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From: schlut@oski.toppoint.de (Olaf Schlueter)
Newsgroups: comp.os.minix
Subject: Re: Vi and alarm.
Message-ID: <79@oski.toppoint.de>
Date: 23 Mar 91 13:33:24 GMT
References: <2996@redstar.cs.qmw.ac.uk>
Organization: Toppoint Mailbox
Lines: 55


zawada@cs.qmw.ac.uk (Simpson) writes:
> Does it gets hate me, or is there really something in it?
> 
> Whenever I run elvis or vi, after a while, I get the message
> 
> Alarm clock......
> 
> Vi aborts, and mucks up the tty!
> 

Vi, as many other unix programms, does the usual version 7 unix trick
for non-blocking read, which is: wind up the alarm-clock, call read,
which returns either one or more chars or EOF, if the alarm call happens
during the call.  If read is not aborted by the alarm clock, an alarm(0)
should stop the clock, but there is a bug in Minix, so this fails.  I
discovered this on MINIX 1.5 ST, as distributed by PH, when I was
programming an uucico.  I got around this bug without fixing the kernel
by setting a SIGALRM interrupt handler dummy routine (which does
nothing) whenever I used alarm for non-blocking IO.  You cannot avoid
the alarm signal but this way you can prevent it from stopping your
program.  The other feature of alarm, that another alarm-call with a
value > 0 resets the alarm clock to the new value, works. 

You can watch the bug working in:

#include <signal.h>

int donothing() {signal(SIGALRM,donothing);}

int main()

{
   char c;
   int n;

#ifdef FOOL_THE_BUG
   signalSIGALRM,donothing);
#endif

   do { 
      alarm(1); n = read (1,&c,1); alarm(0) } while( n > 0);
}

run this program and hold the "a" key pressed. In normal v7 Unix that
programm will stop within a second after releasing "a". In Minix
the program runs 1 second, whether you press "a" or not.

(I keyed that program in from my long-term memory, it has been a couple
of months since I used it to verify the bug - so there maybe a mistake
in it.)

-- 
Olaf Schlueter, Sandkuhle 4-6, 2300 Kiel 1, FRG, schlut@oski.toppoint.de
"Someone is always the evil in the land, and then the good guys are
marching, the eyes full of sand, into a holy war." Konstantin Wecker