write (logname)or(tty*)or(/dev/tty*)

** ------------------------------------------------------------
 **
 **
 *  Enhanced version of write2.c
 *
 *  This version accepts a username on the command line
 *  then searches the utmp file for the first line on which
 *  that user is found.  This program skips the line for the
 *  calling process.  If more than one other tty is found, 
 *  the program says there is more than one choice.
 *
 *	purpose: send messages to another terminal
 *	 method: open the other terminal for output then
 *	 	 copy from stdin to that terminal
 *	  shows: a terminal is just a file supporting regular i/o
 *	  usage: write2 {ttyname|username}
 *
	build: cc write2.c -o write2
 */

/*  几个函数说明

    time_t time(time_t *t)                        得到1970年开始到现在的秒数

    struct utmp *getutent(void)                   从utmp文件中读一行，返回的指针指向这个数据

    void setutent(void)                           打开utmp文件

    void endutent(void)                           关闭utmp文件

    char *ttyname(fd)                             得到fd所指的终端名 /dev/tty*

    int gethostname(char *name,size_t namelen)    得到你所在的终端的主机名    

*/

#include	
#include	
#include	
#include	
#include	
#include	
#include	
#include	
#include	

main( int ac, char *av[] )
{
	int	fd;
	char	buf[BUFSIZ];
	char	*termname, *get_termname(char *);

	/* check args */
	if ( ac != 2 ){
		fprintf(stderr,"usage: write0 {ttyname|username}\n");
		exit(1);
	}

	/* find user or terminal or quit */
	if ( (termname = get_termname(av[1])) == NULL )
		exit(1);

	/* open devices */
	fd = open( termname, O_WRONLY );
	if ( fd == -1 ){
		perror(termname); 
		exit(1);
	}

	/* say hello */
	hello(fd);

	/* loop until EOF on input */
	while( fgets(buf, BUFSIZ, stdin) != NULL )
		if ( write(fd, buf, strlen(buf)) == -1 )
			break;
	close( fd );
}

/*
 * say hello
 */
hello(int fd)
{
	char	       host[256];
	struct passwd  *pwd;
	time_t         now;
	char	       greeting[1024];

	/* gather all the data we need to build the greeting */
	if ( gethostname(host, 256) == -1 )
		strcpy(host,"unknown");

	pwd = getpwuid(getuid());
	time(&now);

	sprintf(greeting,
		"Message from %s@%s on %s at %5.5s ...\n",
			pwd->pw_name, host,
			ttyname(0)+strlen("/dev/"),
			ctime(&now)+11
	);
	write(fd, greeting, strlen(greeting));
}

/*
 * If s is a file in /dev, then use that name.
 * Otherwise search utmp for entries with that user.
 * Find the line from that entry.  Skip lines that
 * match ttyname() (i.e. the caller)
 * If there are more than one, say so but use the first one.
 */

char *
get_termname(char *s)
{
	char	*devname, *is_a_device(char *);
	char	*find_user(char *);

	/* check "s" and "/dev/s" 	*/
	if ( devname = is_a_device(s) )
		return devname;

	/* search for s as username	*/
	devname = find_user(s);

	return devname;
}

/*
 * try the name flat out, then try it with /dev prepended
 * If so, then return the string, else return NULL
 */

char *
is_a_device(char *s)
{
	char	*rv;
	char	*str_add(char*, char*);

	if ( isacdev(s) )		/* try string as is 	*/
		return s;

	rv = str_add("/dev/", s);	/* then try "/dev/s"	*/
	if ( isacdev(rv) )
		return rv;
	return NULL;
}


isacdev(char *s)
{
	struct stat info;

	return( stat(s,&info) != -1 && S_ISCHR(info.st_mode) );
}


/*
 * concatenate two strings into new storage
 * Warning: this function does not always return heap memory
 */
char *
str_add(char *s1, char *s2)
{
	char *rv;
	if ( s1 == NULL )
		return s2;
	if ( s2 == NULL )
		return s1;

	rv = (char *)malloc(1 + strlen(s1) + strlen(s2) );
	if ( rv == NULL ){
		perror("out of memory");
		exit(1);
	}
	sprintf(rv,"%s%s", s1, s2);
	return rv;
}

/*
 * Look through utmp (using getutid()) for a line with the
 * specified user as ut_user.  Skip current tty and report
 * if more than one.  Also, skip terminals with names that
 * start with ':'; those are displays on an X server.
 */

char *
find_user(char *user)
{
	struct utmp *entryp;
	char	*result = NULL;		/* result dev	  */
	int	num = 0;		/* # of matches	  */

	char	*mytty = ttyname(0);	/* begins "/dev/" */
	char	*ttydev = mytty + strlen("/dev/");

	setutent();			/* open utmp	  */
	while( entryp = getutent() )
	{
		/* if user matches and tty is not ours */
		if ( entryp->ut_type == USER_PROCESS 
			&& strcmp(entryp->ut_user,user)==0 
			&& strcmp(entryp->ut_line,ttydev)!=0 
			&& entryp->ut_line[0] != ':' 
			&& is_a_device(entryp->ut_line) 
		   )
		{
			num++;
			if ( num == 1 )
				result = str_add("/dev/", entryp->ut_line );
		}
	}
	endutent();

	if ( num > 1 )
		printf("Warning: %s is logged in %d times, using %s\n",
			user, num, result);
	return result;
}