How do I convert between big-endian and little-endian values in C++?

Question

How do I convert between big-endian and little-endian values in C++?

EDIT: For clarity, I have to translate binary data (double-precision floating point values and 3

Answer 1

Wow, I couldn't believe some of the answers I've read here. There's actually an instruction in assembly which does this faster than anything else. bswap. You could simply write a function like this...

__declspec(naked) uint32_t EndianSwap(uint32 value)
{
    __asm
    {
        mov eax, dword ptr[esp + 4]
        bswap eax
        ret
    }
}

It is MUCH faster than the intrinsics that have been suggested. I've disassembled them and looked. The above function has no prologue/epilogue so virtually has no overhead at all.

unsigned long _byteswap_ulong(unsigned long value);

Doing 16 bit is just as easy, with the exception that you'd use xchg al, ah. bswap only works on 32-bit registers.

64-bit is a little more tricky, but not overly so. Much better than all of the above examples with loops and templates etc.

There are some caveats here... Firstly bswap is only available on 80x486 CPU's and above. Is anyone planning on running it on a 386?!? If so, you can still replace bswap with...

mov ebx, eax
shr ebx, 16
xchg bl, bh
xchg al, ah
shl eax, 16
or eax, ebx

Also inline assembly is only available in x86 code in Visual Studio. A naked function cannot be lined and also isn't available in x64 builds. I that instance, you're going to have to use the compiler intrinsics.