I have written a simple C header for converting the endianness of short integers and long integers. It uses the GCC macro __BYTE_ORDER__ to check the system's byte order and define the macros based on that.

The header creates the macros LITTLE_ENDIAN_SHORT(n), LITTLE_ENDIAN_LONG(n), BIG_ENDIAN_SHORT(n), BIG_ENDIAN_LONG(n) which convert the value n from host endianness to the endianness specified.

Here is the source for endian.h:

#ifndef ENDIAN_H
#define ENDIAN_H

#define REVERSE_SHORT(n) ((unsigned short) (((n & 0xFF) << 8) | \
                                            ((n & 0xFF00) >> 8)))
#define REVERSE_LONG(n) ((unsigned long) (((n & 0xFF) << 24) | \
                                          ((n & 0xFF00) << 8) | \
                                          ((n & 0xFF0000) >> 8) | \
                                          ((n & 0xFF000000) >> 24)))

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
#    define LITTLE_ENDIAN_SHORT(n) (n)
#    define LITTLE_ENDIAN_LONG(n) (n)
#    define BIG_ENDIAN_SHORT(n) REVERSE_SHORT(n)
#    define BIG_ENDIAN_LONG(n) REVERSE_LONG(n)
#elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
#    define LITTLE_ENDIAN_SHORT(n) REVERSE_SHORT(n)
#    define LITTLE_ENDIAN_LONG(n) REVERSE_LONG(n)
#    define BIG_ENDIAN_SHORT(n) (n)
#    define BIG_ENDIAN_LONG(n) (n)
#else
#    error unsupported endianness
#endif

#endif

Is this a good way to implement the macros or is there a better way?