Check Keypress in C++ on Linux

Is there a way to detect if a key has been pressed?

I use the following function for kbhit(). It works fine on g++ compiler in Ubuntu.

#include <termios.h>
#include <unistd.h>
#include <fcntl.h>
int kbhit(void)
{
  struct termios oldt, newt;
  int ch;
  int oldf;

  tcgetattr(STDIN_FILENO, &oldt);
  newt = oldt;
  newt.c_lflag &= ~(ICANON | ECHO);
  tcsetattr(STDIN_FILENO, TCSANOW, &newt);
  oldf = fcntl(STDIN_FILENO, F_GETFL, 0);
  fcntl(STDIN_FILENO, F_SETFL, oldf | O_NONBLOCK);

  ch = getchar();

  tcsetattr(STDIN_FILENO, TCSANOW, &oldt);
  fcntl(STDIN_FILENO, F_SETFL, oldf);

  if(ch != EOF)
  {
    ungetc(ch, stdin);
    return 1;
  }

  return 0;
}

Detecting keyboard key press and release on Linux

Your kbhit (and, apparently, even Windows' original kbhit) doesn't detect whether a key is pressed, but only whether there is something new to read on stdin. This will only be the case 25 times a second or so, depending on your autorepeat setting. Making stdout unbuffered in your example code will make that more obvious (000000W00000000W000000000W)

How can I fix this so that when a keyboard key is kept pressed, kbhit() always returns 0?

This is impossible to do portably. In Linux, it can be done using the device files under /dev/input
See the example program below. It will register all keys, even a lone SHIFT Note that this is effectively a keylogger, so you will have to run as root (or make the program setuid). It will then register all keystrokes, even when it doesn't have keyboard focus. Yikes!

Note: the example below is based on keystate.c by Kevin Cox

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <sysexits.h>
#include <glob.h>
#include <linux/input.h>
#include <sys/stat.h>
#include <fcntl.h>

/* Return -1 if no key is being pressed, or else the lowest keycode 
   (c.f. linux/input-event-codes.h) of all the keys that are being pressed */
int keycode_of_key_being_pressed() { 
  FILE *kbd;
  glob_t kbddev;                                   // Glob structure for keyboard devices
  glob("/dev/input/by-path/*-kbd", 0, 0, &kbddev); // Glob select all keyboards
  int keycode = -1;                                // keycode of key being pressed
  for (int i = 0; i < kbddev.gl_pathc ; i++ ) {    // Loop through all the keyboard devices ...
    if (!(kbd = fopen(kbddev.gl_pathv[i], "r"))) { // ... and open them in turn (slow!)
      perror("Run as root to read keyboard devices"); 
      exit(1);      
    }

    char key_map[KEY_MAX/8 + 1];          // Create a bit array the size of the number of keys
    memset(key_map, 0, sizeof(key_map));  // Fill keymap[] with zero's
    ioctl(fileno(kbd), EVIOCGKEY(sizeof(key_map)), key_map); // Read keyboard state into keymap[]
    for (int k = 0; k < KEY_MAX/8 + 1 && keycode < 0; k++) { // scan bytes in key_map[] from left to right
      for (int j = 0; j <8 ; j++) {       // scan each byte from lsb to msb
        if (key_map[k] & (1 << j)) {      // if this bit is set: key was being pressed
          keycode = 8*k + j ;             // calculate corresponding keycode 
          break;                          // don't scan for any other keys
        }
      }   
    }

    fclose(kbd);
    if (keycode)
      break;                              // don't scan for any other keyboards
  }
  return keycode;
}

void main()
{
  setvbuf(stdout, NULL, _IONBF, 0); // Set stdout unbuffered
  while (1)    {
    int key = keycode_of_key_being_pressed();
    printf((key < 0 ?  "no key\n" : "keycode: %d\n"), key);
    if (key == KEY_X)
      exit(0);
  }
}       

How to handle key pressed in a Linux console in C?

getch() from Curses library perhaps? Also, you will need to use notimeout() to tell getch() not to wait for next keypress.

c programming check if key pressed without stopping program

You can use kbhit() to check if a key is pressed:

#include <stdio.h>
#include <conio.h> /* getch() and kbhit() */

int
main()
{
    char c;

    for(;;){
        printf("hello\n");
        if(kbhit()){
            c = getch();
            printf("%c\n", c);
        }
    }
    return 0;
}

More info here: http://www.programmingsimplified.com/c/conio.h/kbhit

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