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

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

[Robert P. J. Day]

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
========================================================================

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

[Joe Perches]

On Wed, 2012-07-11 at 13:32 -0400, Robert P. J. Day wrote:
[]
> diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
[]
> +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;	\
> +}						\
> +)

Hi Robert.

How about the phrase "GCC's statement expression extension"
or maybe give a link like:
http://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

Please put the opening ({ and closing )} on a single line.
It's shorter and makes grep easier.

#define statement_expression(args)		\
({						\
	etc					\
})

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

[Robert P. J. Day]

On Wed, 11 Jul 2012, Joe Perches wrote:

> On Wed, 2012-07-11 at 13:32 -0400, Robert P. J. Day wrote:
> []
> > diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
> []
> > +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;	\
> > +}						\
> > +)
>
> Hi Robert.
>
> How about the phrase "GCC's statement expression extension"
> or maybe give a link like:
> http://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html

  ok, i'll do that later today.

> Please put the opening ({ and closing )} on a single line.
> It's shorter and makes grep easier.
>
> #define statement_expression(args)		\
> ({						\
> 	etc					\
> })

  i was simply transcribing what was in kernel.h verbatim, but i'll
tuck it together for brevity.

rday

-- 

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

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