1、有些程序处理从特定设备来的数据。这些与特定设备相关的程序必须控制与设备的连接。Unix系统中最常见的设备是终端。

2、终端驱动程序有很多设置。各个设置的特定值决定了终端驱动程序的模式。为用户编写的程序通常需要设置终端驱动程序为特定的模式。

3、键盘输入分为3类，终端驱动程序对这些输入做不同的处理。大多数建代表常规数据，它们从驱动程序传输到程序。有些键调用驱动程序中的编辑函数。如果按下删除键，驱动程序将前一个字符从它的行缓冲中删除，并将命令发送到终端屏幕，使之从显示器中删除字符。最后，有些键调用处理控制函数。Ctrl-C键告诉驱动程序调用内核中某个函数，这个函数给进程发送一个信号。终端驱动程序支持若干种处理控制函数，它们都通过发送信号到进程来实现控制。

man 2 signal

4、信号是从内核发送给进程的一种简短消息。信号可能来自用户、其他进程或内核本身。进程可以告诉内涵，在它收到信号时需要做出怎样的响应。

5、

man 2 fcntl

#include <unistd.h>
#include <fcntl.h>

int fcntl(int fd, int cmd, ... /* arg */);

man 2 signal

#include <signal.h>
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
    /* 
     * signal() returns the previous value of the signal handler,
     * or SIG_ERR on error. In the event of an error,
     * errno is set to indicate the cause.
     */

/*
 * play_aganin04.c
 * purpose: ask if user wants another transaction
 * method: set tty into char-by-char mode and no echo mode,
 *         set tty into no-delay mode
 *         read char, return result
 *         resets terminal modes on SIGINT, ignores SIGQUIT
 * returns: 0->yes, 1->no, 2->timeout
 * better: reset terminal mode on Interrupt
 */

#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
#include <fcntl.h>
#include <string.h>
#include <signal.h>

#define ASK "Do you want another transaction"
#define TRIES (3)           /* max tries */
#define SLEEPTIME (2)       /* time per try */
#define BEEP putchar(‘\a‘)  /* alert user */

int get_response(char *, int);
char get_ok_char(void);
void tty_mode(int);
//void set_crmode(void);
void set_cr_noecho_mode(void);
void set_nodelay_mode(void);

int main(void)
{
    int response;
    void ctrl_c_handler(int);

    tty_mode(0);                    /* save tty mode */
    //set_crmode();                 /* set char-by-char mode */
    set_cr_noecho_mode();
    set_nodelay_mode();
    signal(SIGINT, ctrl_c_handler); /* handle INT */
    signal(SIGQUIT, SIG_IGN);       /* ignore QUIT signals */
    response = get_response(ASK, TRIES);
    tty_mode(1);                    /* restore tty mode */
    printf("\n");
    return response;
}

int get_response(char *question, int maxtries)
/*
 * purpose: ask a question and wait for a y/n answer or maxtries
 * method: use a getchar and ignore non y/n answers
 * returns: 0->yes, 1->no
 */
{
    int input;

    printf("%s (y/n)?", question);
    fflush(stdout);
    while (1)
    {
        sleep(SLEEPTIME);
        input = tolower(get_ok_char());
        if (input == ‘y‘)
            return 0;
        if (input == ‘n‘)
            return 1;
        if (maxtries-- == 0)
            return 2;
        BEEP;
    }
}

/*
 * skip over non-legal chars and return y, Y, n, N or EOF
 */
char get_ok_char(void)
{
    char c;
    while ((c = getchar()) != EOF && strchr("yYnN", c) == NULL)
        ;
    return c;
}

void set_cr_noecho_mode(void)
/*
 * purpose: put file descriptor 0 into char-by-char and noecho mode
 * method: use bits in termios
 */
{
    struct termios ttystate;

    tcgetattr(0, &ttystate);
    ttystate.c_lflag &= ~ICANON;        /* no buffering */
    ttystate.c_lflag &= ~ECHO;          /* no echo either */
    ttystate.c_cc[VMIN] = 1;            /* get 1 char at a time */
    tcsetattr(0, TCSANOW, &ttystate);   /* install settings */
}

void set_nodelay_mode(void)
/*
 * purpose: put file descriptor 0 into no_delay mode
 * method: use fcntl to set bits
 * notes: tcsetattr() will do something similar, but it is complicated
 */
{
    int termflags;
    termflags = fcntl(0, F_GETFL);      /* read curr settings */
    termflags |= O_NDELAY;              /* flip on nodelay bit */
    fcntl(0, F_SETFL, termflags);       /* and install them */
}

/* how == 0 -> save current mode, how == 1 -> restore mode */
/* this version handles termios and fcntl flags */
void tty_mode(int how)
{
    static struct termios original_mode;
    static int original_flags;
    static int stored = 0;
    if (how == 0)
    {
        tcgetattr(0, &original_mode);
        original_flags = fcntl(0, F_GETFL);
        stored = 1;
    }
    else
    {
        tcsetattr(0, TCSANOW, &original_mode);
        fcntl(0, F_SETFL, original_flags);
    }
}

void ctrl_c_handler(int signum)
/*
 * purpose: called if SIGINT is detected
 * action: reset tty and scram
 */
{
    tty_mode(1);
    printf("\nCtrl-C pressed.\n");
    exit(1);
}