Added general yield()-hook for cooperative scheduling development (part 2)

cores/arduino/Arduino.h

@@ -15,7 +15,7 @@
 extern "C"{
 #endif
 
-#define yield()
+void yield(void);
 
 #define HIGH 0x1
 #define LOW  0x0

cores/arduino/hooks.c

@@ -0,0 +1,31 @@
+/*
+  Copyright (c) 2012 Arduino.  All right reserved.
+
+  This library is free software; you can redistribute it and/or
+  modify it under the terms of the GNU Lesser General Public
+  License as published by the Free Software Foundation; either
+  version 2.1 of the License, or (at your option) any later version.
+
+  This library is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  See the GNU Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public
+  License along with this library; if not, write to the Free Software
+  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
+*/
+
+/**
+ * Empty yield() hook.
+ *
+ * This function is intended to be used by library writers to build
+ * libraries or sketches that supports cooperative threads.
+ *
+ * Its defined as a weak symbol and it can be redefined to implement a
+ * real cooperative scheduler.
+ */
+static void __empty() {
+	// Empty
+}
+void yield(void) __attribute__ ((weak, alias("__empty")));

cores/arduino/wiring.c

@@ -111,6 +111,7 @@ void delay(unsigned long ms)
 	uint16_t start = (uint16_t)micros();
 
 	while (ms > 0) {
+		yield();
 		if (((uint16_t)micros() - start) >= 1000) {
 			ms--;
 			start += 1000;