Use accessibility to get the focused window of the front most app on OS X

get_focused_window.m

#import <AppKit/AppKit.h>
#import <Foundation/Foundation.h>
#include <CoreFoundation/CoreFoundation.h>

NSRunningApplication* frontApp = [[NSWorkspace sharedWorkspace] frontmostApplication];
pid_t pid = [frontApp processIdentifier];

AXUIElementRef app = AXUIElementCreateApplication(pid);

AXUIElementRef window;
AXUIElementCopyAttributeValue(app, kAXFocusedWindowAttribute, (CFTypeRef *)&window);