Get Keyboard Interrupt in C

Question

Program:

#include<stdio.h>
void main()
{
    int time=1800;
    while(1){
        system("clear");
        time-=1;
        printf("%d\n",time);
        sleep(1);
    if(time==0)
        pause();
    }
}

The above program stops when the time reaches 0. My requirement is during the runtime of the program, If I press any key like spacebar or any other key, the program gets paused and once again I press the key, the program gets resumed. So for doing this, before execution of while condition, we submit the signal handler for keyboard interrupt. In C how to do this.

What is the function used to get keyboard interrupt. I dont want to get input from the user, I want to handle the interrupt generated by the user through keyboard.

Thanks in Advance..,

Answer 1

The keyboard does not exist in purely standard C99 or C11 ( stdin is not a keyboard, and could be a pipe(7) so is not always a tty(4) ; you might read from /dev/tty ...).

So it is much less simple that what you want it to be, and it is operating system specific. I am focusing on Linux.

Read much more about ttys, notably read the tty demystified . Notice that tty are usually in cooked mode, where the kernel is buffering lines (in addition of stdin being line buffered).

The reasonable way is to use a terminal library like ncurses or readline . These libraries are setting the tty in raw mode (which you might do by yourself, see this and termios(3) ; you'll probably need also to poll(2) ). Be sure to exit properly and reset the tty to cooked mode before exiting.

But your life is probably too short to dive into termios(3) and tty_ioctl(4) so just use ncurses or readline

You could also consider some GUI application (eg above X11 or Wayland ). Then use a toolkit ( GTK , Qt , ...).

Answer 2

My requirement is during the runtime of the program, If I press any key like spacebar or any other key, the program gets paused and once again I press the key, the program gets resumed.

You can achieve this with this type of code

#include <stdio.h>
int main()
{
  char i;int y=0;
  while(1)
  {
     if(!(_kbhit()))
     {
       y=0;
       printf("//key is not pressed");
       //do what you want
     }
     else
     {
       printf("key pressed (execution pause) , press again to resume");
       i=_getch();
       y=1;
     }
     if(y==1)
     {
       getch();
     }
  }
  return 0;
}

Answer 3

You need conio.h for your requirement.It defines kbhit() and getch() both wait for input from keyboard.

Whenever kbhit() is called, it checks the keyboard buffer and returns a nonzero value if the buffer has any keypress otherwise 0 is returned.

The conio.h is used by MSDOS compilers and is not the part of standard C libraries (ISO). It is also not defined in POSIX.

#include<stdio.h>
#include<conio.h>

int main()
{
   while(1)
   {
       while(!kbhit())
       {
          //works continuously until interrupted by keyboard input.
          printf("M Tired. Break Me\n");
       }
       getch();
   }
   return 0;
}

For linux you may use the following snippet to implement kbhit() by using fnctl() from fnctl.h for signal handling:

  #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;
        }