c++ - How do you write (portably) reverse network byte order?

Question

Background

When designing binary file formats, it's generally recommended to write integers in network byte order. For that, there are macros like htonhl(). But for a format such as WAV, actually the little endian format is used.

Question

How do you portably write little endian values, regardless of if the CPU your code runs on is a big endian or little endian architecture? (Ideas: can the standard macros ntohl() and htonl() be used "in reverse" somehow? Or should the code just test runtime if it's running on a little or big endian CPU and choose the appropriate code path?)

_{So the question is not really about file formats, file formats were just an example. It could be any kind of serialization where little endian "on the wire" is required, such as a (heretic) network protocol.}

Answer 1

Warning: This only works on unsigned integers, because signed right shift is implementation defined and can lead to vulnerabilities (https://stackoverflow.com/a/7522498/395029)

C already provides an abstraction over the host's endianness: the number† or int†.

Producing output in a given endianness can be done portably by not trying to be clever: simply interpret the numbers as numbers and use bit shifts to extract each byte:

uint32_t value;
uint8_t lolo = (value >> 0) & 0xFF;
uint8_t lohi = (value >> 8) & 0xFF;
uint8_t hilo = (value >> 16) & 0xFF;
uint8_t hihi = (value >> 24) & 0xFF;

Then you just write the bytes in whatever order you desire.

When you are taking byte sequences with some endianness as input, you can reconstruct them in the host's endianness by again constructing numbers with bit operations:

uint32_t value = (hihi << 24)
               | (hilo << 16)
               | (lohi << 8)
               | (lolo << 0);

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† Only the representations of numbers as byte sequences have endianness; numbers (i.e. quantities) don't.