02. Introduction to Sockets

This chapter is an introduction to socket programming. Readers are assumed to have basic knowledge of computer networking but no experience in network programming. This book does not contain every detail on how to use socket APIs, you are advised to read manpages and other network programming guides while learning from this book. (https://beej.us/ is a good source for socket APIs.)

Redis is an example of the server/client system. Multiple clients connect to a single server, and the server receives requests from TCP connections and sends responses back. There are several Linux system calls we need to learn before we can start socket programming.

The socket() syscall returns an fd. Here is a rough explanation of “fd” if you are unfamiliar with Unix systems: An fd is an integer that refers to something in the Linux kernel, like a TCP connection, a disk file, a listening port, or some other resources, etc.

The bind() and listen() syscall: the bind() associates an address to a socket fd, and the listen() enables us to accept connections to that address.

The accept() takes a listening fd, when a client makes a connection to the listening address, the accept() returns an fd that represents the connection socket. Here is the pseudo-code that explains the typical workflow of a server:

fd = socket()
bind(fd, address)
listen(fd)
while True:
    conn_fd = accept(fd)
    do_something_with(conn_fd)
    close(conn_fd)

The read() syscall receives data from a TCP connection. The write() syscall sends data. The close() syscall destroys the resource referred by the fd and recycles the fd number.

We have introduced the syscalls needed for server-side network programming. For the client side, the connect() syscall takes a socket fd and address and makes a TCP connection to that address. Here is the pseudo-code for the client:

fd = socket()
connect(fd, address)
do_something_with(fd)
close(fd)

The next chapter will help you get started using real code.