This code uses CPU too much until I click the window.

#include <X11/Xlib.h>

static Bool check(Display *dpy, XEvent *ev, XPointer arg)
{
    return ev->type == ButtonPress;
}

int main(void)
{
    Display *dpy = XOpenDisplay(NULL);
    
    Window win = XCreateSimpleWindow(dpy, DefaultRootWindow(dpy), 0, 0, 100, 100, 0, 0, 0);
    
    XSelectInput(dpy, win, ExposureMask | ButtonPressMask);
    
    XMapWindow(dpy, win);
    
    XEvent ev;
    XIfEvent(dpy, &ev, check, NULL);
    
    return 0;
}

X11R7.2
slackware 11.0
linux kernel 2.6.20.6
glibc 2.5
gcc 4.1.2
ThinkPad X40