Sockets: Introduction

56.1 Overview

Sockets exist in a communication domain, which determines:

1. the method of identifying a socket(ipv4 / ipv6)
2. the range of communication (ipv4 / unix domain)

Socket I/O can be performed using the conventional read() and write() system calls, or using a range of socket-specific system calls (e.g., send(), recv(), sendto(), and recvfrom()). By default, these system calls block if the I/O operation can’t be completed immediately. Nonblocking I/O is also possible, by using the fcntl() F_SETFL operation (Section 5.3) to enable the O_NONBLOCK open file status flag.

In the Internet domain, datagram sockets employ the User Datagram Protocol (UDP), and stream sockets (usually) employ the Transmission Control Protocol (TCP). Instead of using the terms Internet domain datagram socket and Internet domain stream socket, we’ll often just use the terms UDP socket and TCP socket, respectively

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