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How are __LITTLE_ENDIAN__ and __BIG_ENDIAN__ defined?

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How are __LITTLE_ENDIAN and BIG_ENDIAN__ defined?

Subject: How are __LITTLE_ENDIAN__ and __BIG_ENDIAN__ defined?
From: Dave Thorup <email@hidden>
Date: Tue, 15 Nov 2005 19:01:39 -0500

I've been working on creating Universal Binaries and I've been using the following where needed:

#if __LITTLE_ENDIAN__
//	do little endian stuff
#else
//	do big endian stuff
#endif

Most of this code is cross-platform C++ that is built for both Windows and Mac OS X. Since neither of these macros are defined by the VisualStudio compiler I need to come up with consistent definitions to be used on the Windows side. It was my initial understanding that both __LITTLE_ENDIAN__ and __BIG_ENDIAN__ are defined no matter if you're compiling for x86 or PPC (one is defined as 1 and the other 0). Given that, I thought I'd put the following at the beginning of a file that uses the endian macros:

#if !defined( __BIG_ENDIAN__ ) || !defined( __LITTLE_ENDIAN__ )
#  error Both __BIG_ENDIAN__ and __LITTLE_ENDIAN__ must be defined.
#endif

However after trying this I found that only one of the Macros is defined. Is this behavior consistent for both GCC and CodeWarrior (CodeWarrior should only worry about __BIG_ENDIAN__ being defined)? If it is then I suppose I'll have to change my error warning to something like:

#if !defined( __BIG_ENDIAN__ ) && !defined( __LITTLE_ENDIAN__ )
#  error Either __BIG_ENDIAN__ or __LITTLE_ENDIAN__ must be defined.
#endif

Also, since only one of them is defined would it be better to use #if as I did above or should I use #ifdef?

#ifdef __LITTLE_ENDIAN__
//	do little endian stuff
#else
//	do big endian stuff
#endif

Thanks!
_____________________________

Dave Thorup
Software Engineer
email@hidden

http://www.kuwan.net
HyperSpell - Check spelling in Mac OS X from any application.


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