Options SO_PASSCRED and SO_PEERCRED
These options are applicable to PF_UNIX (PF_LOCAL) sockets only. These are used to control and pass credentials on sockets that are local to the current host machine. The discussion of credentials will be deferred until Chapter 17, "Passing Credentials and File Descriptors."
This is perhaps the most difficult topic for you to master in this book. For now, simply note that these two options are likely to be of interest to you if you plan to write server programs that serve clients on the same local host.