浅析ptmx代码级open如何运作ptyp,ttyp,pts伪终端
1.对ptmx执行open操作,将创建1对tty主从设备.
tty_init
=>cdev_init(&ptmx_cdev, &ptmx_fops);
=>然后创建/dev/ptmx节点[luther.gliethttp].
所以/dev/ptmx节点的open函数为ptmx_fops.ptmx_open()
static int ptmx_open(struct inode * inode, struct file * filp)
{
...
idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
...
if (index >= pty_limit) {//NR_UNIX98_PTY_DEFAULT也就是4096个
...
}
...
mutex_lock(&tty_mutex);
retval = init_dev(ptm_driver, index, &tty);//以index为pts的设备索引号,创建成对的主从设备ptmx和pts
mutex_unlock(&tty_mutex);
...
retval = ptm_driver->open(tty, filp);
...
}
所以fdm = open("/dev/ptmx", O_RDWR);操作之后,将产生一个成对的ptyp和ttyp主从tty设备,并返回ptyp主设备句柄.
2.调用ioctl获得与fdm主设备配对的ttyp设备号.
tty_ioctl
=>tty->driver->ioctl
=>在unix98_pty_init()中,仅仅对ptmx主设备赋予了ioctl操作：ptm_driver->ioctl = pty_unix98_ioctl;
=>pty_unix98_ioctl
=>
static int pty_unix98_ioctl(struct tty_struct *tty, struct file *file,
unsigned int cmd, unsigned long arg)
{
switch (cmd) {
case TIOCSPTLCK: /* Set PT Lock (disallow slave open) */
return pty_set_lock(tty, (int __user *)arg);
case TIOCGPTN: /* Get PT Number */
return put_user(tty->index, (unsigned int __user *)arg);//当前tty对应的为ptmx结构,它的index就是与之配对的pts
}
return -ENOIOCTLCMD;
}
3.看看glibc库中如何封装ptsname函数,如下为精简版[luther.gliethttp].
char* ptsname( int fd )
{
unsigned int pty_num;
static char buff[64];
if ( ioctl( fd, TIOCGPTN, &pty_num ) != 0 )//最终调用上面的pty_unix98_ioctl获取当前ptmx主设备对应的pty从设备号.
return NULL;
snprintf( buff, sizeof(buff), "/dev/pts/%u", pty_num );//格式化为/dev/pts/0,/dev/pts/1等,即:pts对应的文件全路径.
return buff;
}
4.应用实例
int fdm fds;
char *slavename;
extern char *ptsname();
fdm = open("/dev/ptmx", O_RDWR); /* open master */
grantpt(fdm); /* change permission of slave */
unlockpt(fdm); /* unlock slave */
slavename = ptsname(fdm); /* get name of slave */
fds = open(slavename, O_RDWR); /* open slave */
ioctl(fds, I_PUSH, "ptem"); /* push ptem */
ioctl(fds, I_PUSH, "ldterm"); /* push ldterm */
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