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From: allbery@ncoast.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: Absolutly hanging up a modem
Message-ID: <4947@ncoast.UUCP>
Date: Tue, 27-Oct-87 21:10:51 EST
Article-I.D.: ncoast.4947
Posted: Tue Oct 27 21:10:51 1987
Date-Received: Sat, 31-Oct-87 17:58:50 EST
References: <1548@bloom-beacon.MIT.EDU> <142700004@tiger.UUCP> <292@minya.UUCP>
Reply-To: allbery@ncoast.UUCP (Brandon Allbery)
Followup-To: comp.unix.wizards
Organization: Cleveland Public Access UN*X, Cleveland, Oh
Lines: 85

As quoted from <292@minya.UUCP> by jc@minya.UUCP (John Chambers):
+---------------
| Good suggestion.  Much better than the "Drop DTR" suggestion.  True,
| this is what the modem expects (if it isn't in command mode)  But
| does anyone know the Unix call (for Sys/V and BSD and Xenix and ...)
| that does this?  Presumably the guy wasn't talking about how to hack
| the kernel to make it twiddle the DTR line; he was asking how his
| user-mode program does it.
+---------------

On every Unix and Unix-like system I've used (except Minix, which as yet
does not have a tty driver) you can drop DTR absolutely by setting the baud
rate to 0.  This is nonportable in its lowest-level incarnation, but a
portable version follows:

int hupfd(fd)
int fd; {
	int status;

	switch (fork()) {
	case -1:
		return -1;
	case 0:
		close(0);
		dup(fd);
		close(1);
		dup(fd);
		close(fd);
		execlp("stty", "stty", "0", (char *) 0);
		_exit(-1);
	default:
		wait(&status);
		return status;
	}
}

Pass it a file descriptor of a modem, and it will drop DTR on the modem,
forcing a hangup.

Less portable versions follow:

V7/BSD/Xenix 2.x:
	#include <sgtty.h>

	hupfd(fd) {
		struct sgttyb b;
		
		if (ioctl(fd, TIOCGETP, &b) == -1)
			return -1;
		b.sg_ispeed = b.sg_ospeed = B0;
		return ioctl(fd, TIOCSETP, &b);
	}

System III/System V/Xenix 3/Xenix V:
	#include <termio.h>
	#include <sys/ioctl.h>
	
	hupfd(fd) {
		struct termio b;
		
		if (ioctl(fd, TCGETA, &b) == -1)
			return -1;
		b.c_cflag = (b.c_cflag & ~CBAUD) | B0;
		return ioctl(fd, TCSETAF, &b);
	}

V6 (does anyone still use this?!):

	#include <sgtty.h>
	
	hupfd(fd) {
		struct sgttyb b;
		
		if (gtty(fd, &b) == -1)
			return -1;
		b.sg_ispeed = b.sg_ospeed = B0;
		return stty(fd, &b);
	}

Note that, thanks to "compatibility" routines in both BSD and AT&T Unixes,
the V6 version should work on almost any Unix or lookalike thereof.
-- 
Brandon S. Allbery		     necntc!ncoast!allbery@harvard.harvard.edu
  {{harvard,mit-eddie}!necntc,well!hoptoad,sun!mandrill!hal}!ncoast!allbery
			 "Uncle _who_?" -- Lt. Worf