From 69da7c5b15ad8b87c80ee2a220fa975a0c2571a5 Mon Sep 17 00:00:00 2001
From: Markus Hitter <mah@jump-ing.de>
Date: Sun, 21 Jul 2013 22:43:24 +0200
Subject: [PATCH] Introduce ATOMIC_START and ATOMIC_END.

As a sample application, use it in queue_empty().

There's also ATOMIC_BLOCK() coming with avr-libc, but this requires
a C99 compiler while Arduino IDE insists on running avr-gcc in C89 mode.
---
 dda_queue.c      | 11 ++++-------
 memory_barrier.h |  9 +++++++++
 2 files changed, 13 insertions(+), 7 deletions(-)

diff --git a/dda_queue.c b/dda_queue.c
index 098d985..4fda7e8 100644
--- a/dda_queue.c
+++ b/dda_queue.c
@@ -47,14 +47,11 @@ uint8_t queue_full() {
 
 /// check if the queue is completely empty
 uint8_t queue_empty() {
-	uint8_t save_reg = SREG;
-	cli();
-	CLI_SEI_BUG_MEMORY_BARRIER();
-	
-	uint8_t result = ((mb_tail == mb_head) && (movebuffer[mb_tail].live == 0))?255:0;
+  uint8_t result;
 
-	MEMORY_BARRIER();
-	SREG = save_reg;
+  ATOMIC_START
+    result = ((mb_tail == mb_head) && (movebuffer[mb_tail].live == 0))?255:0;
+  ATOMIC_END
 
 	return result;
 }
diff --git a/memory_barrier.h b/memory_barrier.h
index 8c55575..8aab3f6 100644
--- a/memory_barrier.h
+++ b/memory_barrier.h
@@ -28,4 +28,13 @@
 	#define CLI_SEI_BUG_MEMORY_BARRIER()
 #endif
 
+#define ATOMIC_START { \
+                       uint8_t save_reg = SREG; \
+                       cli(); \
+                       CLI_SEI_BUG_MEMORY_BARRIER();
+
+#define ATOMIC_END   MEMORY_BARRIER(); \
+                     SREG = save_reg; \
+                   }
+
 #endif