[PATCH] Documentation/CodingStyle: Explain using GCC's statement expression extension for defining macros

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

Add an explanation of how to use GCC's statement expression extension
for defining multi-line macros.

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

---

diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
index cb9258b..c0eec33 100644
--- a/Documentation/CodingStyle
+++ b/Documentation/CodingStyle
@@ -600,7 +600,9 @@ 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
+original technique encloses the body of the macro in a do - while block,
+as in:

 #define macrofun(a, b, c) 			\
 	do {					\
@@ -608,6 +610,17 @@ Macros with multiple statements should be enclosed in a do - while block:
 			do_this(b, c);		\
 	} while (0)

+An alternative technique is to use GCC's statement expression extension,
+explained at http://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html,
+which allows you to place statements and declarations in a single expression,
+as with this example from <linux/kernel.h>:
+
+#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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