Errors in Linux Kernel headers - linux

Question

I'm trying to compile a library which includes some headers from kernel-devel package. I linked the appropriate headers, but now I get compilation errors in those header files.
/usr/include/asm-generic/bitops/fls64.h: In function ‘int fls64(__u64)’:
/usr/include/asm-generic/bitops/fls64.h:10: error: ‘fls’ was not declared in this scope
/usr/include/asm-generic/bitops/fls64.h:11: error: ‘fls’ was not declared in this scope
And, here are the code from asm-generic/bitops/fls64.h
#ifndef _ASM_GENERIC_BITOPS_FLS64_H_
#define _ASM_GENERIC_BITOPS_FLS64_H_
#include <asm/types.h>
static inline int fls64(__u64 x)
{
__u32 h = x >> 32;
if (h)
return fls(h) + 32;
return fls(x);
}
#endif /* _ASM_GENERIC_BITOPS_FLS64_H_ */
As you can notice "return fls(h)", there's no definition of fls(). I can resolve this by including "fls.h", but am I suppose to fix such errors in standard Kernel headers??
Any pointers which could explain why is it this way and what can I do to get around such issues?? Btw, the errors I mentioned here are just the tip of the iceberg. There are a lot of such (delcaration missing) errors in multiple such headers.
Help would be greatly appreciated. Thanks!
rgds/R.
PS: some system details:
Linux Distribution: CentOS (5.5)
[raj#localhost common]$ uname -a
Linux localhost.localdomain 2.6.18-238.9.1.el5 #1 SMP Tue Apr 12 18:10:56 EDT 2011 i686 i686 i386 GNU/Linux
[raj#localhost common]$ cat /proc/version
Linux version 2.6.18-238.9.1.el5 (mockbuild#builder10.centos.org) (gcc version 4.1.2 20080704 (Red Hat 4.1.2-50)) #1 SMP Tue Apr 12 18:10:56 EDT 2011

Answer 1

The root of the problem is that you are using a header file <asm-generic/fls64.h> that is part of the internal kernel implementation and not intended to be used by userspace at all. In fact even in the kernel this header file is just supposed to be included by headers like arch/XXX/include/bitops.h to provide a generic implementation of fls64() based on the fls() defined in architecture-specific code.
In other words the library has a problem in that it is depending on kernel internals that are not really exported to userspace for use and therefore may break for various kernel versions; the library may well have built OK against some older kernel but this was just by luck.
The correct fix is really for the library to provide its own fls64 definition rather than relying on getting lucky about what some random version of kernel headers happens to define by accident.