Relay-Version: version B 2.10.2 9/18/84; site lsuc.UUCP
Posting-Version: version B 2.10.2 9/18/84; site lsuc.UUCP
Path: lsuc!dave
From: dave@lsuc.UUCP (David Sherman)
Newsgroups: pe.cust.wanted
Subject: inputpending() routine - why won't this work?
Message-ID: <549@lsuc.UUCP>
Date: 27 Mar 85 12:59:59 GMT
Date-Received: 27 Mar 85 12:59:59 GMT
Organization: Law Society of Upper Canada, Toronto
Lines: 70

I asked in net.lang.c for a mechanism for FIONREAD (non-blocking
read) for v7, and the following was posted in reply:

=From: sjoerd@tjalk.UUCP (Sjoerd Mullender)
=Message-ID: <443@tjalk.UUCP>
=Date: 21 Mar 85 13:00:48 GMT
=Organization: VU Informatica, Amsterdam
=
=Here is a C routine that returns non-zero if there was any input pending.
=This is for a V7 PDP 11 system.  The only requirement is that /dev/kmem
=is readable (which it should NOT be for security).
=
=#include <sys/param.h>
=#include <sys/dir.h>
=#include <sys/user.h>
=
=inputpending()
={
=	static int fd = -2;
=	unsigned c;
=
=	if (fd == -2)	/* uninitialized */
=		if ((fd = open("/dev/kmem", 0)) >= 0) {
=			/* If the open fails, fd will be -1, so we won't
=			 * try again.
=			 * Close the file descriptor on exec.
=			 */
=			ioctl(fd, FIOCLEX, (struct sgttyb *) 0);
=			/* 0140000 is the beginning of the u area
=			 * u_ttyp is the pointer to the tty struct
=			 */
=			lseek(fd, (long) &((struct user *) 0140000)->u_ttyp, 0);
=			read(fd, &c, sizeof c);
=			/* seek to the beginning of the tty struct */
=			lseek(fd, (long) c, 0);
=		}
=	if (fd < 0)
=		return 0;
=	/* the first 2 byte of the tty struct is a pointer to the clist */
=	if (read(fd, &c, sizeof c) < sizeof c)
=		return 0;
=	lseek(fd, - (long) sizeof c, 1);
=	/* there is input when the pointer to the clist is != 0 */
=	return c != 0;
=}

I figured out that the 0140000 should be changed to 0xff0000, or
actually UBASE, which is defined in param.h. But when I tried
compiling this with a test program
	main()
	{
		while(1)
		{
			if(inputpending())
				printf("something\n");
			else
				printf("nothing\n");
			sleep(1);
		}
	}
it always prints "nothing". I changed the unsigned c to int c,
to correspond to the tty.h definition, and that didn't help either.

What am I doing wrong. How can I get this to work? It would be
very helpful for a number of programs, including rn.

Dave Sherman
-- 
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