#include	<stdio.h>#include	<fcntl.h>#include	<utmp.h>/* * write1.c * *	purpose: send messages to another terminal *	 method: open the other terminal for output then *	 	 copy from stdin to that terminal *        usage: write1 username */main( int ac, char *av[] ){	int	fd;	char	buf[BUFSIZ];	char	*get_tty(), *tty_for_user;		if ( ac != 2 ){                                    /* check args */		fprintf(stderr,"usage: write0 logname\n");		exit(1);	}	tty_for_user = get_tty( av[1] );                  /* find user  */	if ( tty_for_user == NULL )		return 1;	sprintf(buf, "/dev/%s", tty_for_user);	          /* open device */	fd = open( buf, O_WRONLY );	if ( fd == -1 ){		perror(buf); exit(1);	}	while( fgets(buf, BUFSIZ, stdin) != NULL )       /* write to user */		if ( write(fd, buf, strlen(buf)) == -1 )			break;	close( fd );}char *get_tty( char *logname )/* * purpose: find the tty at which 'logname' is logged in * returns: a string or NULL if not logged in *  errors: does not handle multiple logins  */{	static struct utmp utrec;	int	      utfd;	int	      namelen = sizeof( utrec.ut_name );	char	      *retval = NULL ;	if ( (utfd = open( UTMP_FILE, O_RDONLY )) == -1 )   /* open utmp */		return NULL;	/* look for a line where the user is logged in */	while( read( utfd, &utrec, sizeof(utrec)) == sizeof(utrec) )		if ( strncmp(logname, utrec.ut_name, namelen ) == 0 )		{			retval = utrec.ut_line ;			break;		}	close(utfd);                                       /* close & go */	return retval;}