Understanding htonl() and ntohl()

As others have mentioned, both htons and ntohs reverse the byte order on a little-endian machine, and are no-ops on big-endian machines.

What wasn't mentioned is that these functions take a 16-bit value and return a 16-bit value. If you want to convert 32-bit values, you want to use htonl and ntohl instead.

The names of these functions come from the traditional sizes of certain datatypes. The s stands for short while the l stands for long. A short is typically 16-bit while on older systems long was 32-bit.

In your code, you don't need to call htonl on rec_addr, because that value was returned by inet_addr, and that function returns the address in network byte order.

You do however need to call htons on rec_port.

"Network byte order" always means big endian.

"Host byte order" depends on architecture of host. Depending on CPU, host byte order may be little endian, big endian or something else. (g)libc adapts to host architecture.

Because Intel architecture is little endian, this means that both functions are doing the same: reversing byte order.