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Date: Sat, 26 Feb 2000 13:03:41 +0000
From: Ben Smithurst <ben@scientia.demon.co.uk>
To: Alfred Perlstein <bright@wintelcom.net>
Cc: cjclark@home.com, Marco Molteni <molter@csl.sri.com>,
	freebsd-chat@FreeBSD.ORG
Subject: Re: how to do this C preprocessor trick?
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Alfred Perlstein wrote:

> * Crist J. Clark <cjc@cc942873-a.ewndsr1.nj.home.com> [000225 22:02] wrote:
>> 
>> Why a,
>> 
>>   do { <stuff> } while(0)
>> 
>> Rather than just,
>> 
>>   { <stuff> }
> 
> I really don't remeber offhand, I know a lot of macros in FreeBSD
> are moved from 
> 
> #define foo { bar; baz; }
> 
> to
> 
> #define foo do{ bar; baz; }while(0)

with the original #define,

if (some_condition)
	foo;
else
	bar();

would expand to,

if (some_condition)
	{ bar; baz; };
else
	bar();

That is a syntax error (extra semicolon before "else"). I used to use
the form without do/while until this bit me. :-) With the second form,
well you can work it out, and the expanded code is perfectly legal.

-- 
Ben Smithurst / ben@scientia.demon.co.uk / PGP: 0x99392F7D


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