Path: utzoo!attcan!uunet!ispi!jbayer
From: jbayer@ispi.UUCP (id for use with uunet/usenet)
Newsgroups: comp.unix.xenix
Subject: Re: Test SCO Xenix IPC reliability
Summary: Add sleep(1) to the ticktock.c program
Message-ID: <166@ispi.UUCP>
Date: 25 Aug 88 14:42:40 GMT
References: <22012@neabbs.UUCP>
Organization: Intelligent Software Products, Inc.
Lines: 106

In article <22012@neabbs.UUCP>, richard@neabbs.UUCP (RICHARD RONTELTAP) writes:
> [ Tested the ticktock.c program ]
>  
> Welllll, I ran the test program on XENIX /386 2.2.1 and 2.2.3 with the
> same results.
>  
> When the program is started the first time only one TICK/TOCK is
> printed. When it is started the second time. TICK/TOCK is infinitely
> printed.
>  
> I think what happens is:
> When the shared memory is created, and the parent process has printed
> TICK, the context is switched to the child process right after the
> 'signal' command and just before the 'pause' command. When the child
> now signals the parent, the signal is caught and the parent goes the
> the next command: pause(), and waits for ever!
>  
> The second time scheduling is different because the shared memory
> doesn't have to be created.
>  
>  

I think Richard is right.  I added two sleep(1) to the program, one in
the child() and one in the parent().  With these additions the program
starts up and prints TICK/TOCK even when creating the shared memory
segment for the first time.  I enclosed the new program below:

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#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <signal.h>

int	zero = 0;
int	*loc = &zero;

int	key = ('v' << 8) | 'o';

catch (sig)
int	sig;
{
	signal (sig, catch);
}

parent ()
{
	while (1) {
		while (*loc)
			;

		write (1, "TICK ....\n", 10);
		*loc = 1;
		sleep(1);			/* added by JB 8/25/88 */
		kill (loc[2], SIGUSR1);
		pause ();
	}
}

child ()
{
	while (1) {
		while (! *loc)
			;

		write (1, ".... TOCK\n", 10);
		*loc = 0;
		sleep(1);			/* added by JB 8/25/88 */
		kill (loc[1], SIGUSR1);
		pause ();
	}
}

main ()
{
	int	id;

	if ((id = shmget (key, 3 * sizeof (int), IPC_CREAT|0666)) == -1) {
		perror ("shmget");
		exit (1);
	}
	if ((loc = (int *) shmat (id, (char *) 0, 0)) == (int *) 0) {
		perror ("shmat");
		exit (1);
	}
	loc[0] = 0;
	switch (fork ()) {
		default:
			loc[1] = getpid ();
			signal (SIGUSR1, catch);
			parent ();
		case 0:
			loc[2] = getpid ();
			signal (SIGUSR1, catch);
			child ();
		case -1:
			perror ("fork");
			exit (1);
	}
	exit (1);
}
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It does work fine now on 386 and 286 Xenix.


Jonathan Bayer