7.1 Introduction

There are various ways to get and set the options that affect a socket:

• The getsockopt and setsockopt functions

• The fcntl function

• The ioctl function

This chapter starts by covering the setsockopt and getsockopt functions, followed by an example that prints the default value of all the options, and then a detailed description of all the socket options. We divide the detailed descriptions into the following categories: generic, IPv4, IPv6, TCP, and SCTP. This detailed coverage can be skipped during a first reading of this chapter, and the individual sections referred to when needed. A few options are discussed in detail in a later chapter, such as the IPv4 and IPv6 multicasting options, which we will describe with multicasting in Section 21.6.

We also describe the fcntl function, because it is the POSIX way to set a socket for nonblocking I/O, signal-driven I/O, and to set the owner of a socket. We save the ioctl function for Chapter 17.