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[PATCH] Documentation: Explain a second alternative for multi-line macros.



  Add an explanation for defining multi-line macros using the ({ })
notation to CodingStyle.

Signed-off-by: Robert P. J. Day <rpjday@mindspring.com>

---

diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
index 9069189..1d0ddb8 100644
--- a/Documentation/CodingStyle
+++ b/Documentation/CodingStyle
@@ -549,13 +549,24 @@ may be named in lower case.

 Generally, inline functions are preferable to macros resembling functions.

-Macros with multiple statements should be enclosed in a do - while block:
+There are two techniques for defining macros that contain multiple
+statements.

-#define macrofun(a, b, c) 			\
-	do {					\
+ (a) Enclose those statements in a do - while block:
+
+	#define macrofun(a, b, c) 		\
+		do {				\
+			if (a == 5)		\
+				do_this(b, c);	\
+		} while (0)
+
+ (b) Use the gcc extension that a compound statement enclosed in
+     parentheses represents an expression:
+
+	#define macrofun(a, b, c) ({		\
 		if (a == 5)			\
 			do_this(b, c);		\
-	} while (0)
+	})

 Things to avoid when using macros:

-
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