From owner-freebsd-current  Fri Mar 13 03:13:43 1998
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Date: Fri, 13 Mar 1998 12:13:30 +0100
From: Eivind Eklund <eivind@yes.no>
To: shimon@simon-shapiro.org, freebsd-current@FreeBSD.ORG
Subject: Re: A question about sys/sys/queue.h
References: <XFMail.980312191745.shimon@simon-shapiro.org>
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In-Reply-To: <XFMail.980312191745.shimon@simon-shapiro.org>; from Simon Shapiro on Thu, Mar 12, 1998 at 07:17:45PM -0800
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On Thu, Mar 12, 1998 at 07:17:45PM -0800, Simon Shapiro wrote:
> Why was the definition of some macros changed
> from:
> 
> #define FOO(a) {   ...   }
> 
> to:
> 
> #define FOO(a) do {  ...  } while(0)
> 
> I thought these are the same...

Imagine these used in a dual if () statement:

if (bar)
	if (baz)
		FOO(1);
	else
		printf ("You loose!\n");

With the former, you get something that (with proper indentation) map
as

if (bar)
	if (baz)
		{ ... };
else
	printf ("You loose!\n");

while with the do {...} while (0) trick, you get

if (bar)
	if (baz)
		do { ... } while(0);
	else
		printf ("You loose!\n");


For any onlookerss: You can't get the correct binding with if ()
tricks, BTW.  Stick with the good old "do { ... } while(0)"

Eivind.

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