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

[KevinHall]

Q: How do I convert between big-endian and little-endian values?

A: The operations are the same in both directions. Here is some code for the common unsigned data types:

inline void endian_swap(unsigned short& x)
{
    x = (x>>8) | 
        (x<<8);
}

inline void endian_swap(unsigned int& x)
{
    x = (x>>24) | 
        ((x<<8) & 0x00FF0000) |
        ((x>>8) & 0x0000FF00) |
        (x<<24);
}

// __int64 for MSVC, "long long" for gcc
inline void endian_swap(unsigned __int64& x)
{
    x = (x>>56) | 
        ((x<<40) & 0x00FF000000000000) |
        ((x<<24) & 0x0000FF0000000000) |
        ((x<<8)  & 0x000000FF00000000) |
        ((x>>8)  & 0x00000000FF000000) |
        ((x>>24) & 0x0000000000FF0000) |
        ((x>>40) & 0x000000000000FF00) |
        (x<<56);
}

[KevinHall]

Q: Why didn't you just use 'ntohl()' and 'htonl()'?

A: These functions

• do not always swap endian-ness and
  
• are not always portable.

For more information please see the FAQ "What do 'ntohl()' and 'htonl()' actually do?".