Question

I need to build a server using Unix domain sockets, and it looks that there are several options to choose the kind of communication.
From man 2 socket:

• SOCK_STREAM
• SOCK_DGRAM
• SOCK_SEQPACKET

So, for what is better suited every one of them? (stream, datagram, packet)

Answer 1

It really depends what kind of server you are going to implement.

If message boundaries are important, then SOCK_DGRAM would be the best choice.
Because recvfrom/recvmsg/select will return when a complete message is received.

With SOCK_STREAM, message receiving is more tricky: One receiving call may return a partial message, or part of two messages, or several messages… etc.

If message boundaries are not important, then SOCK_STREAM could be the best choice.

SOCK_DGRAM of AF_INET is unreliable UDP. But, in most sytems, SOCK_DGRAM of AF_UNIX is reliable.
For example: If queue of receiver is full, sender will be blocked until there is space.