What is SOL_SOCKET used for?

When retrieving a socket option, or setting it, you specify the option name as well as the level. When level = SOL_SOCKET, the item will be searched for in the socket itself.

For example, suppose we want to set the socket option to reuse the address to 1 (on/true), we pass in the "level" SOL_SOCKET and the value we want it set to.

int value = 1;    
setsockopt(mysocket, SOL_SOCKET, SO_REUSEADDR, &value, sizeof(value));

This will set the SO_REUSEADDR in my socket to 1.

I was stuck on this myself, the documentation is extremely cryptic. Slightly more detailed documentation here: http://pubs.opengroup.org/onlinepubs/7908799/xns/getsockopt.html

SOL_SOCKET is the socket layer itself. It is used for options that are protocol independent.

You can read more here.