Messages in this thread | | | Subject | Re: linux-next: build warning in Linus'tree | From | Joakim Tjernlund <> | Date | Wed, 26 May 2010 19:31:06 +0200 |
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Linus Torvalds <torvalds@linux-foundation.org> 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 <stdio.h> > > #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
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