portable_endian.h

// "License": Public Domain
// I, Mathias Panzenb�ck, place this file hereby into the public domain. Use it at your own risk for whatever you like.
// In case there are jurisdictions that don't support putting things in the public domain you can also consider it to
// be "dual licensed" under the BSD, MIT and Apache licenses, if you want to. This code is trivial anyway. Consider it
// an example on how to get the endian conversion functions on different platforms.

#ifndef PORTABLE_ENDIAN_H__
#define PORTABLE_ENDIAN_H__

#if (defined(_WIN16) || defined(_WIN32) || defined(_WIN64)) && !defined(__WINDOWS__)

#	define __WINDOWS__

#endif

#if defined(__linux__) || defined(__CYGWIN__)

#	include <endian.h>

#elif defined(__APPLE__)

#	include <libkern/OSByteOrder.h>

#	define htobe16(x) OSSwapHostToBigInt16(x)
#	define htole16(x) OSSwapHostToLittleInt16(x)
#	define be16toh(x) OSSwapBigToHostInt16(x)
#	define le16toh(x) OSSwapLittleToHostInt16(x)
 
#	define htobe32(x) OSSwapHostToBigInt32(x)
#	define htole32(x) OSSwapHostToLittleInt32(x)
#	define be32toh(x) OSSwapBigToHostInt32(x)
#	define le32toh(x) OSSwapLittleToHostInt32(x)
 
#	define htobe64(x) OSSwapHostToBigInt64(x)
#	define htole64(x) OSSwapHostToLittleInt64(x)
#	define be64toh(x) OSSwapBigToHostInt64(x)
#	define le64toh(x) OSSwapLittleToHostInt64(x)

#	define __BYTE_ORDER    BYTE_ORDER
#	define __BIG_ENDIAN    BIG_ENDIAN
#	define __LITTLE_ENDIAN LITTLE_ENDIAN
#	define __PDP_ENDIAN    PDP_ENDIAN

#elif defined(__OpenBSD__)

#	include <sys/endian.h>

#elif defined(__NetBSD__) || defined(__FreeBSD__) || defined(__DragonFly__)

#	include <sys/endian.h>

#	define be16toh(x) betoh16(x)
#	define le16toh(x) letoh16(x)

#	define be32toh(x) betoh32(x)
#	define le32toh(x) letoh32(x)

#	define be64toh(x) betoh64(x)
#	define le64toh(x) letoh64(x)

#elif defined(__WINDOWS__)

#	include <windows.h>

#	if BYTE_ORDER == LITTLE_ENDIAN

#               if defined(_MSC_VER)
#                       include <stdlib.h>
#			define htobe16(x) _byteswap_ushort(x)
#			define htole16(x) (x)
#			define be16toh(x) _byteswap_ushort(x)
#			define le16toh(x) (x)
                	
#			define htobe32(x) _byteswap_ulong(x)
#			define htole32(x) (x)
#			define be32toh(x) _byteswap_ulong(x)
#			define le32toh(x) (x)
                	
#			define htobe64(x) _byteswap_uint64(x)
#			define htole64(x) (x)
#			define be64toh(x) _byteswap_uint64(x)
#			define le64toh(x) (x)

#               elif defined(__GNUC__) || defined(__clang__)

#			define htobe16(x) __builtin_bswap16(x)
#			define htole16(x) (x)
#			define be16toh(x) __builtin_bswap16(x)
#			define le16toh(x) (x)
                	
#			define htobe32(x) __builtin_bswap32(x)
#			define htole32(x) (x)
#			define be32toh(x) __builtin_bswap32(x)
#			define le32toh(x) (x)
                	
#			define htobe64(x) __builtin_bswap64(x)
#			define htole64(x) (x)
#			define be64toh(x) __builtin_bswap64(x)
#			define le64toh(x) (x)
#               else
#                       error platform not supported
#               endif

#	else

#		error byte order not supported

#	endif

#	define __BYTE_ORDER    BYTE_ORDER
#	define __BIG_ENDIAN    BIG_ENDIAN
#	define __LITTLE_ENDIAN LITTLE_ENDIAN
#	define __PDP_ENDIAN    PDP_ENDIAN

#else

#	error platform not supported

#endif

#endif

There is currently no support for QNX Neutrino. You can add it easily as shown below:

#elif defined(__QNXNTO__)

#	include <gulliver.h>

These macros are inconsistent. Some convert to and from the Host Endianness while others are simple byte swaps. The host is assumed to always be Little Endian, which may not be true.

You are right, thanks for the comment. The correct way is as follows.
# include <net/netbyte.h>