yap-6.3/library/timeout.yap

/*************************************************************************
*									 *
*	 YAP Prolog 							 *
*									 *
*	Yap Prolog was developed at NCCUP - Universidade do Porto	 *
*									 *
* Copyright L.Damas, V.S.Costa and Universidade do Porto 1985-1997	 *
*									 *
**************************************************************************
*									 *
* File:		timeout.yap						 *
* Last rev:	5/12/99							 *
* mods:									 *
* comments:	Goal within timeout					 *
*									 *
*************************************************************************/

:- module(timeout, [
	time_out/3
    ]).


/** @defgroup Timeout Calls With Timeout
@ingroup YAPLibrary
@{

The <tt>time_out/3</tt> command relies on the <tt>alarm/3</tt> built-in to
implement a call with a maximum time of execution. The command is
available with the `use_module(library(timeout))` command.



 @pred time_out(+ _Goal_, + _Timeout_, - _Result_) 


Execute goal  _Goal_ with time limited  _Timeout_, where
 _Timeout_ is measured in milliseconds. If the goal succeeds, unify
 _Result_ with success. If the timer expires before the goal
terminates, unify  _Result_ with <tt>time_out</tt>.

This command is implemented by activating an alarm at procedure
entry. If the timer expires before the goal completes, the alarm will
throw an exception  _timeout_.

One should note that time_out/3 is not reentrant, that is, a goal
called from `time_out` should never itself call
time_out/3. Moreover, time_out/3 will deactivate any previous
alarms set by alarm/3 and vice-versa, hence only one of these
calls should be used in a program.

Last, even though the timer is set in milliseconds, the current
implementation relies on <tt>alarm/3</tt>, and therefore can only offer
precision on the scale of seconds.




 */


:- meta_predicate time_out(0,+,-).

:- use_module(library(hacks), [
	virtual_alarm/3
    ]).


%
% not the nicest program I've ever seen.
%

time_out(Goal, Time, Result) :-
	T is Time//1000,
	UT is (Time mod 1000)*1000,
	catch( ( Result0 = success,
	         setup_call_cleanup(
			virtual_alarm(T.UT,throw(time_out),_),
			Goal,
			virtual_alarm(0,_,RT)),
		 (  var(RT)
		 -> virtual_alarm(0,_,_),
		    (
		      true
		    ;
		      virtual_alarm(T.UT,throw(time_out),_),
		      fail
		    )
		 ;  true
		 )
	       ),
	       time_out,
	       Result0 = time_out ),
	Result = Result0.