Messages in this thread | | | Date | Wed, 13 Sep 2006 17:38:06 +0100 | From | Russell King <> | Subject | Re: [PATCH 4/6] Implement a general log2 facility in the kernel |
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On Wed, Sep 13, 2006 at 10:31:36AM -0600, Matthew Wilcox wrote: > On Wed, Sep 13, 2006 at 06:17:34PM +0200, Adrian Bunk wrote: > > On Wed, Sep 13, 2006 at 02:03:00PM +0100, David Howells wrote: > > > From: David Howells <dhowells@redhat.com> > > > > > > This facility provides three entry points: > > > > > > log2() Log base 2 of u32 > > >... > > > > Considering that several arch maintainers have vetoed my patch to revert > > the -ffreestanding removal Andi sneaked in with his usual trick to hide > > generic patches as "x86_64 patch", such a usage of a libc function name > > with a signature different from the one defined in ISO/IEC 9899:1999 is > > a namespace violation that mustn't happen. > > log2 is only defined if math.h gets included. If we're including math.h > at any point in the kernel itself (excluding the bootloader and similar), > we're already screwed six ways from sunday.
Adrian's point is that gcc without -ffreestanding may decide to implement log2() itself by issuing the appropriate floating point instructions rather than using a function call into a library to do this operation.
Therefore, re-using "log2()" is about as bad as re-using the "strcmp()" name to implement a function which copies strings.
And, sure enough, try throwing this at a compiler:
int log2(int foo) { return foo; }
you get:
t.c:2: warning: conflicting types for built-in function 'log2'
but not if you use -ffreestanding.
Don't re-use C standard library identifiers (or use -ffreestanding.)
-- Russell King Linux kernel 2.6 ARM Linux - http://www.arm.linux.org.uk/ maintainer of: 2.6 Serial core - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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