For a Rube Goldberg device I needed a way to know if a mouse button was pressed on the root window of an X11 screen. Here is the code for it:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <unistd.h>

#include <X11/Xlib.h>
#include <X11/Xutil.h>

int main() {

        Display *dsp = XOpenDisplay( NULL );
        if( !dsp ){ return 1; }

        int screenNumber = DefaultScreen(dsp);

        long eventMask = StructureNotifyMask;
        XSelectInput( dsp, DefaultRootWindow(dsp), eventMask );

        XEvent evt;

        eventMask = ButtonPressMask|ButtonReleaseMask;
        XSelectInput(dsp, DefaultRootWindow(dsp), eventMask); // override prev 

        do{
                XNextEvent( dsp, &evt );   // calls XFlush()
                if ( (evt.type == ButtonRelease) || (evt.type == ButtonPress) )
                exit(2);
        } while( 1 );

return 0;
}

Compile: gcc -lX11 -L/usr/local/include x11.c -o x11-mousebutton

Upon a mouse button being pressed it will exit 2, so using that return value in some script you can execute a command. For example, to open a CD rom drive so a domino falls over.