Filesystems, serial ports, consoles, and sound cards all use the client/server model. A C language application program takes on the role of the client and sends requests to these servers. The servers perform whatever work was specified, and reply with the answer.
Some of these traditional "client/server" servers may in fact actually be reply-driven (server/subserver) servers! This is because, to the ultimate client, they appear as a standard server, even though the server itself uses server/subserver methods to get the work done. What I mean by that is, the client still sends a message to what it thinks is the "service providing process." What actually happens is that the "service providing process" simply delegates the client's work to a different process (the subserver).