Re: [boost] BOOST_BIG_ENDIAN

From: Jody Hagins (jody-boost-011304_at_[hidden])
Date: 2004-07-24 12:50:47

On Sat, 24 Jul 2004 11:57:50 +0100
"John Maddock" <john_at_[hidden]> wrote:

> I think this one has come up before: in general there is no way tell
> what the endianness is without doing a runtime test, there's also the
> problem that there are more than two orderings possible, for example
> for 4 byte types the byte ordering could be any of:

Thanks, John!

Interesting.  I am unaware of the impossibility of it.  The only
complaints I know of with detecting architecture through macros is
having to modify the code for each architecture (ala detail/limits.hpp).
 It is not the best in the world, but that is mostly because of the
maintenance issues.

> 
> 1 2 3 4
> 4 3 2 1
> 2 1 4 3
> 3 4 1 2

Of the above, I only know of three that are actually in use, and the
third (3412) has not been around since the early PDP machines.  Of
course, I am not familiar with all architectures.

> throw in 8 or 16 byte types, plus floating point types, and things get
> complicated :-(

Again, I think the possibilities outstrip reality, and reality is the
only thing that counts here...  as long as each architecture has a
specific #define that can be detected at compile time.  The worst thing
is that a particular architecture may not be supported, and a #error can
be issued.

I imagine architectures are consistent themselves, but something like
this would help if that were necessary:

#if defined(__i386__)
#define BOOST_BYTEORDER_16 12
#define BOOST_BYTEORDER_32 1234
#define BOOST_BYTEORDER_64 12345678
#elif defined(__sparc__)
#define BOOST_BYTEORDER_16 21
#define BOOST_BYTEORDER_32 4321
#define BOOST_BYTEORDER_64 87654321
#elif defined(__dinosaur__)
#define BOOST_BYTEORDER_16 21
#define BOOST_BYTEORDER_32 4321
#define BOOST_BYTEORDER_64 87654321
#else
#error Byte ordering undetected
#endif