Add an explanation of how to use GCC's statement expression extension
for defining multi-line macros.
Signed-off-by: Robert P. J. Day <rpj...@crashcourse.ca>
---
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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