Started writing the same but for Mac OS X. This time it's not tested yet; will test now.

alloc_darwin.m

@@ -0,0 +1,31 @@
+// 4 december 2014
+#include "ui_darwin.h"
+
+void *uiAlloc(size_t size)
+{
+	void *out;
+
+	out = malloc(size);
+	NSAssert(out != NULL, @"out of memory in uiAlloc()");
+	memset(out, 0, size);
+	return out;
+}
+
+void *uiRealloc(void *p, size_t size)
+{
+	void *out;
+
+	if (p == NULL)
+		return uiAlloc(size);
+	out = realloc(p, size);
+	NSAssert(out != NULL, @"out of memory in uiRealloc()");
+	// TODO zero the extra memory
+	return out;
+}
+
+void uiFree(void *p)
+{
+	if (p == NULL)
+		return;
+	free(p);
+}

init_darwin.m

@@ -0,0 +1,38 @@
+// 6 april 2015
+#include "ui_darwin.m"
+
+@interface uiApplication : NSApplication
+@end
+
+@implementation uiApplication
+
+// hey look! we're overriding terminate:!
+// we're going to make sure we can go back to main() whether Cocoa likes it or not!
+// and just how are we going to do that, hm?
+// (note: this is called after applicationShouldTerminate:)
+- (void)terminate:(id)sender
+{
+	// yes that's right folks: DO ABSOLUTELY NOTHING.
+	// the magic is [NSApp run] will just... stop.
+}
+
+@end
+
+// TODO applicationShouldTerminateAfterLastWindowClosed
+
+uiInitError *uiInit(uiInitOptions *o)
+{
+	[uiApplication sharedApplication];
+	// don't check for a NO return; something (launch services?) causes running from application bundles to always return NO when asking to change activation policy, even if the change is to the same activation policy!
+	// see https://github.com/andlabs/ui/issues/6
+	[NSApp setActivationPolicy:NSApplicationActivationPolicyRegular];
+	return NULL;
+}
+
+const char *uiInitErrorMessage(uiInitError *err)
+{
+}
+
+void uiInitErrorFree(uiInitError *err)
+{
+}

main_darwin.m

@@ -0,0 +1,28 @@
+// 6 april 2015
+#include "ui_darwin.h"
+
+// TODO merge with init_darwin.m?
+
+void uiMain(void)
+{
+	[NSApp run];
+}
+
+void uiQuit(void)
+{
+	NSEvent *e;
+
+	[NSApp stop:NSApp];
+	// stop: won't register until another event has passed; let's synthesize one
+	e = [NSEvent otherEventWithType:NSApplicationDefined
+		location:NSZeroPoint
+		modifierFlags:0
+		timestamp:[[NSProcessInfo processInfo] systemUptime]
+		windowNumber:0
+		context:[NSGraphicsContext currentContext]
+		subtype:0
+		data1:0
+		data2:0];
+	[NSApp postEvent:e atStart:NO];		// let pending events take priority
+	// TODO really wait?
+}

ui_darwin.h

@@ -0,0 +1,16 @@
+// 6 january 2015
+
+#ifndef __UI_UI_DARWIN_H__
+#define __UI_UI_DARWIN_H__
+
+#define MAC_OS_X_VERSION_MIN_REQUIRED MAC_OS_X_VERSION_10_7
+#define MAC_OS_X_VERSION_MAX_ALLOWED MAC_OS_X_VERSION_10_7
+#import <Cocoa/Cocoa.h>
+#import "ui.h"
+
+// alloc_darwin.m
+extern void *uiAlloc(size_t);
+extern void *uiRealloc(void *, size_t);
+extern void uiFree(void *);
+
+#endif

window_darwin.m

@@ -0,0 +1,78 @@
+// 6 april 2015
+#include "ui_darwin.h"
+
+@interface uiWindowDelegate : NSWindowDelegate
+@property void (*onClosing)(uiWindow *, void *);
+@property void *onClosingData;
+@end
+
+@implementation uiWindowDelegate
+
+// TODO will this *destroy* the window?
+- (BOOL)windowShouldClose:(id)win
+{
+	// return exact constants to be safe
+	if ((*(self.onClosing))(self.onClosingData))
+		return YES;
+	return NO;
+}
+
+@end
+
+struct uiWindow {
+	NSWindow *w;
+	uiWindowDelegate *d;
+};
+
+static int defaultOnClosing(uiWindow *w, void *data)
+{
+	return 1;
+}
+
+uiWindow *uiNewWindow(char *title, int width, int height)
+{
+	uiWindow *w;
+
+	w = (uiWindow *) uiAlloc(sizeof (uiWindow));
+
+	w->w = [[NSWindow alloc] initWithContentRect:NSMakeRect(0, 0, (CGFloat) width, (CGFloat) height)
+		styleMask:(NSTitledWindowMask | NSClosableWindowMask | NSMiniaturizableWindowMask | NSResizableWindowMask)
+		backing:NSBackingStoreBuffered
+		defer:YES];
+	// TODO substitutions
+
+	w->d = [uiWindowDelegate new];
+	w->d.onClosing = defaultOnClosing;
+	[w->w setDelegate:w->d];
+
+	return w;
+}
+
+void uiWindowDestroy(uiWindow *w)
+{
+	// TODO
+	// TODO will w->d be destroyed?
+}
+
+uintptr_t uiWindowHandle(uiWindow *w)
+{
+	return (uintptr_t) (w->w);
+}
+
+// TODO titles
+
+void uiWindowShow(uiWindow *w)
+{
+	[w->w makeKeyAndOrderFront:w->w];
+}
+
+void uiWindowHide(uiWindow *w)
+{
+	[w->w orderOut:w->w];
+}
+
+void uiWindowOnClosing(uiWindow *w, int (*)(uiWindow *f, void *), void *data)
+{
+	w->d.onClosing = f;
+	w->d.onClosingData = data;
+}