linux-kernel - Re: [PATCH] BUILD_BUG

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Date:	Sat, 16 Aug 2008 13:07:51 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
cc:	Alexey Dobriyan <adobriyan@...il.com>,
	torvalds@...uxfoundation.org, akpm@...uxfoundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] BUILD_BUG_ON sucks

On Sat, 16 Aug 2008, Rusty Russell wrote:
> 
> Interesting idea, but I've come to actually like the semantic explicitness of 
> BUILD_BUG_ON.  There's a difference between "we should never get here" 
> and "this should never exist".

Agreed. I think Alexey's patch is broken.

The thing is, BUILD_BUG_ON() is a different thing. It says "this is a 
build error", while BUG_ON() says "this is an error if we reach it".

Very different.

The fact that you broke BUG_ON(1) should have made you think. Sometimes 
the "1" isn't necessarily a constant one. It might be

	if (something_that_can_never_happen_in_some_configuration) {
		...
		BUG_ON(CONFIG_XYZZY);
		...
	}

where the BUG_ON(1) is absolutely *not* the same thing as BUILD_BUG_ON().

			Linus
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