Why does `ioctl(fd, EVIOCGRAB, 1)` cause key spam sometimes?

Question

I'm trying to write my own "keyboard driver" (without actually writing a kernel module), by grabbing the keyboard at what I assume is the lowest level of abstraction in userland: /dev/input/event*.

The following code does the grabbing, provided you change the first ocurrence of ioctl(fd, EVIOCGRAB, UNGRAB) to ioctl(fd, EVIOCGRAB, GRAB).

// gcc main.c -o main

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <linux/input.h>
#include <fcntl.h>
#include <errno.h>

#define EXIT_KEY  KEY_ESC
#define UNGRAB    0
#define GRAB      1

const char* kbd_device = "/dev/input/event4";

// ------------------------------------------------------------------------------------------------
int main(void){
  int fd = open(kbd_device, O_RDONLY);
  if(fd == -1){
    printf("Cannot open %s. %s.\n", kbd_device, strerror(errno));
    return -1;
  }

  if(ioctl(fd, EVIOCGRAB, UNGRAB))
    printf("Couldn't grab %s. %s.\n", kbd_device, strerror(errno));
  else
    printf("Grabbed %s!\n", kbd_device);

  while(1){
    struct input_event event;
    read(fd, &event, sizeof(event));
    if (event.type == EV_KEY && event.value >= 0 && event.value <= 2){
      printf("%d %3d\n", event.value, event.code);

      if(event.code == EXIT_KEY){
        ioctl(fd, EVIOCGRAB, UNGRAB);
        close(fd);
        return 0;
      }

    }
  }
}

Problem

• If I run gcc main.c -o main && sudo ./main, everything works as expected.
• If first compile and then I run sudo ./main, however, the terminal scrolls down nonstop, as if the RETURN key was held down.

Why does happen?

Notes

• I'm running Ubuntu 14.04
• On my platform, /dev/input/event4 happens to be the keyboard

Motivation

I'm trying to write a keyboard "driver" that works both on X and not on X (eg. a TTY).

I understand X11's keyboard library/extension is XKB. I think the TTY's keyboard library is linux/divers/tty/vt/keyboard.c (source), the initial keyboard map it uses is in linux/drivers/tty/vt/defkeymap.map (source), and it can be modified by using loadkeys (source here). Do correct me if I'm wrong.

Answer 1

When you type

gcc main.c -o main && sudo ./main ↵

GCC takes some time, so the ↵ key has been released by the time ./main runs.

When you type

sudo ./main ↵

the terminal sends the shell a newline as soon as you push down ↵, and starts executing ./main. Then the ↵ released event is seen by your program, but not by your terminal, because your program has grabbed the input device. Thus, to the terminal it looks like ↵ is stuck down, so it continues to produce newlines.