/************************************************************************* * * * YAP Prolog * * * * Yap Prolog was developed at NCCUP - Universidade do Porto * * * * Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997 * * * ************************************************************************** * * * File: signals.pl * * Last rev: * * mods: * * comments: signal handling in YAP * * * *************************************************************************/ %%! @addtogroup OS %% @{ :- system_module( '$_signals', [alarm/3, on_exception/3, on_signal/3, raise_exception/1, read_sig/0], []). :- use_system_module( '$_boot', ['$meta_call'/2]). :- use_system_module( '$_debug', ['$trace'/1]). :- use_system_module( '$_threads', ['$thread_gfetch'/1]). /** @pred alarm(+ _Seconds_,+ _Callable_,+ _OldAlarm_) Arranges for YAP to be interrupted in _Seconds_ seconds, or in [ _Seconds_| _MicroSeconds_]. When interrupted, YAP will execute _Callable_ and then return to the previous execution. If _Seconds_ is `0`, no new alarm is scheduled. In any event, any previously set alarm is canceled. The variable _OldAlarm_ unifies with the number of seconds remaining until any previously scheduled alarm was due to be delivered, or with `0` if there was no previously scheduled alarm. Note that execution of _Callable_ will wait if YAP is executing built-in predicates, such as Input/Output operations. The next example shows how _alarm/3_ can be used to implement a simple clock: ~~~~~ loop :- loop. ticker :- write('.'), flush_output, get_value(tick, yes), alarm(1,ticker,_). :- set_value(tick, yes), alarm(1,ticker,_), loop. ~~~~~ The clock, `ticker`, writes a dot and then checks the flag `tick` to see whether it can continue ticking. If so, it calls itself again. Note that there is no guarantee that the each dot corresponds a second: for instance, if the YAP is waiting for user input, `ticker` will wait until the user types the entry in. The next example shows how alarm/3 can be used to guarantee that a certain procedure does not take longer than a certain amount of time: ~~~~~ loop :- loop. :- catch((alarm(10, throw(ball), _),loop), ball, format('Quota exhausted.~n',[])). ~~~~~ In this case after `10` seconds our `loop` is interrupted, `ball` is thrown, and the handler writes `Quota exhausted`. Execution then continues from the handler. Note that in this case `loop/0` always executes until the alarm is sent. Often, the code you are executing succeeds or fails before the alarm is actually delivered. In this case, you probably want to disable the alarm when you leave the procedure. The next procedure does exactly so: ~~~~~ once_with_alarm(Time,Goal,DoOnAlarm) :- catch(execute_once_with_alarm(Time, Goal), alarm, DoOnAlarm). execute_once_with_alarm(Time, Goal) :- alarm(Time, alarm, _), ( call(Goal) -> alarm(0, alarm, _) ; alarm(0, alarm, _), fail). ~~~~~ The procedure `once_with_alarm/3` has three arguments: the _Time_ to wait before the alarm is sent; the _Goal_ to execute; and the goal _DoOnAlarm_ to execute if the alarm is sent. It uses catch/3 to handle the case the `alarm` is sent. Then it starts the alarm, calls the goal _Goal_, and disables the alarm on success or failure. */ /** @pred on_signal(+ _Signal_,? _OldAction_,+ _Callable_) Set the interrupt handler for soft interrupt _Signal_ to be _Callable_. _OldAction_ is unified with the previous handler. Only a subset of the software interrupts (signals) can have their handlers manipulated through on_signal/3. Their POSIX names, YAP names and default behavior is given below. The "YAP name" of the signal is the atom that is associated with each signal, and should be used as the first argument to on_signal/3. It is chosen so that it matches the signal's POSIX name. on_signal/3 succeeds, unless when called with an invalid signal name or one that is not supported on this platform. No checks are made on the handler provided by the user. + sig_up (Hangup) SIGHUP in Unix/Linux; Reconsult the initialization files ~/.yaprc, ~/.prologrc and ~/prolog.ini. + sig_usr1 and sig_usr2 (User signals) SIGUSR1 and SIGUSR2 in Unix/Linux; Print a message and halt. A special case is made, where if _Callable_ is bound to `default`, then the default handler is restored for that signal. A call in the form `on_signal( _S_, _H_, _H_)` can be used to retrieve a signal's current handler without changing it. It must be noted that although a signal can be received at all times, the handler is not executed while YAP is waiting for a query at the prompt. The signal will be, however, registered and dealt with as soon as the user makes a query. Please also note, that neither POSIX Operating Systems nor YAP guarantee that the order of delivery and handling is going to correspond with the order of dispatch. */ :- meta_predicate on_signal(+,?,:), alarm(+,:,-). '$creep'(G) :- % get the first signal from the mask '$first_signal'(Sig), !, % process it '$do_signal'(Sig, G). '$creep'([M|G]) :- % noise, just go on with our life. '$execute'(M:G). '$do_signal'(sig_wake_up, G) :- '$awoken_goals'(LG), % if more signals alive, set creep flag '$continue_signals', '$wake_up_goal'(G, LG). % never creep on entering system mode!!! % don't creep on meta-call. '$do_signal'(sig_creep, MG) :- '$disable_debugging', '$start_creep'(MG, creep). '$do_signal'(sig_iti, [M|G]) :- '$thread_gfetch'(Goal), % if more signals alive, set creep flag '$continue_signals', '$current_module'(M0), '$execute0'(Goal,M0), '$execute'(M:G). '$do_signal'(sig_trace, [M|G]) :- '$continue_signals', trace, '$execute'(M:G). '$do_signal'(sig_debug, [M|G]) :- '$continue_signals', debug, '$execute'(M:G). '$start_creep'([Mod|G], _WhereFrom) :- '$trace'(Mod:G). '$no_creep_call'('$execute_clause'(G,Mod,Ref,CP),_) :- !, '$enable_debugging', '$execute_clause'(G,Mod,Ref,CP). '$no_creep_call'('$execute_nonstop'(G, M),_) :- !, '$enable_debugging', '$execute_nonstop'(G, M). '$no_creep_call'(G, M) :- '$enable_debugging', '$execute_nonstop'(G, M). '$execute_goal'(G, Mod) :- ( '$is_metapredicate'(G, Mod) -> '$meta_call'(G,Mod) ; '$execute_nonstop'(G,Mod) ). '$signal_do'(Sig, Goal) :- recorded('$signal_handler', action(Sig,Goal), _), !. '$signal_do'(Sig, Goal) :- '$signal_def'(Sig, Goal). % reconsult init files. % '$signal_def'(sig_hup, (( exists('~/.yaprc') -> [-'~/.yaprc'] ; true ), ( exists('~/.prologrc') -> [-'~/.prologrc'] ; true ), ( exists('~/prolog.ini') -> [-'~/prolog.ini'] ; true ))). % die on signal default. % '$signal_def'(sig_usr1, throw(signal(usr1,[]))). '$signal_def'(sig_usr2, throw(signal(usr2,[]))). '$signal_def'(sig_pipe, throw(signal(pipe,[]))). '$signal_def'(sig_fpe, throw(signal(fpe,[]))). % ignore sig_alarm by default % '$signal_def'(sig_alarm, true). '$signal'(sig_hup). '$signal'(sig_usr1). '$signal'(sig_usr2). '$signal'(sig_pipe). '$signal'(sig_alarm). '$signal'(sig_vtalarm). '$signal'(sig_fpe). on_signal(Signal,OldAction,NewAction) :- var(Signal), !, (nonvar(OldAction) -> throw(error(instantiation_error,on_signal/3)) ; true), '$signal'(Signal), on_signal(Signal, OldAction, NewAction). on_signal(Signal,OldAction,default) :- '$reset_signal'(Signal, OldAction). on_signal(_Signal,_OldAction,Action) :- var(Action), !, throw(error('SYSTEM_ERROR_INTERNAL','Somehow the meta_predicate declarations of on_signal are subverted!')). on_signal(Signal,OldAction,Action) :- Action = (_:Goal), var(Goal), !, '$check_signal'(Signal, OldAction), Goal = OldAction. on_signal(Signal,OldAction,Action) :- '$reset_signal'(Signal, OldAction), % 13211-2 speaks only about callable % ( Action = M:Goal -> true ; throw(error(type_error(callable,Action),on_signal/3)) ), % the following disagrees with 13211-2:6.7.1.4 which disagrees with 13211-1:7.12.2a % % but the following agrees with 13211-1:7.12.2a % ( nonvar(M) -> true ; throw(error(instantiation_error,on_signal/3)) ), ( atom(M) -> true ; throw(error(type_error(callable,Action),on_signal/3)) ), ( nonvar(Goal) -> true ; throw(error(instantiation_error,on_signal/3)) ), recordz('$signal_handler', action(Signal,Action), _). '$reset_signal'(Signal, OldAction) :- recorded('$signal_handler', action(Signal,OldAction), Ref), !, erase(Ref). '$reset_signal'(_, default). '$check_signal'(Signal, OldAction) :- recorded('$signal_handler', action(Signal,OldAction), _), !. '$check_signal'(_, default). alarm(Interval, Goal, Left) :- Interval == 0, !, '$alarm'(0, 0, Left0, _), on_signal(sig_alarm, _, Goal), Left = Left0. alarm(Interval, Goal, Left) :- integer(Interval), !, on_signal(sig_alarm, _, Goal), '$alarm'(Interval, 0, Left, _). alarm(Number, Goal, Left) :- float(Number), !, Secs is integer(Number), USecs is integer((Number-Secs)*1000000) mod 1000000, on_signal(sig_alarm, _, Goal), '$alarm'(Secs, USecs, Left, _). alarm([Interval|USecs], Goal, [Left|LUSecs]) :- on_signal(sig_alarm, _, Goal), '$alarm'(Interval, USecs, Left, LUSecs). raise_exception(Ball) :- throw(Ball). on_exception(Pat, G, H) :- catch(G, Pat, H). read_sig :- recorded('$signal_handler',X,_), writeq(X),nl, fail. read_sig. % % % make thes predicates non-traceable. % :- '$set_private'(current_choicepoint(_DCP), yap_hacks). :- '$set_private'('$current_choice_point'(_DCP), _). :- '$set_private'('$$cut_by'(_DCP), prolog). :- '$set_private'(true, yap_hacks). :- '$set_private'(true, prolog). :- '$set_private'('$call'(_,_,_,_), prolog). :- '$set_private'('$execute_nonstop'(_,_), prolog). :- '$set_private'('$execute_clause'(_,_,_,_), prolog). :- '$set_private'('$restore_regs'(_,_), prolog). :- '$set_private'('$undefp'(_,_), prolog). :- '$set_private'('$Error'(_), prolog). :- '$set_private'('$LoopError'(_,_), prolog). :- '$set_private'('$TraceError'(_,_,_,_,_), prolog). %%! @}