Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uwm.edu!bionet!arisia!quintus!don
From: don@quintus.UUCP (Don Ferguson)
Newsgroups: comp.lang.prolog
Subject: Re: Interrupt Handler for Quintus Prolog
Message-ID: <1326@quintus.UUCP>
Date: 8 Mar 90 21:58:14 GMT
References: <2030@laura.UUCP>
Reply-To: don@quintus.UUCP (Don Ferguson)
Organization: Quintus Computer Systems, Inc.
Lines: 49

In article <2030@laura.UUCP> waldmann@exunido.UUCP (Uwe Waldmann) writes:
>I am trying to write an interrupt handler for Quintus Prolog (Rel. 2.4.2 on
>a Sun-3 and/or a Sun-4).  I wanted to take the demo example from the
>"Quintus Prolog System-dependent Features Manual" (p. 15) as a starting
>point, but unfortunately even this (unmodified) example does not work
>properly:

The problem you report refects a SunOS bug.  Apparently getc() is no longer
re-entrant.  If getc() is interrupted and the interrupt handler calls getc(),
then, on return, the original getc() will yield bad values - it's internal
buffer has become corrupted.  I proved this to myself by modifying the example
to contain only C code.  The code ran correctly under SunOS 2.3, but not under
SunOS 3.2 or 4.0.3.

Avoid the problem by calling read() from the interrupt handler instead of
getchar. (NOTE: getchar is just a macro that calls getc(stdin)).

#include <signal.h>
#include <stdio.h>
#include "/goedel/quintus/install/q2.4.2/library/qpaction.h"

int my_handler()
{
    char c, c1;

    for (;;) {
        printf("\nWell? ");
	fflush(stdin);
	read(0, &c, 1);
	c1 = c;
	while (c1 != '\n') {
		read(0, &c1, 1);
	}
	fflush(stdin);
        switch (c) {
            case 'a': QP_action(QP_ABORT);  /* abort */
            case 'e': QP_action(QP_EXIT);   /* exit */
            case 'c': return;               /* continue */
            default: printf("a, c or e, please");
        }
    }
}

Thanks for pointing out this discrepancy in our manual.  We will update
our documenatation accordingly.


    -Don Ferguson
     Manager, Technical Services