6.1 Introduction

In Section 5.12, we saw our TCP client handling two inputs at the same time: standard input and a TCP socket. We encountered a problem when the client was blocked in a call to fgets (on standard input) and the server process was killed. The server TCP correctly sent a FIN to the client TCP, but since the client process was blocked reading from standard input, it never saw the EOF until it read from the socket (possibly much later). What we need is the capability to tell the kernel that we want to be notified if one or more I/O conditions are ready (i.e., input is ready to be read, or the descriptor is capable of taking more output). This capability is called I/O multiplexing and is provided by the select and poll functions. We will also cover a newer POSIX variation of the former, called pselect.

Some systems provide more advanced ways for processes to wait for a list of events. A poll device is one mechanism provided in different forms by different vendors. This mechanism will be described in Chapter 14.

I/O multiplexing is typically used in networking applications in the following scenarios:

• When a client is handling multiple descriptors (normally interactive input and a network socket), I/O multiplexing should be used. This is the scenario we described previously.

• It is possible, but rare, for a client to handle multiple sockets at the same time. We will show an example of this using select in Section 16.5 in the context of a Web client.

• If a TCP server handles both a listening socket and its connected sockets, I/O multiplexing is normally used, as we will show in Section 6.8.

• If a server handles both TCP and UDP, I/O multiplexing is normally used. We will show an example of this in Section 8.15.

• If a server handles multiple services and perhaps multiple protocols (e.g., the inetd daemon that we will describe in Section 13.5), I/O multiplexing is normally used.

I/O multiplexing is not limited to network programming. Many nontrivial applications find a need for these techniques.