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rcsid[] and sccsid[] and copyright[] are essentially unmaintained (and
unmaintainable).  these days, people use source.  these id's do not provide
any benefit, and do hurt the small install media
(the 33,000 line diff is essentially mechanical)
ok with the idea millert, ok dms

/*	$OpenBSD: write.c,v 1.26 2009/10/27 23:59:50 deraadt Exp $	*/
/*	$NetBSD: write.c,v 1.5 1995/08/31 21:48:32 jtc Exp $	*/

/*
 * Copyright (c) 1989, 1993
 *	The Regents of the University of California.  All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * Jef Poskanzer and Craig Leres of the Lawrence Berkeley Laboratory.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/param.h>
#include <sys/stat.h>
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>
#include <time.h>
#include <fcntl.h>
#include <paths.h>
#include <pwd.h>
#include <unistd.h>
#include <utmp.h>
#include <err.h>
#include <vis.h>

void done(int sig);
void do_write(char *, char *, uid_t);
void wr_fputs(char *);
void search_utmp(char *, char *, int, char *, uid_t);
int term_chk(char *, int *, time_t *, int);
int utmp_chk(char *, char *);

int
main(int argc, char *argv[])
{
	char tty[MAXPATHLEN], *mytty, *cp;
	int msgsok, myttyfd;
	time_t atime;
	uid_t myuid;

	/* check that sender has write enabled */
	if (isatty(fileno(stdin)))
		myttyfd = fileno(stdin);
	else if (isatty(fileno(stdout)))
		myttyfd = fileno(stdout);
	else if (isatty(fileno(stderr)))
		myttyfd = fileno(stderr);
	else
		errx(1, "can't find your tty");
	if (!(mytty = ttyname(myttyfd)))
		errx(1, "can't find your tty's name");
	if ((cp = strrchr(mytty, '/')))
		mytty = cp + 1;
	if (term_chk(mytty, &msgsok, &atime, 1))
		exit(1);
	if (!msgsok)
		warnx("you have write permission turned off");

	myuid = getuid();

	/* check args */
	switch (argc) {
	case 2:
		search_utmp(argv[1], tty, sizeof tty, mytty, myuid);
		do_write(tty, mytty, myuid);
		break;
	case 3:
		if (!strncmp(argv[2], _PATH_DEV, sizeof(_PATH_DEV) - 1))
			argv[2] += sizeof(_PATH_DEV) - 1;
		if (utmp_chk(argv[1], argv[2]))
			errx(1, "%s is not logged in on %s",
			    argv[1], argv[2]);
		if (term_chk(argv[2], &msgsok, &atime, 1))
			exit(1);
		if (myuid && !msgsok)
			errx(1, "%s has messages disabled on %s",
			    argv[1], argv[2]);
		do_write(argv[2], mytty, myuid);
		break;
	default:
		(void)fprintf(stderr, "usage: write user [ttyname]\n");
		exit(1);
	}
	done(0);

	/* NOTREACHED */
	return (0);
}

/*
 * utmp_chk - checks that the given user is actually logged in on
 *     the given tty
 */
int
utmp_chk(char *user, char *tty)
{
	struct utmp u;
	int ufd;

	if ((ufd = open(_PATH_UTMP, O_RDONLY)) < 0)
		return(1);	/* no utmp, cannot talk to users */

	while (read(ufd, (char *) &u, sizeof(u)) == sizeof(u))
		if (strncmp(user, u.ut_name, sizeof(u.ut_name)) == 0 &&
		    strncmp(tty, u.ut_line, sizeof(u.ut_line)) == 0) {
			(void)close(ufd);
			return(0);
		}

	(void)close(ufd);
	return(1);
}

/*
 * search_utmp - search utmp for the "best" terminal to write to
 *
 * Ignores terminals with messages disabled, and of the rest, returns
 * the one with the most recent access time.  Returns as value the number
 * of the user's terminals with messages enabled, or -1 if the user is
 * not logged in at all.
 *
 * Special case for writing to yourself - ignore the terminal you're
 * writing from, unless that's the only terminal with messages enabled.
 */
void
search_utmp(char *user, char *tty, int ttyl, char *mytty, uid_t myuid)
{
	struct utmp u;
	time_t bestatime, atime;
	int ufd, nloggedttys, nttys, msgsok, user_is_me;
	char atty[UT_LINESIZE + 1];

	if ((ufd = open(_PATH_UTMP, O_RDONLY)) < 0)
		err(1, "%s", _PATH_UTMP);

	nloggedttys = nttys = 0;
	bestatime = 0;
	user_is_me = 0;
	while (read(ufd, (char *) &u, sizeof(u)) == sizeof(u))
		if (strncmp(user, u.ut_name, sizeof(u.ut_name)) == 0) {
			++nloggedttys;
			(void)strncpy(atty, u.ut_line, UT_LINESIZE);
			atty[UT_LINESIZE] = '\0';
			if (term_chk(atty, &msgsok, &atime, 0))
				continue;	/* bad term? skip */
			if (myuid && !msgsok)
				continue;	/* skip ttys with msgs off */
			if (strcmp(atty, mytty) == 0) {
				user_is_me = 1;
				continue;	/* don't write to yourself */
			}
			++nttys;
			if (atime > bestatime) {
				bestatime = atime;
				(void)strlcpy(tty, atty, ttyl);
			}
		}

	(void)close(ufd);
	if (nloggedttys == 0)
		errx(1, "%s is not logged in", user);
	if (nttys == 0) {
		if (user_is_me) {		/* ok, so write to yourself! */
			(void)strlcpy(tty, mytty, ttyl);
			return;
		}
		errx(1, "%s has messages disabled", user);
	} else if (nttys > 1)
		warnx("%s is logged in more than once; writing to %s",
		    user, tty);
}

/*
 * term_chk - check that a terminal exists, and get the message bit
 *     and the access time
 */
int
term_chk(char *tty, int *msgsokP, time_t *atimeP, int showerror)
{
	struct stat s;
	char path[MAXPATHLEN];

	(void)snprintf(path, sizeof(path), "%s%s", _PATH_DEV, tty);
	if (stat(path, &s) < 0) {
		if (showerror)
			warn("%s", path);
		return(1);
	}
	*msgsokP = (s.st_mode & S_IWGRP) != 0;	/* group write bit */
	*atimeP = s.st_atime;
	return(0);
}

/*
 * do_write - actually make the connection
 */
void
do_write(char *tty, char *mytty, uid_t myuid)
{
	char *login, *nows;
	struct passwd *pwd;
	time_t now;
	char path[MAXPATHLEN], host[MAXHOSTNAMELEN], line[512];
	gid_t gid;

	/* Determine our login name before the we reopen() stdout */
	if ((login = getlogin()) == NULL) {
		if ((pwd = getpwuid(myuid)))
			login = pwd->pw_name;
		else
			login = "???";
	}

	(void)snprintf(path, sizeof(path), "%s%s", _PATH_DEV, tty);
	if ((freopen(path, "w", stdout)) == NULL)
		err(1, "%s", path);

	/* revoke privs, now that we have opened the tty */
	gid = getgid();
	if (setresgid(gid, gid, gid) == -1)
		err(1, "setresgid");

	(void)signal(SIGINT, done);
	(void)signal(SIGHUP, done);

	/* print greeting */
	if (gethostname(host, sizeof(host)) < 0)
		(void)strlcpy(host, "???", sizeof host);
	now = time((time_t *)NULL);
	nows = ctime(&now);
	nows[16] = '\0';
	(void)printf("\r\n\007\007\007Message from %s@%s on %s at %s ...\r\n",
	    login, host, mytty, nows + 11);

	while (fgets(line, sizeof(line), stdin) != NULL)
		wr_fputs(line);
}

/*
 * done - cleanup and exit
 */
void
done(int sig)
{
	(void)write(STDOUT_FILENO, "EOF\r\n", 5);
	if (sig)
		_exit(0);
	else
		exit(0);
}

/*
 * wr_fputs - like fputs(), but makes control characters visible and
 *     turns \n into \r\n
 */
void
wr_fputs(char *s)
{
	u_char c;
	char visout[5], *s2;

#define	PUTC(c)	if (putchar(c) == EOF) goto err;

	for (; *s != '\0'; ++s) {
		c = toascii(*s);
		if (c == '\n') {
			PUTC('\r');
			PUTC('\n');
			continue;
		}
		vis(visout, c, VIS_SAFE|VIS_NOSLASH, s[1]);
		for (s2 = visout; *s2; s2++)
			PUTC(*s2);
	}
	return;

err:	err(1, NULL);
#undef PUTC
}