Detecting Endianness

As stated earlier, the only "real" way to detect Big Endian is to use runtime tests.

However, sometimes, a macro might be preferred.

Unfortunately, I've not found a single "test" to detect this situation, rather a collection of them.

For example, GCC recommends : __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ . However, this only works with latest versions, and earlier versions (and other compilers) will give this test a false value "true", since NULL == NULL. So you need the more complete version : defined(__BYTE_ORDER__)&&(__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__)

OK, now this works for newest GCC, but what about other compilers ?

You may try __BIG_ENDIAN__ or __BIG_ENDIAN or _BIG_ENDIAN which are often defined on big endian compilers.

This will improve detection. But if you specifically target PowerPC platforms, you can add a few more tests to improve even more detection. Try _ARCH_PPC or __PPC__ or __PPC or PPC or __powerpc__ or __powerpc or even powerpc. Bind all these defines together, and you have a pretty fair chance to detect big endian systems, and powerpc in particular, whatever the compiler and its version.

So, to summarize, there is no such thing as a "standard pre-defined macros" which guarantees to detect big-endian CPU on all platforms and compilers, but there are many such pre-defined macros which, collectively, give a high probability of correctly detecting big endian under most circumstances.

At compile time in C you can't do much more than trusting preprocessor #defines, and there are no standard solutions because the C standard isn't concerned with endianness.

Still, you could add an assertion that is done at runtime at the start of the program to make sure that the assumption done when compiling was true:

inline int IsBigEndian()
{
    int i=1;
    return ! *((char *)&i);
}

/* ... */

#ifdef COMPILED_FOR_BIG_ENDIAN
assert(IsBigEndian());
#elif COMPILED_FOR_LITTLE_ENDIAN
assert(!IsBigEndian());
#else
#error "No endianness macro defined"
#endif

(where COMPILED_FOR_BIG_ENDIAN and COMPILED_FOR_LITTLE_ENDIAN are macros #defined previously according to your preprocessor endianness checks)