Run the server:

[gby@shu unix_domain_sockets]$ ./server 
demo_server going into background... 

The clients needs a serve name and a message:
[gby@shu unix_domain_sockets]$ ./client 
Usage: ./client [server name] [message]

The demo server understands the following commands: 

"c" will send back the current value of the counter:

[gby@shu unix_domain_sockets]$ ./client demo_server c
demo_server# Counter is 13

[gby@shu unix_domain_sockets]$ ./client demo_server c
demo_server# Counter is 15
[gby@shu unix_domain_sockets]$ ./client demo_server c
demo_server# Counter is 16

"l" will send the current value of the counter to 
the system log:

[gby@shu unix_domain_sockets]$ ./client demo_server l
demo_server# Wrote counter value to log.

This will show up in the system log as the following
messages:

Feb 12 18:30:53 shu demo_server[20079]: Counter is 17

"q" will ask the server to terminate gracefully:

[gby@shu unix_domain_sockets]$ ./client demo_server q
demo_server# Server quits.

When the server quits because of a user request or
a signal, it will log a message as follows to the 
system log:

Feb 12 18:32:40 shu demo_server[20200]: Server terminated on user request
 or:
Feb 12 18:34:44 shu demo_server[20225]: Server terminated on signal request


Anything else will get you an error message:

[gby@shu unix_domain_sockets]$ ./client demo_server 0
demo_server# Unknown command

Trying to contact a non running server will produce 
the following error:

[gby@shu unix_domain_sockets]$ ./client demo_server l
Connecting: No such file or directory