linux-kernel - Re: linux-next: build warning in Linus'tree

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Date:	Wed, 26 May 2010 09:46:11 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Joakim Tjernlund <joakim.tjernlund@...nsmode.se>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-next@...r.kernel.org,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: linux-next: build warning in Linus'tree

On Wed, 26 May 2010, Joakim Tjernlund wrote:
> 
> 1) It silently breaks when neither of {__LITTLE_,__BIG}_ENDIAN (or both)are
>    defined depending on the endianess of the target CPU.
>    The glibc model generates a compile error if you forget to include __BYTE_ORDER.

Umm. Except when it doesn't (yes, Linux has the "Wundefined" thing, and 
has had for a long time). I've seen the glibc model do the wrong thing 
exactly because traditional C semantics is "undefined symbol is 0 in 
evaluations"

Try compiling this

	#include <stdio.h>

	#if NOT_HERE == NOT_THERE
	int main()
	{
		printf("Hello world!\n");
	}
	#endif

and even with -Wall it compiles perfectly happily.

So no. The glibc model is _not_ any better in practice.

> 2) It clashes with user space so one cannot use it in exported header files.

Which is annoying, I agree. But you shouldn't generally use kernel headers 
for user space anyway, much less export anything that is byteorder- 
specific. So anybody who has this problem is likely doing something iffy 
to begin with.

Besides, you can solve it cleanly by simply avoiding the crazy glibc 
semantics entirely. IOW, the CONFIG_BIG_ENDIAN option I suggested (and 
again, you should damn well not export things that depend on it to user 
space - there are architectures where user-space might be switchable)

			Linus
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