How to identify shift/ctrl keys were pressed in the linux world (without using GL and X11/XKBlib.h) in C++?

Question

I used to do in windows:

SHORT key1 = ::GetKeyState(VK_LSHIFT)& 0xF0;

SHORT key2 = ::GetKeyState(VK_RSHIFT)& 0xF0;

SHORT key3 = ::GetKeyState(VK_LCONTROL)& 0xF0;

SHORT key4 = ::GetKeyState(VK_RCONTROL)& 0xF0;


bShift = (key1 != 0) || (key2 != 0);

bCtrl = (key3 != 0) || (key4 != 0);

How to detect if the shift/ctrl keys were pressed in the linux world? Without using GL(it can be pressed not in the graphic window..) or X11.. Is there something general to retrieve it directly form the OS?

Thanks, Vladimir.

score 1 · Answer 1 · edited May 23 '17 at 12:03

I would do this using Linux Input API. Take a look at Youarefunny's answer here to see how you can check current key state (pressed or released).

Such check may take noticeable amount of time especially if you need to call it very often. So once you determine the initial state you may monitor for changes by reading input events from device file, like this (I skipped error-checking for brevity):

#include <stdio.h>
#include <linux/input.h>

int main (int argc, char *argv[])
{
  struct input_event ev;
  FILE *kbd = fopen("/dev/input/by-id/usb-Dell_Dell_QuietKey_Keyboard-event-kbd", "r");
  while (fread(&ev, sizeof(ev), 1, kbd) == 1)
  {
    if (ev.type == EV_KEY && (ev.code == KEY_LEFTSHIFT || ev.code == KEY_RIGHTSHIFT))
    {
      switch (ev.value)
      {
        case 0: printf("Shift released\n"); break;
        case 1: printf("Shift pressed\n"); break;
        case 2: printf("Shift repeated\n"); break;
        default: break;
      }
    }
    // similarly for KEY_LEFTCTRL, KEY_RIGHTCTRL, etc.
  }
  fclose(kbd);
  return 0;
}

Note that reading /dev/input/* files will probably require root privileges (unless you run chmod before) since default access mode is 640 and the files belong to root.

How to identify shift/ctrl keys were pressed in the linux world (without using GL and X11/XKBlib.h) in C++?

1 Answers1