伪终端pty的原理及使用
1. what is pseudo termina
伪终端(Pseudo Terminal)是成对的逻辑终端设备(即master和slave设备, 对master的操作会反映到slave上)。例 如/dev/ptyp3和/dev/ttyp3(或者在设备文件系统中分别是/dev/pty/m3和 /dev/pty/s3)。它们与实际物理设备并不直接相关。如果一个程序把ptyp3(master设备)看作是一个串行端口设备，则它对该端口的读/ 写操作会反映在该逻辑终端设备对应的另一个ttyp3(slave设备)上面。而ttyp3则是另一个程序用于读写操作的逻辑设备。
这 样，两个程序就可以通过这种逻辑设备进行互相交流，而其中一个使用ttyp3的程序则认为自己正在与一个串行端口进行通信。这很象是逻辑设备对之间的管道 操作。对于ttyp3(s3)，任何设计成使用一个串行端口设备的程序都可以使用该逻辑设备。但对于使用ptyp3的程序，则需要专门设计来使用 ptyp3(m3)逻辑设备。
/***************************************/
支持两种类型的pty:
CONFIG_UNIX98_PTYS=y
CONFIG_LEGACY_PTYS=y
CONFIG_LEGACY_PTY_COUNT=16
/***************************************/
2. How to implement pty in kernel
drivers/tty/pty.c
module_init(pty_init);
static int __init pty_init(void)
{
legacy_pty_init();
unix98_pty_init();
return 0;
}
这里只关注legacy pty,两者本质上是一样的，只是细节上不同而已:
/***************************************/
2.1 legacy_pty_init
static void __init legacy_pty_init(void)
{
struct tty_driver *pty_driver, *pty_slave_driver;
1]有关全局变量legacy_count
static int legacy_count = CONFIG_LEGACY_PTY_COUNT;if (legacy_count <= 0)
return;
2]分配一对tty_driver: pty_deiver and pty_slave_derver
并初始化，注意其中的other成员，两者的tty_operations基本相同，只是master多了一个ioctl函数
/*
static const struct tty_operations master_pty_ops_bsd = {
.install = pty_install,
.open = pty_open,
.close = pty_close,
.write = pty_write,
.write_room = pty_write_room,
.flush_buffer = pty_flush_buffer,
.chars_in_buffer = pty_chars_in_buffer,
.unthrottle = pty_unthrottle,
.set_termios = pty_set_termios,
.ioctl = pty_bsd_ioctl,
.resize = pty_resize
};
static const struct tty_operations slave_pty_ops_bsd = {
.install = pty_install,
.open = pty_open,
.close = pty_close,
.write = pty_write,
.write_room = pty_write_room,
.flush_buffer = pty_flush_buffer,
.chars_in_buffer = pty_chars_in_buffer,
.unthrottle = pty_unthrottle,
.set_termios = pty_set_termios,
.resize = pty_resize
};
*/
pty_driver = alloc_tty_driver(legacy_count);
if (!pty_driver)panic("Couldn't allocate pty driver");
pty_slave_driver = alloc_tty_driver(legacy_count);
if (!pty_slave_driver)
panic("Couldn't allocate pty slave driver");
pty_driver->driver_name = "pty_master";
pty_driver->name = "pty";
pty_driver->major = PTY_MASTER_MAJOR;
pty_driver->minor_start = 0;
pty_driver->type = TTY_DRIVER_TYPE_PTY;
pty_driver->subtype = PTY_TYPE_MASTER;
pty_driver->init_termios = tty_std_termios;
pty_driver->init_termios.c_iflag = 0;
pty_driver->init_termios.c_oflag = 0;
pty_driver->init_termios.c_cflag = B38400 | CS8 | CREAD;
pty_driver->init_termios.c_lflag = 0;
pty_driver->init_termios.c_ispeed = 38400;
pty_driver->init_termios.c_ospeed = 38400;
pty_driver->flags = TTY_DRIVER_RESET_TERMIOS | TTY_DRIVER_REAL_RAW;
pty_driver->other = pty_slave_driver;
tty_set_operations(pty_driver, &master_pty_ops_bsd);
pty_slave_driver->driver_name = "pty_slave";
pty_slave_driver->name = "ttyp";
pty_slave_driver->major = PTY_SLAVE_MAJOR;
pty_slave_driver->minor_start = 0;
pty_slave_driver->type = TTY_DRIVER_TYPE_PTY;
pty_slave_driver->subtype = PTY_TYPE_SLAVE;
pty_slave_driver->init_termios = tty_std_termios;
pty_slave_driver->init_termios.c_cflag = B38400 | CS8 | CREAD;
pty_slave_driver->init_termios.c_ispeed = 38400;
pty_slave_driver->init_termios.c_ospeed = 38400;
pty_slave_driver->flags = TTY_DRIVER_RESET_TERMIOS |
TTY_DRIVER_REAL_RAW;
pty_slave_driver->other = pty_driver;
tty_set_operations(pty_slave_driver, &slave_pty_ops_bsd);
3]注册具体的tty_driver到tty子系统
根据flags,这里创建了dev/pty* and dev/ttyp*if (tty_register_driver(pty_driver))
panic("Couldn't register pty driver");
if (tty_register_driver(pty_slave_driver))
panic("Couldn't register pty slave driver");
}
2.2 pty_install
/*当open 设备文件时，调用函数pty_install，该函数创建了成对的tty_struct*/static int pty_install(struct tty_driver *driver, struct tty_struct *tty)
{
struct tty_struct *o_tty;
int idx = tty->index;
int retval;
o_tty = alloc_tty_struct();
initialize_tty_struct(o_tty, driver->other, idx);
/* We always use new tty termios data so we can do this
the easy way .. */
retval = tty_init_termios(tty);
retval = tty_init_termios(o_tty);
/*
* Everything allocated ... set up the o_tty structure.
*/
driver->other->ttys[idx] = o_tty;
tty_driver_kref_get(driver->other);
if (driver->subtype == PTY_TYPE_MASTER)
o_tty->count++;
/* Establish the links in both directions */
tty->link = o_tty;
o_tty->link = tty;
tty_driver_kref_get(driver);
tty->count++;
driver->ttys[idx] = tty;
return 0;
}
2.3 pty 设备的写操作：写数据到虚拟设备对的另一端
/*pty 设备的写操作：写数据到虚拟设备对的另一端*/
/**
* pty_write - write to a pty
* @tty: the tty we write from
* @buf: kernel buffer of data
* @count: bytes to write
*
* Our "hardware" write method. Data is coming from the ldisc which
* may be in a non sleeping state. We simply throw this at the other
* end of the link as if we were an IRQ handler receiving stuff for
* the other side of the pty/tty pair.
*/
static int pty_write(struct tty_struct *tty, const unsigned char *buf, int c)
{
struct tty_struct *to = tty->link;
if (tty->stopped)
return 0;
if (c > 0) {
/* Stuff the data into the input queue of the other end */
c = tty_insert_flip_string(to, buf, c);
/* And shovel */
if (c) {
tty_flip_buffer_push(to);
tty_wakeup(tty);
}
}
return c;
}
2.4 pty 设备的读操作：虚拟设备的另一端会写数据到 readbuffe
/*pty 设备的读操作：虚拟设备的另一端会写数据到 readbuffer*/
也就是说函数tty_insert_flip_string等被调用
3 使用虚拟串口的例子：
3.1] 打开master side 端口：cat ptypb &
注意打开 slave side port 会失败,为什么请查一下？3.1.1 如果先打开slave port的例子
root@android:/dev # cat ttyp6pty_open: should open master first, otherwise error
/system/bin/sh: cat: ttyp6: I/O error
root@android:/dev # cat ptyp6
3.1.2 为什么先打开slave 出错
pty_install -> /*如果打开pty设备的类型是 master,则 count++*/if (driver->subtype == PTY_TYPE_MASTER)
o_tty->count++;
pty_open -> /*如果pty对的另一方 count != 1, 则 return error*/
if (tty->link->count != 1){
pr_err("pty_open: should open master first, otherwise error\n");
goto out;
}
3.2] 打开slave side 端口cat > ttypb
root@android:/dev # cat > ttypb3.3] 输入：
在ttypb设备上输入，在设备ptypb就读出相同的内容qqq
qqq
qqq
qqq
111111
111111
4444
4444