparison and Unification of Terms
- Control Predicates
- Meta-Call Predicates
- Delimited continuations
- Exception handling
- Printing messages
- Handling signals
- DCG Grammar rules
- Declaring predicate properties
- Examining the program
- Input and output
- Status of streams
- Primitive character I/O
- Term reading and writing
- Analysing and Constructing Terms
- Analysing and Constructing Atoms
- Localization (locale) support
- Character properties
- Character Conversion
- Misc arithmetic support predicates
- Built-in list operations
- Finding all Solutions to a Goal
- Formatted Write
- Global variables
- Terminal Control
- Operating System Interaction
- File System Interaction
- User Top-level Manipulation
- Creating a Protocol of the User Interaction
- Debugging and Tracing Programs
- Debugging and declaring determinism
- Obtaining Runtime Statistics
- Execution profiling
- Memory Management
- Windows DDE interface
- Built-in Predicates
- Reference manual
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 12.4.16).
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 how Signal is processed. Old is unified
with the old behaviour, while the behaviour is switched to New.
As with similar environment control predicates, the current value is
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.
Four names have special meaning.
throwimplies Prolog will map the signal onto a Prolog exception as described in section 4.10,
ignorecauses Prolog to ignore the signal entirely,
debugspecifies the debug interrupt prompt that is initially bound to
defaultresets 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
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:
- Prompts the user, allowing to inspect the current state of the process and start the tracer.
- 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.
- 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).
- 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:
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 3281TBD: the system should support the Unix realtime signals. Installing a signal outside the limited set of supported signals in MS-Windows crashes the application.
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
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 12.4.16 provides the option
PL_SIGSYNCto select either safe synchronous or unsafe asynchronous delivery.
- Time of delivery
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.