From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joakim Tjernlund Subject: Re: linux-next: build warning in Linus'tree Date: Wed, 26 May 2010 19:31:06 +0200 Message-ID: References: <20100526110506.f2f4f22c.sfr@canb.auug.org.au> <20100525182040.f1882d0a.akpm@linux-foundation.org> <20100526140900.5b091c16.sfr@canb.auug.org.au> <20100525234116.71889c71.akpm@linux-foundation.org> <20100526171424.447fac18.sfr@canb.auug.org.au> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Return-path: In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Linus Torvalds Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-next@vger.kernel.org, Stephen Rothwell List-Id: linux-next.vger.kernel.org Linus Torvalds wrote on 2010/05/26 18:46:11: > > 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 > > #if NOT_HERE == NOT_THERE > int main() > { > printf("Hello world!\n"); > } > #endif > > and even with -Wall it compiles perfectly happily. Ouch! But here -Wundef really helps. > > So no. The glibc model is _not_ any better in practice. In the kernel it is since it breaks the compile. The breakage my patch introduced is a sign of that, right? > > > 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- Not in general, but my case could have been avoided, I sure there are others too. Why else does some header files bother with __BYTE_ORDER? > specific. So anybody who has this problem is likely doing something iffy > to begin with. hmm, so then I guess the existing use of __BYTE_ORDER in the kernel should be removed? > > Besides, you can solve it cleanly by simply avoiding the crazy glibc > semantics entirely. IOW, the CONFIG_BIG_ENDIAN option I suggested (and CONFIG_BIG_ENDIAN would have helped me with my lib/crc32.c problem but it does not prevent silent breakage so I figured the glibc model would be better. Is it such a big difference, readability wise, between #ifdef CONFIG_BIG_ENDIAN and #if __BYTE_ORDER == __BIG_ENDIAN that you rather risk silent breakage? > again, you should damn well not export things that depend on it to user > space - there are architectures where user-space might be switchable) Such arch exists but does any of them run linux in both modes? Jocke