Index: src/threading.h
==================================================================
--- src/threading.h
+++ src/threading.h
@@ -51,11 +51,11 @@
 typedef CRITICAL_SECTION of_mutex_t;
 typedef struct {
 	HANDLE event;
 	int count;
 } of_condition_t;
-typedef volatile LONG of_once_t;
+typedef volatile int of_once_t;
 # define OF_ONCE_INIT 0
 #else
 # error No threads available!
 #endif
 

Index: src/threading.m
==================================================================
--- src/threading.m
+++ src/threading.m
@@ -23,10 +23,30 @@
 #elif defined(OF_WINDOWS)
 # include "threading_winapi.m"
 #else
 # error No threads available!
 #endif
+
+#ifndef OF_HAVE_PTHREADS
+void
+of_once(of_once_t *control, void (*func)(void))
+{
+	if (of_atomic_int_cmpswap(control, 0, 1)) {
+		func();
+		of_atomic_int_inc(control);
+	} else {
+		while (*control == 1)
+# if defined(HAVE_SCHED_YIELD)
+			sched_yield();
+# elif defined(OF_WINDOWS)
+			Sleep(0);
+# else
+			;
+# endif
+	}
+}
+#endif
 
 #if !defined(OF_HAVE_RECURSIVE_PTHREAD_MUTEXES) && !defined(OF_WINDOWS)
 bool
 of_rmutex_new(of_rmutex_t *rmutex)
 {

Index: src/threading_winapi.m
==================================================================
--- src/threading_winapi.m
+++ src/threading_winapi.m
@@ -88,25 +88,10 @@
 void
 of_thread_set_name(of_thread_t thread, const char *name)
 {
 }
 
-void
-of_once(of_once_t *control, void (*func)(void))
-{
-	switch (InterlockedCompareExchange(control, 1, 0)) {
-	case 0:
-		func();
-		InterlockedIncrement(control);
-		break;
-	case 1:
-		while (*control == 1)
-			Sleep(0);
-		break;
-	}
-}
-
 bool
 of_mutex_new(of_mutex_t *mutex)
 {
 	InitializeCriticalSection(mutex);