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

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Message-ID: <Pine.LNX.4.64.0612311447030.18368@localhost.localdomain>
Date:	Sun, 31 Dec 2006 14:49:48 -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 02:32:25PM -0500, Robert P. J. Day wrote:
>
> >  Generally, inline functions are preferable to macros resembling
> >  functions.
>
> This should be stressed, IMHO. We have too many macros which have no
> reason to live.
>
> > -Macros with multiple statements should be enclosed in a do - while block:
> > +There are two techniques for defining macros that contain multiple
> > +statements.
> >
> > -#define macrofun(a, b, c) 			\
> > -	do {					\
> > + (a) Enclose those statements in a do - while block:
> > +
> > +	#define macrofun(a, b, c) 		\
> > +		do {				\
> > +			if (a == 5)		\
> > +				do_this(b, c);	\
> > +		} while (0)
> > +
> > + (b) Use the gcc extension that a compound statement enclosed in
> > +     parentheses represents an expression:
> > +
> > +	#define macrofun(a, b, c) ({		\
> >  		if (a == 5)			\
> >  			do_this(b, c);		\
> > -	} while (0)
> > +	})
>
> When giving two alternatives, the reader will thank you if you
> explain when each should be used. In this case, the second form
> should be used when the macro needs to return a value (and you can't
> use an inline function for whatever reason), whereas the first form
> should be used at all other times.

that's a fair point, although it's certainly not the coding style
that's in play now.  for example,

  #define setcc(cc) ({ \
    partial_status &= ~(SW_C0|SW_C1|SW_C2|SW_C3); \
    partial_status |= (cc) & (SW_C0|SW_C1|SW_C2|SW_C3); })

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.

rday
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