default.nix

with import <nixpkgs> {};

stdenv.mkDerivation {
  name = "x11idle";
  installPhase = ''
    mkdir -p $out/bin
    gcc -lX11 -lXss ${./x11idle.c} -o $out/bin/x11idle
  '';
  buildInputs = with xlibs; [ libXScrnSaver xproto libX11 ];
  unpackPhase = ":";
}

x11idle.c

#include <X11/extensions/scrnsaver.h>
#include <stdio.h>
 
/* Based on code from
 * http://coderrr.wordpress.com/2008/04/20/getting-idle-time-in-unix/
 *
 * compile with 'gcc -l Xss x11idle.c -o x11idle' and copy x11idle into your
 * path
 */
 
int main() {
    XScreenSaverInfo *info = XScreenSaverAllocInfo();
    //open the display specified by the DISPLAY environment variable
    Display *display = XOpenDisplay(0);
 
    //display could be null if there is no X server running
    if (info == NULL || display == NULL) {
    return -1;
    }
 
    //X11 is running, try to retrieve info
    if (XScreenSaverQueryInfo(display, DefaultRootWindow(display), info) == 0) {
    return -1;
    }
 
    //info was retrieved successfully, print idle time
    printf("%lu\n", info->idle);
    return 0;
}