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SO_PEERCRED is simple way to get pid/uid/gid of connected AF_UNIX stream socket, SCM_CREDENTIALS is more or less the same, but more complex (various ancillary messages). Links to example showing both ways.
What should I use?
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tl,dr: AF_UNIX is for sockets(and they use files) while AF_INET is for binding to ip addresses, and creating communications on its various forms(unicast, multicast, broadcast...).
Unix sockets are bidirectional. This means that every side can perform both read and write operations. While, FIFOs are unidirectional: it has a writer peer and a reader peer. Unix sockets create less overhead and communication is faster, than by localhost IP sockets.
SO_PRIORITY Set the protocol-defined priority for all packets to be sent on this socket. Linux uses this value to order the networking queues: packets with a higher priority may be processed first depending on the selected device queueing discipline.
Unix domain sockets are often twice as fast as a TCP socket when both peers are on the same host. The Unix domain protocols are not an actual protocol suite, but a way of performing client/server communication on a single host using the same API that is used for clients and servers on different hosts.
If I understand correctly, there is a subtle difference between the two. SO_PEERCRED retrieves the credentials of the peer process, without requiring any interaction from the peer process. In contrast, SCM_CREDENTIALS is a mechanism to send / receive credentials of the peer process, which are then checked by the kernel. This subtle difference may matter when a process is running as UID 0. SCM_CREDENTIALS allows a process running as UID 0, to declare itself less privileged (e.g., UID 50), whereas this would not be possible with SO_PEERCRED.
See above. I guess using SCM_CREDENTIALS is encouraged and SO_PEERCRED is only supported for compatibility.
The dbus daemon seems to use SO_PEERCRED and getpeereid(). I think it is best to copy their code in order to portably get the credentials.
http://cgit.freedesktop.org/dbus/dbus/tree/dbus/dbus-sysdeps-unix.c?id=edaa6fe253782dda959d78396b43e9fd71ea77e3
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