Getch() function in Linux¶

/*
 *  kbhit(), a keyboard lookahead monitor
 *  *getch*(), a blocking single character input from stdin
 *
 *  Plus a demo main() to illustrate usage.
 */

#include <stdio.h>
#include <unistd.h>
#include <termios.h>
#include <sys/ioctl.h>
#include <sys/time.h>
#include <sys/types.h>

int getch(void);
int kbhit(void);

#define CMIN  1

#ifdef CTIME
#undef CTIME
#endif

#define CTIME 1

/*
 *  kbhit()  --  a keyboard lookahead monitor
 *
 *  returns the number of characters available to read.
 */
int kbhit(void) {
  int                   cnt = 0;
  int                   error;
  static struct termios Otty, Ntty;

  tcgetattr(0, &Otty);
  Ntty = Otty;

  Ntty.c_iflag          = 0;       /* input mode                */
  Ntty.c_oflag          = 0;       /* output mode               */
  Ntty.c_lflag         &= ~ICANON; /* line mode                 */
  Ntty.c_cc[VMIN]       = CMIN;    /* minimum time to wait      */
  Ntty.c_cc[VTIME]      = CTIME;   /* minimum characters to wait for */

  if (0 == (error = tcsetattr(0, TCSANOW, &Ntty))) {
    struct timeval      tv;
    error     += ioctl(0, FIONREAD, &cnt);
    error     += tcsetattr(0, TCSANOW, &Otty);
    tv.tv_sec  = 0;
    tv.tv_usec = 100;
    select(1, NULL, NULL, NULL, &tv);  /* a small time delay */
  }

  return (error == 0 ? cnt : -1 );
}


/*
 *  *getch*()  --  a blocking single character input from stdin
 *
 *  Returns a character, or -1 if an input error occurs.
 *
 *
 *  Conditionals allow compiling with or without echoing of
 *  the input characters, and with or without flushing pre-existing
 *  existing  buffered input before blocking.
 *
 */
int getch(void) {
  char                  ch;
  int                   error;
  static struct termios Otty, Ntty;

  fflush(stdout);
  tcgetattr(0, &Otty);
  Ntty = Otty;

  Ntty.c_iflag  =  0;           /* input mode           */
  Ntty.c_oflag  =  0;           /* output mode          */
  Ntty.c_lflag &= ~ICANON;  /* line settings        */

#if 1
  /* disable echoing the char as it is typed */
  Ntty.c_lflag &= ~ECHO;    /* disable echo         */
#else
  /* enable echoing the char as it is typed */
  Ntty.c_lflag |=  ECHO;        /* enable echo          */
#endif

  Ntty.c_cc[VMIN]  = CMIN;      /* minimum time to wait */
  Ntty.c_cc[VTIME] = CTIME;     /* minimum chars to wait for */

#if 1
/*
 * use this to flush the input buffer before blocking for new input
 */
#define FLAG TCSAFLUSH
#else
/*
 * use this to return a char from the current input buffer, or block if
 * no input is waiting.
 */
#define FLAG TCSANOW

#endif

 if (0 == (error = tcsetattr(0, FLAG, &Ntty))) {
    error  = read(0, &ch, 1 );                /* get char from stdin */
    error += tcsetattr(0, FLAG, &Otty);   /* restore old settings */
  }

  return (error == 1 ? (int) ch : -1 );

}

/*
 * a cutsie main() to demo how *getch*() and kbhit() work.
 */
#include <ctype.h>

int main(void) {
  int ch, cnt;
  static struct termios Otty, Ntty;

  tcgetattr(0, &Otty);
  Ntty = Otty;
  Ntty.c_lflag &= ~ECHO;

  printf("You must enter 10 characters to get\nthis program to continue:");
  fflush(stdout);
  /* collect 10 characters */
  while (1) {
    if (10 <= (cnt = kbhit())) {
      break;
    }
  }

  tcsetattr(0, TCSANOW, &Ntty); /* disable echoing of further input */
  printf("\nSTOP!");
  fflush(stdout);
  printf("\nNow type <Enter> to continue!");
  fflush(stdout);
  ch = getchar();
  tcsetattr(0, TCSANOW, &Otty); /* enable echoing of further input */
  printf("\n\nThe first five characters are:  \"");
  /*
   * print a few chars, note that calling *getch*() will flush
   * remaining buffered input.
   */
  cnt = 4;
  do {
    printf("%c", ch);
    ch = getchar();
  } while  (--cnt);
  printf("%c\"\n\n", ch);

  printf("\n\n      ***  Demo Menu  ***\n\n");
  printf("      Option       Action\n");
  printf("        A          Exit and print an A word\n");
  printf("        B          Exit and print a  B word\n");
  printf("        C          Exit and print a  C word\n");
  printf("\n      Enter your choice:  ?\b");
  fflush(stdout);

  while(1) {
    switch (ch = getch()) {
    case 'a': case 'A':
      printf("%c\n\nThat is an *awesome* function!\n", toupper(ch));
      break;
    case 'b': case 'B':
      printf("%c\n\nThat is a *beneficial* function!\n", toupper(ch));
      break;
    case 'c': case 'C':
      printf("%c\n\nThat is a *critical* function!\n", toupper(ch));
      break;
    default:
      continue;
    }
    break;
  }
  return 0;

}