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

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Message-ID: <Pine.LNX.4.64.0612311513070.18796@localhost.localdomain>
Date:	Sun, 31 Dec 2006 15:13:59 -0500 (EST)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Muli Ben-Yehuda <muli@...ibm.com>
cc:	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	Randy Dunlap <randy.dunlap@...cle.com>, trivial@...nel.org
Subject: Re: [PATCH] Documentation: Explain a second alternative for multi-line
 macros.

On Sun, 31 Dec 2006, Muli Ben-Yehuda wrote:

> On Sun, Dec 31, 2006 at 02:49:48PM -0500, Robert P. J. Day wrote:
>
> > there would appear to be *lots* of cases where the ({ }) notation
> > is used when nothing is being returned.  i'm not sure you can be
> > that adamant about that distinction at this point.
>
> IMHO, the main point of CodingStyle is to clarify how new code
> should be written and old code should've been written.

ok, how about this as a patch:

diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
index 9069189..064a13e 100644
--- a/Documentation/CodingStyle
+++ b/Documentation/CodingStyle
@@ -549,13 +549,26 @@ 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:
-
-#define macrofun(a, b, c) 			\
-	do {					\
-		if (a == 5)			\
-			do_this(b, c);		\
-	} while (0)
+There are two techniques for defining macros that contain multiple
+statements, depending on whether you're returning a value or not:
+
+ (a) If there is no return value from the macro, you should enclose
+     the statements in a do - while block, as in:
+
+	#define macrofun(a, b, c) 		\
+		do {				\
+			if (a == 5)		\
+				do_this(b, c);	\
+		} while (0)
+
+ (b) If the macro is designed to return a value, you should use the
+     gcc extension that a compound statement enclosed in parentheses
+     represents an expression, as in:
+
+	#define maxint(a, b) ({			\
+		int _a = (a), _b = (b);		\
+		_a > _b ? _a : _b;		\
+	})

 Things to avoid when using macros:

-
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