This construct is not legal in C, as far as I understand it. It is an
aliasing violation to store data in one field of a union and read it
from another. The compiler can do crazy things that would make you
sad.
There's a series of macros that are the 'blessed' way of figuring out
endianness. Here's what I do in one project I have:
// This is all to figure out endian-ness and byte-swapping on various systems
#if defined(HAVE_ENDIAN_H)
#include <endian.h> // for the __BYTE_ORDER use below
#elif defined(HAVE_SYS_ENDIAN_H)
#include <sys/endian.h> // location on FreeBSD
#elif defined(HAVE_MACHINE_ENDIAN_H)
#include <machine/endian.h> // location on OS X
#endif
#if defined(HAVE_SYS_BYTEORDER_H)
#include <sys/byteorder.h> // BSWAP_32 on Solaris 10
#endif
#ifdef HAVE_SYS_ISA_DEFS_H
#include <sys/isa_defs.h> // _BIG_ENDIAN/_LITTLE_ENDIAN on Solaris 10
#endif
In linux (glibc), the macro __BYTE_ORDER is set to either
__LITTLE_ENDIAN or __BIG_ENDIAN. Other systems do something similar.
I can't guarantee all systems have a macro for this, though.
craig
