Inter-process communication using BSD Sockets and TCP

Tutorial on client-server programming using Berkeley sockets

Introduction

On Unix/Linux and BSD operating systems socket API is part of Standard C Library. It means that no any external dependencies are needed to use it in our C programs. It only needs to include required header files which are available in the system by default.

TCP vs UDP

In this tutorial we are going to consider only TCP. There is a great article about socket programming using UDP by Glenn Fiedler [2].

Opening and closing sockets

To start working with a soket, regardless it it's sending or receiving data, we need to the following:

1. create a socket using socket() function
2. bind it to an address (and port) using bind() function

After the work with a socket is done, it has to be closed using close() function.

Creating a socket: socket() function

The function to create a socket has the following prototype:

int socket(int domain, int type, int protocol);

domain parameter

The first parameter domain specifies the protocol family.

The POSIX standard [1] defines only 3 possible values:

• AF_UNIX - UNIX internal protocols for local communications.
• AF_INET - Internet Protocol version 4 (IPv4) communications.
• AF_INET6 - Internet Protocol version 6 (IPv6) communications.

On Linux systems there are many more other values defined. In this article we are interested only in the last two values: AF_INET and AF_INET6.

Further we will see that we don't have to specify these values directly. By using the function getaddrinfo() we can create IPv4/IPv6 protocol-independent sockets.

Instead of AF_* constants, some documentation and source code use corresponding PF_* constants (i.e. PF_INET, PF_INET6). For example, man-pages on FreeBSD uses them. The PF prefix stands for "protocol family", while AF stands for "address family". Some people say that using PF_* constants in the socket() call is the right way. However the POSIX standard defines only AF_ constants and requires to use them. In fact, the corresponding AF_* and PF_* constant have the same values.

type parameter

The second paramater, type, is the socket type. There are different types of sockets. The two most common ones are stream sockets and datagram sockets.

In this article we are going to consider only stream sockets. For stream sockets the value of the parameter type should be specified as SOCK_STREAM.

protocol parameter

The third parameter, protocol, specifies the protocol for a given socket type.

Binding a socket

Before listening for incoming connections, a socket should be bound to a local interface address using bind() function.

It is possible to start listening on an unbound socket. In such case the socket is automatically bound to a random free port with the local address set to INADDR_ANY.

Closing a socket

Example: server listening for incoming connections

Let's create a small program that accepts incoming TCP connections.

References

[1] IEEE Std 1003.1-2017 (POSIX.1-2017)

[2] Glenn Fiedler, Sending and Receiving Packets

[3] Beej's Guide to Network Programming