Re: Confusion in usr/include/asm-generic/fcntl.h

[H. Peter Anvin]

David Miller wrote:
> From: Jaswinder Singh Rajput <jaswinder@xxxxxxxxxx>
> Date: Wed, 21 Jan 2009 05:34:17 +0530
>
> > usr/include/asm-generic/fcntl.h is giving 2 'make headers_check' warnings:
> >  usr/include/asm-generic/fcntl.h:127: leaks CONFIG_64BIT to userspace where it is not valid
> >  usr/include/asm-generic/fcntl.h:149: leaks CONFIG_64BIT to userspace where it is not valid
> >
> > usr/include/asm-generic/fcntl.h:
> ...
> > #ifndef CONFIG_64BIT will always be true for userspace. So what is the use of #ifndef CONFIG_64BIT ?
>
> Good catch.
>
> This file needs to test for 64-bit'ness using some non-CONFIG_*
> test.  And the standard built-in CPP macros which can be used
> to check for that are different on every platform.

There are a few ways to check for 64-bitness that are 
platform-independent, unfortunately each of them have drawbacks.

a) the gcc-specific way:

	#if __SIZEOF_POINTER__ == 8

   or

	#ifdef __LP64__

   or any other number of variants.

   It has the obvious disadvantage of being gcc-specific, although
   it seems rather likely that other Linux-supporting compilers might
   also define these macros.

b) the standard C way:

	#include <limits.h>

	#if LONG_MAX > 2147483647L

   it has the obvious disadvantage of needing to include <limits.h>...

We can, of course, also do our own thing as DaveM suggested; this is 
probably the best option.  FWIW, glibc has _WORDSIZE which is equivalent 
to what we call BITS_PER_LONG.  We might as well export it under that 
name (presumably under #ifndef).

	-hpa
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