Question

I have seen two examples that illustrate how the client socket can receive messages from server.

Example 1:

server code
http://man7.org/tlpi/code/online/book/sockets/ud_ucase_sv.c.html

client code
http://man7.org/tlpi/code/online/book/sockets/ud_ucase_cl.c.html

The client program creates a socket and binds the socket to an address, so that the server can send its reply.

if (bind(sfd, (struct sockaddr *) &claddr, sizeof(struct sockaddr_un)) == -1)
    errExit("bind"); // snippet from ud_ucase_cl.c

Example 2:

server code
http://man7.org/tlpi/code/online/book/sockets/i6d_ucase_sv.c.html

client code
http://man7.org/tlpi/code/online/book/sockets/i6d_ucase_cl.c.html

In example 2, client code doesn’t bind its socket with an address.

Question:

Is it necessary for the client code to bind the socket with an address in order to receive message from server?

Why in the first example, we have to bind the client socket with an address, why we don’t have to in the second example?

Answer 1

The difference is the socket family – first example uses AF_UNIX, while the second does AF_INET6. According to Stevens UNP you need to explicitly bind pathname to Unix client socket so that the server has a pathname to which it can send its reply:

… sending a datagram to an unbound Unix domain datagram socket does not implicitly bind a pathname to the socket. Therefore, if we omit this step, the server’s call to recvfrom … returns a null pathname …

This is not required for INET{4,6} sockets since they are “auto-bound” to an ephemeral port.