linux X11/Xlib.h

#include <stdio.h>

#include <X11/Xlib.h>

int main()

{

  Display *d;

  Window inwin;      /* root window the pointer is in */

  Window inchildwin; /* child win the pointer is in */

  int rootx, rooty; /* relative to the "root" window; we are not interested in these,

                       but we can't pass NULL */

  int childx, childy;  /* the values we are interested in */

  Atom atom_type_prop; /* not interested */

  int actual_format;   /* should be 32 after the call */

  unsigned int mask;   /* status of the buttons */

  unsigned long n_items, bytes_after_ret;

  Window *props; /* since we are interested just in the first value, which is

        a Window id */

  /* default DISPLAY */

  d = XOpenDisplay(NULL); 

  /* ask for active window (no error check); the client must be freedesktop

     compliant */

  (void)XGetWindowProperty(d, DefaultRootWindow(d),

         XInternAtom(d, "_NET_ACTIVE_WINDOW", True),

         0, 1, False, AnyPropertyType,

         &atom_type_prop, &actual_format,

         &n_items, &bytes_after_ret, (unsigned char**)&props);

  XQueryPointer(d, props[0], &inwin,  &inchildwin,

    &rootx, &rooty, &childx, &childy, &mask);

  printf("relative to active window: %d,%d\n", childx, childy);

  XFree(props);           /* free mem */

  (void)XCloseDisplay(d); /* and close the display */

  return 0;

}