[PATCH] Documentation/CodingStyle: Mention multi-line macros using expressions

["Robert P. J. Day" <rpjday@crashcourse.ca>]

Since defining multi-line macros using statements and declarations in
expressions is fairly common in the kernel, add this to CodingStyle.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>

---
diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
index cb9258b..7eb0734 100644
--- a/Documentation/CodingStyle
+++ b/Documentation/CodingStyle
@@ -600,7 +600,8 @@ 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:
+Macros with multiple statements can be defined in one of two ways.  The
+earlier technique enclosed the macro in a do - while block, as in:

 #define macrofun(a, b, c) 			\
 	do {					\
@@ -608,6 +609,17 @@ Macros with multiple statements should be enclosed in a do - while block:
 			do_this(b, c);		\
 	} while (0)

+A newer technique is to use the GCC extension of being able to place
+statements and declarations in an expression, as with this example from
+the <linux/kernel.h> header file:
+
+#define roundup(x, y) (				\
+{						\
+        const typeof(y) __y = y;		\
+        (((x) + (__y - 1)) / __y) * __y;	\
+}						\
+)
+
 Things to avoid when using macros:

 1) macros that affect control flow:


-- 

========================================================================
Robert P. J. Day                                 Ottawa, Ontario, CANADA
                        http://crashcourse.ca

Twitter:                                       http://twitter.com/rpjday
LinkedIn:                               http://ca.linkedin.com/in/rpjday
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