TTY终端的输入过程

绝地灬酷狼 2022-05-29 02:08 593阅读 0赞

TTY终端的输入过程
    =================
    1) 当用户按压键盘时, 键盘中断处理程序将经过转换的键盘功能码用tty_insert_flip_char()放入到当前打开终端的翻转缓冲区之中, 
    然后将缓冲区输出任务函数(flush_to_ldisc)添加到控制台任务队列(con_task_queue)并激活控制台软中断执行该任务函数. flush_to_ldisc()翻转读写缓冲区,
     将缓冲区接收数据传递给tty终端规程的n_tty_receive_buf()接收函数, n_tty_receive_buf()处理输入字符, 将输出字符缓冲在终端的循环缓冲区(read_buf)之中.
     用户通过tty规程的read_chan()读取read_buf中的字符. 当多个进程同时读取同一终端时, 使用tty->atomic_read信号灯来竞争读取权. 
    
    
    2) 每一打开的终端接口中包含终端的参数结构(termios), 当终端打开时, 它将继承终端驱动设备参数表中参数结构, 系统缺省的终端参数为tty_std_termios,
     驱动设备的初始终端参数为init_termios. 终端参数可由用户通过ioctl()调整, 用来控制终端接口的行为. c_iflag描述对输入字符的识别模式, 
    c_oflag描述对输出字符的处理模式, c_cflag描述终端接口的控制模式, c_lflag描述字符处理过程中各种本地行为, 还有由19个字符组成的c_cc字符参数表, 
    描述各类控制字符以及辅助参数. 
    
    3) 当本地模式具有规范输入标志(ICANON)时, 输出数据按行拷贝给用户. 在接收到换行符(或者文件结束符c_cc[VEOF],行结束符c_cc[VEOL]和c_cc[VEOL2])之前, 
    可以使用编辑控制符完成简单的编辑. c_cc[VERASE]删除前一字符, c_cc[VKILL]删除整行, c_cc[VWERASE]删除前一单词. 
    
    4) 当本地模式具有回显标志(ECHO)时, 输入字符按一定的规则回显到输出设备上. 当本地模式具有信号标志(ISIG)时, 特定的输入字符可以在终端当前进程组中产生信号.
     c_cc[VINTR]产生中断信号, c_cc[VQUIT]产生退出信息, c_cc[VSUSP]产生暂停信号. c_cc[VTIME]定义了用户读取数据的等待超时, 单位为0.1秒, 
    c_cc[VMIN]为所期望的字节数. 在非ICANON模式下, 如果在c_cc[VTIME]时间内接收到c_cc[VMIN]数量的字符, 读过程返回. c_cc[VSTOP]暂停驱动设备输出, 
    c_cc[VSTART]启动驱动设备输出. 
    
    5) 内核初始化结束后, 打开控制台终端设备/dev/console作为输入输出去运行/sbin/init. /dev/console的设备号是0x0501, 
    在打开时被定向到命令行(console=)指定的终端设备, 缺省情况下为/dev/tty1, 其设备号为0x0401. init程序完成用户初始化完成后启动若干个终端守护程序(getty).
     getty在打开所守护的终端文件之前先使用setsid()创建会话组并成为会话组的首领进程(leader=1), 首领进程的会话号(session)和进程组号(pgrp)等于进程号(pid).
     首领进程打开终端文件后成为终端的控制进程, 其tty指针指向该终端打开结构, 该终端也成为本次会话的控制终端, 
    其session号和pgrp号分别等于进程的session号和pgrp号. 当用户从终端登录后, 登录bash程序置换getty进程成为首领进程. 
    进程的会话号和组号随着进程的复制而传递, 子进程的首领标志复位.
     bash在用execve()执行用户程序之前将用setpgid()为每一程序建立属于它自已的进程组(使进程组号等于进程号), 
    再用对终端的(TIOCSPGRP)设备控制将终端进程组号设为将该进程号, 使终端切换到新的进程组. 
    终端进程组以外的进程组如果进行读操作将在其进程组中生成SIGTTIN信号, 缺省的信号处理器使进程进入暂停. 
    
    ; drivers/char/keyboard.c:
    
    void put_queue(int ch)
    {
    	wake_up(&keypress_wait);
    	if (tty) {
    		tty_insert_flip_char(tty, ch, 0);
    		con_schedule_flip(tty);
    	}
    }
    static void puts_queue(char *cp)
    {
    	wake_up(&keypress_wait);
    	if (!tty)
    		return;
    
    	while (*cp) {
    		tty_insert_flip_char(tty, *cp, 0);
    		cp++;
    	}
    	con_schedule_flip(tty);
    }
    _INLINE_ void tty_insert_flip_char(struct tty_struct *tty,
    				   unsigned char ch, char flag)
    {
    	if (tty->flip.count < TTY_FLIPBUF_SIZE) {
    		tty->flip.count++;
    		*tty->flip.flag_buf_ptr++ = flag;
    		*tty->flip.char_buf_ptr++ = ch;
    	}
    }
    extern inline void con_schedule_flip(struct tty_struct *t)
    {
    	queue_task(&t->flip.tqueue, &con_task_queue);
    	tasklet_schedule(&console_tasklet);
    }
    
    
    ; drivers/char/tty_io.c:
    
    /*
     * When a break, frame error, or parity error happens, these codes are
     * stuffed into the flags buffer.
     */
    #define TTY_NORMAL	0
    #define TTY_BREAK	1
    #define TTY_FRAME	2
    #define TTY_PARITY	3
    #define TTY_OVERRUN	4
    
    #define INTR_CHAR(tty) ((tty)->termios->c_cc[VINTR])
    #define QUIT_CHAR(tty) ((tty)->termios->c_cc[VQUIT])
    #define ERASE_CHAR(tty) ((tty)->termios->c_cc[VERASE])
    #define KILL_CHAR(tty) ((tty)->termios->c_cc[VKILL])
    #define EOF_CHAR(tty) ((tty)->termios->c_cc[VEOF])
    #define TIME_CHAR(tty) ((tty)->termios->c_cc[VTIME])
    #define MIN_CHAR(tty) ((tty)->termios->c_cc[VMIN])
    #define SWTC_CHAR(tty) ((tty)->termios->c_cc[VSWTC])
    #define START_CHAR(tty) ((tty)->termios->c_cc[VSTART])
    #define STOP_CHAR(tty) ((tty)->termios->c_cc[VSTOP])
    #define SUSP_CHAR(tty) ((tty)->termios->c_cc[VSUSP])
    #define EOL_CHAR(tty) ((tty)->termios->c_cc[VEOL])
    #define REPRINT_CHAR(tty) ((tty)->termios->c_cc[VREPRINT])
    #define DISCARD_CHAR(tty) ((tty)->termios->c_cc[VDISCARD])
    #define WERASE_CHAR(tty) ((tty)->termios->c_cc[VWERASE])
    #define LNEXT_CHAR(tty)	((tty)->termios->c_cc[VLNEXT])
    #define EOL2_CHAR(tty) ((tty)->termios->c_cc[VEOL2])
    
    #define _I_FLAG(tty,f)	((tty)->termios->c_iflag & (f))
    #define _O_FLAG(tty,f)	((tty)->termios->c_oflag & (f))
    #define _C_FLAG(tty,f)	((tty)->termios->c_cflag & (f))
    #define _L_FLAG(tty,f)	((tty)->termios->c_lflag & (f))
    
    #define I_IGNBRK(tty)	_I_FLAG((tty),IGNBRK)
    #define I_BRKINT(tty)	_I_FLAG((tty),BRKINT)
    #define I_IGNPAR(tty)	_I_FLAG((tty),IGNPAR)
    #define I_PARMRK(tty)	_I_FLAG((tty),PARMRK)
    #define I_INPCK(tty)	_I_FLAG((tty),INPCK)
    #define I_ISTRIP(tty)	_I_FLAG((tty),ISTRIP)
    #define I_INLCR(tty)	_I_FLAG((tty),INLCR)
    #define I_IGNCR(tty)	_I_FLAG((tty),IGNCR)
    #define I_ICRNL(tty)	_I_FLAG((tty),ICRNL)
    #define I_IUCLC(tty)	_I_FLAG((tty),IUCLC)
    #define I_IXON(tty)	_I_FLAG((tty),IXON)
    #define I_IXANY(tty)	_I_FLAG((tty),IXANY)
    #define I_IXOFF(tty)	_I_FLAG((tty),IXOFF)
    #define I_IMAXBEL(tty)	_I_FLAG((tty),IMAXBEL)
    
    #define O_OPOST(tty)	_O_FLAG((tty),OPOST)
    #define O_OLCUC(tty)	_O_FLAG((tty),OLCUC)
    #define O_ONLCR(tty)	_O_FLAG((tty),ONLCR)
    #define O_OCRNL(tty)	_O_FLAG((tty),OCRNL)
    #define O_ONOCR(tty)	_O_FLAG((tty),ONOCR)
    #define O_ONLRET(tty)	_O_FLAG((tty),ONLRET)
    #define O_OFILL(tty)	_O_FLAG((tty),OFILL)
    #define O_OFDEL(tty)	_O_FLAG((tty),OFDEL)
    #define O_NLDLY(tty)	_O_FLAG((tty),NLDLY)
    #define O_CRDLY(tty)	_O_FLAG((tty),CRDLY)
    #define O_TABDLY(tty)	_O_FLAG((tty),TABDLY)
    #define O_BSDLY(tty)	_O_FLAG((tty),BSDLY)
    #define O_VTDLY(tty)	_O_FLAG((tty),VTDLY)
    #define O_FFDLY(tty)	_O_FLAG((tty),FFDLY)
    
    #define C_BAUD(tty)	_C_FLAG((tty),CBAUD)
    #define C_CSIZE(tty)	_C_FLAG((tty),CSIZE)
    #define C_CSTOPB(tty)	_C_FLAG((tty),CSTOPB)
    #define C_CREAD(tty)	_C_FLAG((tty),CREAD)
    #define C_PARENB(tty)	_C_FLAG((tty),PARENB)
    #define C_PARODD(tty)	_C_FLAG((tty),PARODD)
    #define C_HUPCL(tty)	_C_FLAG((tty),HUPCL)
    #define C_CLOCAL(tty)	_C_FLAG((tty),CLOCAL)
    #define C_CIBAUD(tty)	_C_FLAG((tty),CIBAUD)
    #define C_CRTSCTS(tty)	_C_FLAG((tty),CRTSCTS)
    
    #define L_ISIG(tty)	_L_FLAG((tty),ISIG)
    #define L_ICANON(tty)	_L_FLAG((tty),ICANON)
    #define L_XCASE(tty)	_L_FLAG((tty),XCASE)
    #define L_ECHO(tty)	_L_FLAG((tty),ECHO)
    #define L_ECHOE(tty)	_L_FLAG((tty),ECHOE)
    #define L_ECHOK(tty)	_L_FLAG((tty),ECHOK)
    #define L_ECHONL(tty)	_L_FLAG((tty),ECHONL)
    #define L_NOFLSH(tty)	_L_FLAG((tty),NOFLSH)
    #define L_TOSTOP(tty)	_L_FLAG((tty),TOSTOP)
    #define L_ECHOCTL(tty)	_L_FLAG((tty),ECHOCTL)
    #define L_ECHOPRT(tty)	_L_FLAG((tty),ECHOPRT)
    #define L_ECHOKE(tty)	_L_FLAG((tty),ECHOKE)
    #define L_FLUSHO(tty)	_L_FLAG((tty),FLUSHO)
    #define L_PENDIN(tty)	_L_FLAG((tty),PENDIN)
    #define L_IEXTEN(tty)	_L_FLAG((tty),IEXTEN)
    
    /* number of characters left in xmit buffer before select has we have room */
    #define WAKEUP_CHARS 256
    
    typedef unsigned char	cc_t;
    typedef unsigned int	speed_t;
    typedef unsigned int	tcflag_t;
    
    #define NCCS 19
    struct termios {
    	tcflag_t c_iflag;		/* input mode flags */
    	tcflag_t c_oflag;		/* output mode flags */
    	tcflag_t c_cflag;		/* control mode flags */
    	tcflag_t c_lflag;		/* local mode flags */
    	cc_t c_line;			/* line discipline */
    	cc_t c_cc[NCCS];		/* control characters */
    };
    
    /*	intr=^C		quit=^\		erase=del	kill=^U
    	eof=^D		vtime=\0	vmin=\1		sxtc=\0
    	start=^Q	stop=^S		susp=^Z		eol=\0
    	reprint=^R	discard=^U	werase=^W	lnext=^V
    	eol2=\0
    */
    #define INIT_C_CC "\003\034\177\025\004\0\1\0\021\023\032\0\022\017\027\026\0"
    
    /* c_cc characters */
    #define VINTR 0
    #define VQUIT 1
    #define VERASE 2
    #define VKILL 3
    #define VEOF 4
    #define VTIME 5
    #define VMIN 6
    #define VSWTC 7
    #define VSTART 8
    #define VSTOP 9
    #define VSUSP 10
    #define VEOL 11
    #define VREPRINT 12
    #define VDISCARD 13
    #define VWERASE 14
    #define VLNEXT 15
    #define VEOL2 16
    
    /* c_iflag bits */
    #define IGNBRK	0000001
    #define BRKINT	0000002
    #define IGNPAR	0000004
    #define PARMRK	0000010
    #define INPCK	0000020
    #define ISTRIP	0000040
    #define INLCR	0000100
    #define IGNCR	0000200
    #define ICRNL	0000400
    #define IUCLC	0001000
    #define IXON	0002000
    #define IXANY	0004000
    #define IXOFF	0010000
    #define IMAXBEL	0020000
    
    /* c_oflag bits */
    #define OPOST	0000001
    #define OLCUC	0000002
    #define ONLCR	0000004
    #define OCRNL	0000010
    #define ONOCR	0000020
    #define ONLRET	0000040
    #define OFILL	0000100
    #define OFDEL	0000200
    #define NLDLY	0000400
    #define   NL0	0000000
    #define   NL1	0000400
    #define CRDLY	0003000
    #define   CR0	0000000
    #define   CR1	0001000
    #define   CR2	0002000
    #define   CR3	0003000
    #define TABDLY	0014000
    #define   TAB0	0000000
    #define   TAB1	0004000
    #define   TAB2	0010000
    #define   TAB3	0014000
    #define   XTABS	0014000
    #define BSDLY	0020000
    #define   BS0	0000000
    #define   BS1	0020000
    #define VTDLY	0040000
    #define   VT0	0000000
    #define   VT1	0040000
    #define FFDLY	0100000
    #define   FF0	0000000
    #define   FF1	0100000
    
    /* c_cflag bit meaning */
    #define CBAUD	0010017
    #define  B0	0000000		/* hang up */
    #define  B50	0000001
    #define  B75	0000002
    #define  B110	0000003
    #define  B134	0000004
    #define  B150	0000005
    #define  B200	0000006
    #define  B300	0000007
    #define  B600	0000010
    #define  B1200	0000011
    #define  B1800	0000012
    #define  B2400	0000013
    #define  B4800	0000014
    #define  B9600	0000015
    #define  B19200	0000016
    #define  B38400	0000017
    #define EXTA B19200
    #define EXTB B38400
    #define CSIZE	0000060
    #define   CS5	0000000
    #define   CS6	0000020
    #define   CS7	0000040
    #define   CS8	0000060
    #define CSTOPB	0000100
    #define CREAD	0000200
    #define PARENB	0000400
    #define PARODD	0001000
    #define HUPCL	0002000
    #define CLOCAL	0004000
    #define CBAUDEX 0010000
    #define    B57600 0010001
    #define   B115200 0010002
    #define   B230400 0010003
    #define   B460800 0010004
    #define   B500000 0010005
    #define   B576000 0010006
    #define   B921600 0010007
    #define  B1000000 0010010
    #define  B1152000 0010011
    #define  B1500000 0010012
    #define  B2000000 0010013
    #define  B2500000 0010014
    #define  B3000000 0010015
    #define  B3500000 0010016
    #define  B4000000 0010017
    #define CIBAUD	  002003600000	/* input baud rate (not used) */
    #define CMSPAR	  010000000000		/* mark or space (stick) parity */
    #define CRTSCTS	  020000000000		/* flow control */
    
    /* c_lflag bits */
    #define ISIG	0000001
    #define ICANON	0000002
    #define XCASE	0000004
    #define ECHO	0000010
    #define ECHOE	0000020
    #define ECHOK	0000040
    #define ECHONL	0000100
    #define NOFLSH	0000200
    #define TOSTOP	0000400
    #define ECHOCTL	0001000
    #define ECHOPRT	0002000
    #define ECHOKE	0004000
    #define FLUSHO	0010000
    #define PENDIN	0040000
    #define IEXTEN	0100000
    
    /*
     * This defines the low- and high-watermarks for throttling and
     * unthrottling the TTY driver.  These watermarks are used for
     * controlling the space in the read buffer.
     */
    #define TTY_THRESHOLD_THROTTLE		128 /* now based on remaining room */
    #define TTY_THRESHOLD_UNTHROTTLE 	128
    
    static struct file_operations tty_fops = {
    	llseek:		tty_lseek,
    	read:		tty_read,
    	write:		tty_write,
    	poll:		tty_poll,
    	ioctl:		tty_ioctl,
    	open:		tty_open,
    	release:	tty_release,
    	fasync:		tty_fasync,
    };
    
    void __init console_init(void)
    {
    	/* Setup the default TTY line discipline. */
    	memset(ldiscs, 0, sizeof(ldiscs));
    	(void) tty_register_ldisc(N_TTY, &tty_ldisc_N_TTY);
    
    	/*
    	 * Set up the standard termios.  Individual tty drivers may 
    	 * deviate from this; this is used as a template.
    	 */
    	memset(&tty_std_termios, 0, sizeof(struct termios));
    	memcpy(tty_std_termios.c_cc, INIT_C_CC, NCCS);
    	tty_std_termios.c_iflag = ICRNL | IXON;
    	tty_std_termios.c_oflag = OPOST | ONLCR;
    	tty_std_termios.c_cflag = B38400 | CS8 | CREAD | HUPCL;
    	tty_std_termios.c_lflag = ISIG | ICANON | ECHO | ECHOE | ECHOK |
    		ECHOCTL | ECHOKE | IEXTEN;
    
    	...
    }
    
    static ssize_t tty_read(struct file * file, char * buf, size_t count, 
    			loff_t *ppos)
    {
    	int i;
    	struct tty_struct * tty;
    	struct inode *inode;
    
    	/* Can't seek (pread) on ttys.  */
    	if (ppos != &file->f_pos)
    		return -ESPIPE;
    
    	tty = (struct tty_struct *)file->private_data;
    	inode = file->f_dentry->d_inode;
    	if (tty_paranoia_check(tty, inode->i_rdev, "tty_read")) 校验tty结构是否完整
    		return -EIO;
    	if (!tty || (test_bit(TTY_IO_ERROR, &tty->flags)))
    		return -EIO;
    
    	/* This check not only needs to be done before reading, but also
    	   whenever read_chan() gets woken up after sleeping, so I've
    	   moved it to there.  This should only be done for the N_TTY
    	   line discipline, anyway.  Same goes for write_chan(). -- jlc. */
    #if 0
    	if ((inode->i_rdev != CONSOLE_DEV) && /* don't stop on /dev/console */
    	    (tty->pgrp > 0) &&
    	    (current->tty == tty) &&
    	    (tty->pgrp != current->pgrp))
    		if (is_ignored(SIGTTIN) || is_orphaned_pgrp(current->pgrp))
    			return -EIO;
    		else {
    			(void) kill_pg(current->pgrp, SIGTTIN, 1);
    			return -ERESTARTSYS;
    		}
    #endif
    	lock_kernel();
    	if (tty->ldisc.read)
    		i = (tty->ldisc.read)(tty,file,buf,count);
    	else
    		i = -EIO;
    	unlock_kernel();
    	if (i > 0)
    		inode->i_atime = CURRENT_TIME;
    	return i;
    }
    
    /*
     * This routine is called out of the software interrupt to flush data
     * from the flip buffer to the line discipline.
     */
    static void flush_to_ldisc(void *private_)
    {
    	struct tty_struct *tty = (struct tty_struct *) private_;
    	unsigned char	*cp;
    	char		*fp;
    	int		count;
    	unsigned long flags;
    
    	if (test_bit(TTY_DONT_FLIP, &tty->flags)) { 如果不允许翻转读写缓冲区
    		queue_task(&tty->flip.tqueue, &tq_timer); 将任务延迟一个时钟中断
    		return;
    	}
    	if (tty->flip.buf_num) {
    		cp = tty->flip.char_buf + TTY_FLIPBUF_SIZE;
    		fp = tty->flip.flag_buf + TTY_FLIPBUF_SIZE;
    		tty->flip.buf_num = 0;
    
    		save_flags(flags); cli();
    		tty->flip.char_buf_ptr = tty->flip.char_buf;
    		tty->flip.flag_buf_ptr = tty->flip.flag_buf;
    	} else {
    		cp = tty->flip.char_buf;
    		fp = tty->flip.flag_buf;
    		tty->flip.buf_num = 1;
    
    		save_flags(flags); cli();
    		tty->flip.char_buf_ptr = tty->flip.char_buf + TTY_FLIPBUF_SIZE;
    		tty->flip.flag_buf_ptr = tty->flip.flag_buf + TTY_FLIPBUF_SIZE;
    	}
    	count = tty->flip.count;
    	tty->flip.count = 0;
    	restore_flags(flags);
    	
    	tty->ldisc.receive_buf(tty, cp, fp, count);
    }
    void start_tty(struct tty_struct *tty)
    {
    	if (!tty->stopped || tty->flow_stopped)
    		return;
    	tty->stopped = 0;
    	if (tty->link && tty->link->packet) {
    		tty->ctrl_status &= ~TIOCPKT_STOP;
    		tty->ctrl_status |= TIOCPKT_START;
    		wake_up_interruptible(&tty->link->read_wait);
    	}
    	if (tty->driver.start)
    		(tty->driver.start)(tty);
    	if ((test_bit(TTY_DO_WRITE_WAKEUP, &tty->flags)) &&
    	    tty->ldisc.write_wakeup)
    		(tty->ldisc.write_wakeup)(tty);
    	wake_up_interruptible(&tty->write_wait);
    }
    void stop_tty(struct tty_struct *tty)
    {
    	if (tty->stopped)
    		return;
    	tty->stopped = 1;
    	if (tty->link && tty->link->packet) { 
    		tty->ctrl_status &= ~TIOCPKT_START; 
    		tty->ctrl_status |= TIOCPKT_STOP; 
    		wake_up_interruptible(&tty->link->read_wait);
    	}
    	if (tty->driver.stop)
    		(tty->driver.stop)(tty);
    }
    ; drivers/char/n_tty.c:
    
    struct tty_ldisc tty_ldisc_N_TTY = {
    	TTY_LDISC_MAGIC,	/* magic */
    	"n_tty",		/* name */
    	0,			/* num */
    	0,			/* flags */
    	n_tty_open,		/* open */
    	n_tty_close,		/* close */
    	n_tty_flush_buffer,	/* flush_buffer */
    	n_tty_chars_in_buffer,	/* chars_in_buffer */
    	read_chan,		/* read */
    	write_chan,		/* write */
    	n_tty_ioctl,		/* ioctl */
    	n_tty_set_termios,	/* set_termios */
    	normal_poll,		/* poll */
    	n_tty_receive_buf,	/* receive_buf */
    	n_tty_receive_room,	/* receive_room */
    	0			/* write_wakeup */
    };
    
    static ssize_t read_chan(struct tty_struct *tty, struct file *file,
    			 unsigned char *buf, size_t nr)
    {
    	unsigned char *b = buf;
    	DECLARE_WAITQUEUE(wait, current);
    	int c;
    	int minimum, time;
    	ssize_t retval = 0;
    	ssize_t size;
    	long timeout;
    	unsigned long flags;
    
    do_it_again:
    
    	if (!tty->read_buf) {
    		printk("n_tty_read_chan: called with read_buf == NULL?!?\n");
    		return -EIO;
    	}
    
    	/* Job control check -- must be done at start and after
    	   every sleep (POSIX.1 7.1.1.4). */
    	/* NOTE: not yet done after every sleep pending a thorough
    	   check of the logic of this change. -- jlc */
    	/* don't stop on /dev/console */
    	if (file->f_dentry->d_inode->i_rdev != CONSOLE_DEV && 打开的不是/dev/tty设备
    	    file->f_dentry->d_inode->i_rdev != SYSCONS_DEV && 打开的不是/dev/console设备
    	    current->tty == tty) { 
    		if (tty->pgrp <= 0)
    			printk("read_chan: tty->pgrp <= 0!\n");
    		else if (current->pgrp != tty->pgrp) { 如果当前进程不属于终端的当前进程组
    			if (is_ignored(SIGTTIN) || 如果SIGTTIN信号被忽略
    			    is_orphaned_pgrp(current->pgrp)) 如果当前进程组的父进程已退出
    				return -EIO;
    			kill_pg(current->pgrp, SIGTTIN, 1); 
    			return -ERESTARTSYS;
    		}
    	}
    
    	minimum = time = 0;
    	timeout = MAX_SCHEDULE_TIMEOUT;
    	if (!tty->icanon) { 如果不是规范输入模式
    		time = (HZ / 10) * TIME_CHAR(tty); 计算超时时间, 时间字节的单位为0.1秒
    		minimum = MIN_CHAR(tty); 返回的最少字符数
    		if (minimum) { 
    			if (time) 
    				tty->minimum_to_wake = 1; 接收到1个字符后就唤醒
    			else if (!waitqueue_active(&tty->read_wait) ||
    				 (tty->minimum_to_wake > minimum))
    				tty->minimum_to_wake = minimum;
    		} else { 如果最少返回字符为零
    			timeout = 0; 
    			if (time) {
    				timeout = time;
    				time = 0;
    			}
    			tty->minimum_to_wake = minimum = 1;
    		}
    	}
    
    	if (file->f_flags & O_NONBLOCK) {
    		if (down_trylock(&tty->atomic_read))
    			return -EAGAIN;
    	}
    	else {
    		if (down_interruptible(&tty->atomic_read))
    			return -ERESTARTSYS;
    	}
    
    	add_wait_queue(&tty->read_wait, &wait);
    	set_bit(TTY_DONT_FLIP, &tty->flags); 禁止缓冲区翻转
    	while (nr) {
    		/* First test for status change. */
    		if (tty->packet && tty->link->ctrl_status) {
    			unsigned char cs;
    			if (b != buf)
    				break;
    			cs = tty->link->ctrl_status;
    			tty->link->ctrl_status = 0;
    			put_user(cs, b++); 
    			nr--;
    			break; 
    		}
    		/* This statement must be first before checking for input
    		   so that any interrupt will set the state back to
    		   TASK_RUNNING. */
    		set_current_state(TASK_INTERRUPTIBLE);
    		
    		if (((minimum - (b - buf)) < tty->minimum_to_wake) &&
    		    ((minimum - (b - buf)) >= 1))
    			tty->minimum_to_wake = (minimum - (b - buf));
    		
    		if (!input_available_p(tty, 0)) {
    			if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
    				retval = -EIO;
    				break;
    			}
    			if (tty_hung_up_p(file))
    				break;
    			if (!timeout)
    				break;
    			if (file->f_flags & O_NONBLOCK) {
    				retval = -EAGAIN;
    				break;
    			}
    			if (signal_pending(current)) {
    				retval = -ERESTARTSYS;
    				break;
    			}
    			clear_bit(TTY_DONT_FLIP, &tty->flags);
    			timeout = schedule_timeout(timeout);
    			set_bit(TTY_DONT_FLIP, &tty->flags);
    			continue;
    		}
    		current->state = TASK_RUNNING;
    
    		/* Deal with packet mode. */
    		if (tty->packet && b == buf) {
    			put_user(TIOCPKT_DATA, b++);
    			nr--;
    		}
    
    		if (tty->icanon) {
    			/* N.B. avoid overrun if nr == 0 */
    			while (nr && tty->read_cnt) {
     				int eol;
    
    				eol = test_and_clear_bit(tty->read_tail,
    						&tty->read_flags);
    				c = tty->read_buf[tty->read_tail];
    				spin_lock_irqsave(&tty->read_lock, flags);
    				tty->read_tail = ((tty->read_tail+1) &
    						  (N_TTY_BUF_SIZE-1));
    				tty->read_cnt--;
    				if (eol) {
    					/* this test should be redundant:
    					 * we shouldn't be reading data if
    					 * canon_data is 0
    					 */
    					if (--tty->canon_data < 0)
    						tty->canon_data = 0;
    				}
    				spin_unlock_irqrestore(&tty->read_lock, flags);
    
    				if (!eol || (c != __DISABLED_CHAR)) {
    					put_user(c, b++);
    					nr--;
    				}
    				if (eol)
    					break;
    			}
    		} else {
    			int uncopied;
    			uncopied = copy_from_read_buf(tty, &b, &nr);
    			uncopied += copy_from_read_buf(tty, &b, &nr);
    			if (uncopied) {
    				retval = -EFAULT;
    				break;
    			}
    		}
    
    		/* If there is enough space in the read buffer now, let the
    		 * low-level driver know. We use n_tty_chars_in_buffer() to
    		 * check the buffer, as it now knows about canonical mode.
    		 * Otherwise, if the driver is throttled and the line is
    		 * longer than TTY_THRESHOLD_UNTHROTTLE in canonical mode,
    		 * we won't get any more characters.
    		 */
    		if (n_tty_chars_in_buffer(tty) <= TTY_THRESHOLD_UNTHROTTLE)
    			check_unthrottle(tty); 如果读缓冲区的字符小于128, 打开节流的输入设备
    
    		if (b - buf >= minimum)
    			break;
    		if (time)
    			timeout = time;
    	}
    	clear_bit(TTY_DONT_FLIP, &tty->flags);
    	up(&tty->atomic_read);
    	remove_wait_queue(&tty->read_wait, &wait);
    
    	if (!waitqueue_active(&tty->read_wait))
    		tty->minimum_to_wake = minimum;
    
    	current->state = TASK_RUNNING;
    	size = b - buf;
    	if (size) {
    		retval = size;
    		if (nr)
    	       		clear_bit(TTY_PUSH, &tty->flags);
    	} else if (test_and_clear_bit(TTY_PUSH, &tty->flags))
    		 goto do_it_again;
    
    	return retval;
    }
    int is_ignored(int sig)
    {
    	return (sigismember(¤t->blocked, sig) ||
    	        current->sig->action[sig-1].sa.sa_handler == SIG_IGN);
    }
    
    static inline int input_available_p(struct tty_struct *tty, int amt)
    {
    	if (tty->icanon) {
    		if (tty->canon_data)
    			return 1;
    	} else if (tty->read_cnt >= (amt ? amt : 1))
    		return 1;
    
    	return 0;
    }
    int tty_hung_up_p(struct file * filp)
    {
    	return (filp->f_op == &hung_up_tty_fops);
    }
    /*
     * Helper function to speed up read_chan.  It is only called when
     * ICANON is off; it copies characters straight from the tty queue to
     * user space directly.  It can be profitably called twice; once to
     * drain the space from the tail pointer to the (physical) end of the
     * buffer, and once to drain the space from the (physical) beginning of
     * the buffer to head pointer.
     */
    static inline int copy_from_read_buf(struct tty_struct *tty,
    				      unsigned char **b,
    				      size_t *nr)
    
    {
    	int retval;
    	ssize_t n;
    	unsigned long flags;
    
    	retval = 0;
    	spin_lock_irqsave(&tty->read_lock, flags);
    	n = MIN(*nr, MIN(tty->read_cnt, N_TTY_BUF_SIZE - tty->read_tail));
    	spin_unlock_irqrestore(&tty->read_lock, flags);
    	if (n) {
    		mb();
    		retval = copy_to_user(*b, &tty->read_buf[tty->read_tail], n);
    		n -= retval;
    		spin_lock_irqsave(&tty->read_lock, flags);
    		tty->read_tail = (tty->read_tail + n) & (N_TTY_BUF_SIZE-1);
    		tty->read_cnt -= n;
    		spin_unlock_irqrestore(&tty->read_lock, flags);
    		*b += n;
    		*nr -= n;
    	}
    	return retval;
    }
    /*
     * Return number of characters buffered to be delivered to user
     */
    ssize_t n_tty_chars_in_buffer(struct tty_struct *tty)
    {
    	unsigned long flags;
    	ssize_t n = 0;
    
    	spin_lock_irqsave(&tty->read_lock, flags);
    	if (!tty->icanon) {
    		n = tty->read_cnt;
    	} else if (tty->canon_data) {
    		n = (tty->canon_head > tty->read_tail) ?
    			tty->canon_head - tty->read_tail :
    			tty->canon_head + (N_TTY_BUF_SIZE - tty->read_tail);
    	}
    	spin_unlock_irqrestore(&tty->read_lock, flags);
    	return n;
    }
    
    /* 
     * Check whether to call the driver.unthrottle function.
     * We test the TTY_THROTTLED bit first so that it always
     * indicates the current state.
     */
    static void check_unthrottle(struct tty_struct * tty)
    {
    	if (tty->count &&
    	    test_and_clear_bit(TTY_THROTTLED, &tty->flags) && 
    	    tty->driver.unthrottle)
    		tty->driver.unthrottle(tty);
    }
    
    static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
    			      char *fp, int count)
    {
    	const unsigned char *p;
    	char *f, flags = TTY_NORMAL;
    	int	i;
    	char	buf[64];
    	unsigned long cpuflags;
    
    	if (!tty->read_buf)
    		return;
    
    	if (tty->real_raw) { 如果终端处于原始接收模式, 直接输出翻转缓冲区字符
    		spin_lock_irqsave(&tty->read_lock, cpuflags);
    		i = MIN(count, MIN(N_TTY_BUF_SIZE - tty->read_cnt,
    				   N_TTY_BUF_SIZE - tty->read_head));
    		memcpy(tty->read_buf + tty->read_head, cp, i);
    		tty->read_head = (tty->read_head + i) & (N_TTY_BUF_SIZE-1);
    		tty->read_cnt += i;
    		cp += i;
    		count -= i;
    
    		i = MIN(count, MIN(N_TTY_BUF_SIZE - tty->read_cnt,
    			       N_TTY_BUF_SIZE - tty->read_head));
    		memcpy(tty->read_buf + tty->read_head, cp, i);
    		tty->read_head = (tty->read_head + i) & (N_TTY_BUF_SIZE-1);
    		tty->read_cnt += i;
    		spin_unlock_irqrestore(&tty->read_lock, cpuflags);
    	} else {
    		for (i=count, p = cp, f = fp; i; i--, p++) {
    			if (f)
    				flags = *f++;
    			switch (flags) {
    			case TTY_NORMAL:
    				n_tty_receive_char(tty, *p);
    				break;
    			case TTY_BREAK:
    				n_tty_receive_break(tty);
    				break;
    			case TTY_PARITY:
    			case TTY_FRAME:
    				n_tty_receive_parity_error(tty, *p);
    				break;
    			case TTY_OVERRUN:
    				n_tty_receive_overrun(tty);
    				break;
    			default:
    				printk("%s: unknown flag %d\n",
    				       tty_name(tty, buf), flags);
    				break;
    			}
    		}
    		if (tty->driver.flush_chars)
    			tty->driver.flush_chars(tty);
    	}
    
    	if (!tty->icanon && (tty->read_cnt >= tty->minimum_to_wake)) {
    		kill_fasync(&tty->fasync, SIGIO, POLL_IN); 异步信号读写通知
    		if (waitqueue_active(&tty->read_wait))
    			wake_up_interruptible(&tty->read_wait);
    	}
    
    	/*
    	 * Check the remaining room for the input canonicalization
    	 * mode.  We don't want to throttle the driver if we're in
    	 * canonical mode and don't have a newline yet!
    	 */
    	if (n_tty_receive_room(tty) < TTY_THRESHOLD_THROTTLE) { 如果剩余空间小于128字节
    		/* check TTY_THROTTLED first so it indicates our state */
    		if (!test_and_set_bit(TTY_THROTTLED, &tty->flags) &&
    		    tty->driver.throttle)
    			tty->driver.throttle(tty); 进行节流处理
    	}
    }
    static inline void n_tty_receive_break(struct tty_struct *tty)
    {
    	if (I_IGNBRK(tty)) 如果忽略中断字符
    		return;
    	if (I_BRKINT(tty)) { 如果中断字符产生中信号
    		isig(SIGINT, tty, 1); 给终端所在的进程组发送INT信号
    		return;
    	}
    	if (I_PARMRK(tty)) { 断点标记
    		put_tty_queue('\377', tty);
    		put_tty_queue('\0', tty);
    	}
    	put_tty_queue('\0', tty);
    	wake_up_interruptible(&tty->read_wait);
    }
    
    static inline void n_tty_receive_overrun(struct tty_struct *tty)
    {
    	char buf[64];
    
    	tty->num_overrun++;
    	if (time_before(tty->overrun_time, jiffies - HZ)) {
    		printk("%s: %d input overrun(s)\n", tty_name(tty, buf),
    		       tty->num_overrun);
    		tty->overrun_time = jiffies;
    		tty->num_overrun = 0;
    	}
    }
    
    static inline void n_tty_receive_parity_error(struct tty_struct *tty,
    					      unsigned char c)
    {
    	if (I_IGNPAR(tty)) {
    		return;
    	}
    	if (I_PARMRK(tty)) {
    		put_tty_queue('\377', tty);
    		put_tty_queue('\0', tty);
    		put_tty_queue(c, tty);
    	} else	if (I_INPCK(tty)) 允许奇偶校验
    		put_tty_queue('\0', tty);
    	else
    		put_tty_queue(c, tty);
    	wake_up_interruptible(&tty->read_wait);
    }
    
    static int n_tty_receive_room(struct tty_struct *tty)
    {
    	int	left = N_TTY_BUF_SIZE - tty->read_cnt - 1;
    
    	/*
    	 * If we are doing input canonicalization, and there are no
    	 * pending newlines, let characters through without limit, so
    	 * that erase characters will be handled.  Other excess
    	 * characters will be beeped.
    	 */
    	if (tty->icanon && !tty->canon_data)
    		return N_TTY_BUF_SIZE;
    
    	if (left > 0)
    		return left;
    	return 0;
    }
    static inline void put_tty_queue_nolock(unsigned char c, struct tty_struct *tty)
    {
    	if (tty->read_cnt < N_TTY_BUF_SIZE) {
    		tty->read_buf[tty->read_head] = c;
    		tty->read_head = (tty->read_head + 1) & (N_TTY_BUF_SIZE-1);
    		tty->read_cnt++;
    	}
    }
    static inline void n_tty_receive_char(struct tty_struct *tty, unsigned char c)
    {
    	unsigned long flags;
    
    	if (tty->raw) {
    		put_tty_queue(c, tty);
    		return;
    	}
    	
    	if (tty->stopped && !tty->flow_stopped &&
    	    I_IXON(tty) && I_IXANY(tty)) { 如果软件流控制
    		start_tty(tty);
    		return;
    	}
    	
    	if (I_ISTRIP(tty)) 如果去除字符最高位
    		c &= 0x7f;
    	if (I_IUCLC(tty) && L_IEXTEN(tty)) 如果允许小写转换扩展模式
    		c=tolower(c);
    
    	if (tty->closing) { 如果正在关闭终端
    		if (I_IXON(tty)) {
    			if (c == START_CHAR(tty))
    				start_tty(tty);
    			else if (c == STOP_CHAR(tty))
    				stop_tty(tty);
    		}
    		return;
    	}
    
    	/*
    	 * If the previous character was LNEXT, or we know that this
    	 * character is not one of the characters that we'll have to
    	 * handle specially, do shortcut processing to speed things
    	 * up.
    	 */
    	if (!test_bit(c, &tty->process_char_map) || tty->lnext) {
    		finish_erasing(tty);
    		tty->lnext = 0;
    		if (L_ECHO(tty)) { 如果输入字符回显
    			if (tty->read_cnt >= N_TTY_BUF_SIZE-1) { 如果输入缓冲区满
    				put_char('\a', tty); /* beep if no space */ 向显示终端发送响铃字符
    				return;
    			}
    			/* Record the column of first canon char. */
    			if (tty->canon_head == tty->read_head)
    				tty->canon_column = tty->column;
    			echo_char(c, tty);
    		}
    		if (I_PARMRK(tty) && c == (unsigned char) '\377')
    			put_tty_queue(c, tty);
    		put_tty_queue(c, tty);
    		return;
    	}
    		
    	if (c == '\r') {
    		if (I_IGNCR(tty)) 忽略回车符
    			return;
    		if (I_ICRNL(tty)) 回车转换为换行符
    			c = '\n';
    	} else if (c == '\n' && I_INLCR(tty)) 换行符转换成回车符
    		c = '\r';
    	if (I_IXON(tty)) { 如果为软件流控
    		if (c == START_CHAR(tty)) { 如果为启动字符
    			start_tty(tty); 打开终端
    			return;
    		}
    		if (c == STOP_CHAR(tty)) { 如果为停止字符
    			stop_tty(tty); 暂停终端
    			return;
    		}
    	}
    	if (L_ISIG(tty)) { 如果允许触发信号
    		int signal;
    		signal = SIGINT;
    		if (c == INTR_CHAR(tty)) 如果为中断字符
    			goto send_signal;
    		signal = SIGQUIT;
    		if (c == QUIT_CHAR(tty)) 如果为退出字符
    			goto send_signal;
    		signal = SIGTSTP;
    		if (c == SUSP_CHAR(tty)) { 如果为暂停字符
    send_signal:
    			isig(signal, tty, 0); 向终端所在的进程组发送相应的信号
    			return;
    		}
    	}
    	if (tty->icanon) { 
    		if (c == ERASE_CHAR(tty) || c == KILL_CHAR(tty) ||
    		    (c == WERASE_CHAR(tty) && L_IEXTEN(tty))) { 
    			eraser(c, tty);
    			return;
    		}
    		if (c == LNEXT_CHAR(tty) && L_IEXTEN(tty)) {
    			tty->lnext = 1;
    			if (L_ECHO(tty)) {
    				finish_erasing(tty);
    				if (L_ECHOCTL(tty)) {
    					put_char('^', tty);
    					put_char('\b', tty);
    				}
    			}
    			return;
    		}
    		if (c == REPRINT_CHAR(tty) && L_ECHO(tty) &&
    		    L_IEXTEN(tty)) {
    			unsigned long tail = tty->canon_head;
    
    			finish_erasing(tty);
    			echo_char(c, tty);
    			opost('\n', tty);
    			while (tail != tty->read_head) {
    				echo_char(tty->read_buf[tail], tty);
    				tail = (tail+1) & (N_TTY_BUF_SIZE-1);
    			}
    			return;
    		}
    		if (c == '\n') {
    			if (L_ECHO(tty) || L_ECHONL(tty)) {
    				if (tty->read_cnt >= N_TTY_BUF_SIZE-1) {
    					put_char('\a', tty);
    					return;
    				}
    				opost('\n', tty);
    			}
    			goto handle_newline;
    		}
    		if (c == EOF_CHAR(tty)) {
    		        if (tty->canon_head != tty->read_head)
    			        set_bit(TTY_PUSH, &tty->flags);
    			c = __DISABLED_CHAR;
    			goto handle_newline;
    		}
    		if ((c == EOL_CHAR(tty)) ||
    		    (c == EOL2_CHAR(tty) && L_IEXTEN(tty))) {
    			/*
    			 * XXX are EOL_CHAR and EOL2_CHAR echoed?!?
    			 */
    			if (L_ECHO(tty)) {
    				if (tty->read_cnt >= N_TTY_BUF_SIZE-1) {
    					put_char('\a', tty);
    					return;
    				}
    				/* Record the column of first canon char. */
    				if (tty->canon_head == tty->read_head)
    					tty->canon_column = tty->column;
    				echo_char(c, tty);
    			}
    			/*
    			 * XXX does PARMRK doubling happen for
    			 * EOL_CHAR and EOL2_CHAR?
    			 */
    			if (I_PARMRK(tty) && c == (unsigned char) '\377')
    				put_tty_queue(c, tty);
    
    		handle_newline:
    			spin_lock_irqsave(&tty->read_lock, flags);
    			set_bit(tty->read_head, &tty->read_flags);
    			put_tty_queue_nolock(c, tty);
    			tty->canon_head = tty->read_head;
    			tty->canon_data++;
    			spin_unlock_irqrestore(&tty->read_lock, flags);
    			kill_fasync(&tty->fasync, SIGIO, POLL_IN);
    			if (waitqueue_active(&tty->read_wait))
    				wake_up_interruptible(&tty->read_wait);
    			return;
    		}
    	}
    	
    	finish_erasing(tty);
    	if (L_ECHO(tty)) {
    		if (tty->read_cnt >= N_TTY_BUF_SIZE-1) {
    			put_char('\a', tty); /* beep if no space */
    			return;
    		}
    		if (c == '\n')
    			opost('\n', tty);
    		else {
    			/* Record the column of first canon char. */
    			if (tty->canon_head == tty->read_head)
    				tty->canon_column = tty->column;
    			echo_char(c, tty);
    		}
    	}
    
    	if (I_PARMRK(tty) && c == (unsigned char) '\377')
    		put_tty_queue(c, tty);
    
    	put_tty_queue(c, tty);
    }	
    static inline void put_char(unsigned char c, struct tty_struct *tty)
    {
    	tty->driver.put_char(tty, c);
    }
    
    /* Must be called only when L_ECHO(tty) is true. */
    
    static void echo_char(unsigned char c, struct tty_struct *tty)
    {
    	if (L_ECHOCTL(tty) && iscntrl(c) && c != '\t') {
    		put_char('^', tty); 回显控制字符
    		put_char(c ^ 0100, tty);
    		tty->column += 2;
    	} else
    		opost(c, tty); 回显字符
    }
    
    static inline void finish_erasing(struct tty_struct *tty)
    {
    	if (tty->erasing) {
    		put_char('/', tty);
    		tty->column += 2;
    		tty->erasing = 0;
    	}
    }
    static inline void isig(int sig, struct tty_struct *tty, int flush)
    {
    	if (tty->pgrp > 0)
    		kill_pg(tty->pgrp, sig, 1);
    	if (flush || !L_NOFLSH(tty)) {
    		n_tty_flush_buffer(tty);
    		if (tty->driver.flush_buffer)
    			tty->driver.flush_buffer(tty);
    	}
    }
    static void eraser(unsigned char c, struct tty_struct *tty)
    {
    	enum { ERASE, WERASE, KILL } kill_type;
    	int head, seen_alnums;
    	unsigned long flags;
    
    	if (tty->read_head == tty->canon_head) { 如果输入为空
    		/* opost('\a', tty); */		/* what do you think? */
    		return;
    	}
    	if (c == ERASE_CHAR(tty))
    		kill_type = ERASE;
    	else if (c == WERASE_CHAR(tty))
    		kill_type = WERASE;
    	else { 
    		if (!L_ECHO(tty)) { 如果禁止回显
    			spin_lock_irqsave(&tty->read_lock, flags);
    			tty->read_cnt -= ((tty->read_head - tty->canon_head) &
    					  (N_TTY_BUF_SIZE - 1));
    			tty->read_head = tty->canon_head;
    			spin_unlock_irqrestore(&tty->read_lock, flags);
    			return;
    		}
    		if (!L_ECHOK(tty) || !L_ECHOKE(tty) || !L_ECHOE(tty)) {
    			spin_lock_irqsave(&tty->read_lock, flags);
    			tty->read_cnt -= ((tty->read_head - tty->canon_head) &
    					  (N_TTY_BUF_SIZE - 1));
    			tty->read_head = tty->canon_head;
    			spin_unlock_irqrestore(&tty->read_lock, flags);
    			finish_erasing(tty);
    			echo_char(KILL_CHAR(tty), tty);
    			/* Add a newline if ECHOK is on and ECHOKE is off. */
    			if (L_ECHOK(tty))
    				opost('\n', tty);
    			return;
    		}
    		kill_type = KILL;
    	}
    
    	seen_alnums = 0;
    	while (tty->read_head != tty->canon_head) {
    		head = (tty->read_head - 1) & (N_TTY_BUF_SIZE-1); 
    		c = tty->read_buf[head];
    		if (kill_type == WERASE) {
    			/* Equivalent to BSD's ALTWERASE. */
    			if (isalnum(c) || c == '_')
    				seen_alnums++;
    			else if (seen_alnums)
    				break;
    		}
    		spin_lock_irqsave(&tty->read_lock, flags);
    		tty->read_head = head;
    		tty->read_cnt--;
    		spin_unlock_irqrestore(&tty->read_lock, flags);
    		if (L_ECHO(tty)) {
    			if (L_ECHOPRT(tty)) {
    				if (!tty->erasing) {
    					put_char('\\', tty);
    					tty->column++;
    					tty->erasing = 1;
    				}
    				echo_char(c, tty);
    			} else if (kill_type == ERASE && !L_ECHOE(tty)) {
    				echo_char(ERASE_CHAR(tty), tty);
    			} else if (c == '\t') {
    				unsigned int col = tty->canon_column;
    				unsigned long tail = tty->canon_head;
    
    				/* Find the column of the last char. */
    				while (tail != tty->read_head) {
    					c = tty->read_buf[tail];
    					if (c == '\t')
    						col = (col | 7) + 1;
    					else if (iscntrl(c)) {
    						if (L_ECHOCTL(tty))
    							col += 2;
    					} else
    						col++;
    					tail = (tail+1) & (N_TTY_BUF_SIZE-1);
    				}
    
    				/* should never happen */
    				if (tty->column > 0x80000000)
    					tty->column = 0; 
    
    				/* Now backup to that column. */
    				while (tty->column > col) {
    					/* Can't use opost here. */
    					put_char('\b', tty);
    					if (tty->column > 0)
    						tty->column--;
    				}
    			} else {
    				if (iscntrl(c) && L_ECHOCTL(tty)) {
    					put_char('\b', tty);
    					put_char(' ', tty);
    					put_char('\b', tty);
    					if (tty->column > 0)
    						tty->column--;
    				}
    				if (!iscntrl(c) || L_ECHOCTL(tty)) {
    					put_char('\b', tty);
    					put_char(' ', tty);
    					put_char('\b', tty);
    					if (tty->column > 0)
    						tty->column--;
    				}
    			}
    		}
    		if (kill_type == ERASE)
    			break;
    	}
    	if (tty->read_head == tty->canon_head)
    		finish_erasing(tty);
    }
    /*
     * Perform OPOST processing.  Returns -1 when the output device is
     * full and the character must be retried.
     */
    static int opost(unsigned char c, struct tty_struct *tty) 输出预处理
    {
    	int	space, spaces;
    
    	space = tty->driver.write_room(tty); 取输出设备剩余缓冲区空间
    	if (!space)
    		return -1;
    
    	if (O_OPOST(tty)) { 如果允许输出预处理
    		switch (c) {
    		case '\n':
    			if (O_ONLRET(tty)) 换行时是否回车
    				tty->column = 0; 
    			if (O_ONLCR(tty)) {
    				if (space < 2)
    					return -1;
    				tty->driver.put_char(tty, '\r');
    				tty->column = 0;
    			}
    			tty->canon_column = tty->column;
    			break;
    		case '\r':
    			if (O_ONOCR(tty) && tty->column == 0) 禁止重复输出回车符
    				return 0;
    			if (O_OCRNL(tty)) { 回车符转换为换行符
    				c = '\n';
    				if (O_ONLRET(tty))
    					tty->canon_column = tty->column = 0;
    				break;
    			}
    			tty->canon_column = tty->column = 0;
    			break;
    		case '\t':
    			spaces = 8 - (tty->column & 7);
    			if (O_TABDLY(tty) == XTABS) {
    				if (space < spaces)
    					return -1;
    				tty->column += spaces;
    				tty->driver.write(tty, 0, "        ", spaces); 
    				return 0;
    			}
    			tty->column += spaces;
    			break;
    		case '\b':
    			if (tty->column > 0)
    				tty->column--; 退格
    			break;
    		default:
    			if (O_OLCUC(tty)) 小写转换成大写
    				c = toupper(c); 
    			if (!iscntrl(c)) 如果不是控制符
    				tty->column++;
    			break;
    		}
    	}
    	tty->driver.put_char(tty, c);
    	return 0;
    }
    
    /*
     * Reset the read buffer counters, clear the flags, 
     * and make sure the driver is unthrottled. Called
     * from n_tty_open() and n_tty_flush_buffer().
     */
    static void reset_buffer_flags(struct tty_struct *tty)
    {
    	unsigned long flags;
    
    	spin_lock_irqsave(&tty->read_lock, flags);
    	tty->read_head = tty->read_tail = tty->read_cnt = 0;
    	spin_unlock_irqrestore(&tty->read_lock, flags);
    	tty->canon_head = tty->canon_data = tty->erasing = 0;
    	memset(&tty->read_flags, 0, sizeof tty->read_flags);
    	check_unthrottle(tty);
    }
    
    static int n_tty_open(struct tty_struct *tty)
    {
    	if (!tty)
    		return -EINVAL;
    
    	if (!tty->read_buf) {
    		tty->read_buf = alloc_buf();
    		if (!tty->read_buf)
    			return -ENOMEM;
    	}
    	memset(tty->read_buf, 0, N_TTY_BUF_SIZE);
    	reset_buffer_flags(tty);
    	tty->column = 0;
    	n_tty_set_termios(tty, 0);
    	tty->minimum_to_wake = 1;
    	tty->closing = 0;
    	return 0;
    }
    
    /*
     * This character is the same as _POSIX_VDISABLE: it cannot be used as
     * a c_cc[] character, but indicates that a particular special character
     * isn't in use (eg VINTR has no character etc)
     */
    #define __DISABLED_CHAR '\0'
    
    static void n_tty_set_termios(struct tty_struct *tty, struct termios * old)
    {
    	if (!tty)
    		return;
    	
    	tty->icanon = (L_ICANON(tty) != 0);
    	if (test_bit(TTY_HW_COOK_IN, &tty->flags)) {
    		tty->raw = 1;
    		tty->real_raw = 1;
    		return;
    	}
    	if (I_ISTRIP(tty) || I_IUCLC(tty) || I_IGNCR(tty) ||
    	    I_ICRNL(tty) || I_INLCR(tty) || L_ICANON(tty) ||
    	    I_IXON(tty) || L_ISIG(tty) || L_ECHO(tty) ||
    	    I_PARMRK(tty)) {
    		cli();
    		memset(tty->process_char_map, 0, 256/8);
    
    		if (I_IGNCR(tty) || I_ICRNL(tty))
    			set_bit('\r', &tty->process_char_map);
    		if (I_INLCR(tty))
    			set_bit('\n', &tty->process_char_map);
    
    		if (L_ICANON(tty)) {
    			set_bit(ERASE_CHAR(tty), &tty->process_char_map);
    			set_bit(KILL_CHAR(tty), &tty->process_char_map);
    			set_bit(EOF_CHAR(tty), &tty->process_char_map);
    			set_bit('\n', &tty->process_char_map);
    			set_bit(EOL_CHAR(tty), &tty->process_char_map);
    			if (L_IEXTEN(tty)) {
    				set_bit(WERASE_CHAR(tty),
    					&tty->process_char_map);
    				set_bit(LNEXT_CHAR(tty),
    					&tty->process_char_map);
    				set_bit(EOL2_CHAR(tty),
    					&tty->process_char_map);
    				if (L_ECHO(tty))
    					set_bit(REPRINT_CHAR(tty),
    						&tty->process_char_map);
    			}
    		}
    		if (I_IXON(tty)) {
    			set_bit(START_CHAR(tty), &tty->process_char_map);
    			set_bit(STOP_CHAR(tty), &tty->process_char_map);
    		}
    		if (L_ISIG(tty)) {
    			set_bit(INTR_CHAR(tty), &tty->process_char_map);
    			set_bit(QUIT_CHAR(tty), &tty->process_char_map);
    			set_bit(SUSP_CHAR(tty), &tty->process_char_map);
    		}
    		clear_bit(__DISABLED_CHAR, &tty->process_char_map);
    		sti();
    		tty->raw = 0;
    		tty->real_raw = 0;
    	} else {
    		tty->raw = 1;
    		if ((I_IGNBRK(tty) || (!I_BRKINT(tty) && !I_PARMRK(tty))) &&
    		    (I_IGNPAR(tty) || !I_INPCK(tty)) &&
    		    (tty->driver.flags & TTY_DRIVER_REAL_RAW))
    			tty->real_raw = 1;
    		else
    			tty->real_raw = 0;
    	}
    }