Advanced Programming in the UNIX Environment: Second Edition [Electronic resources] نسخه متنی

14.6. Asynchronous I/O

Using select and poll, as described in the previous section, is a synchronous form of notification. The system doesn't tell us anything until we ask (by calling either select or poll). As we saw in Chapter 10, signals provide an asynchronous form of notification that something has happened. All systems derived from BSD and System V provide some form of asynchronous I/O, using a signal (SIGPOLL in System V; SIGIO in BSD) to notify the process that something of interest has happened on a descriptor.

We saw that select and poll work with any descriptors. But with asynchronous I/O, we now encounter restrictions. On systems derived from System V, asynchronous I/O works only with STREAMS devices and STREAMS pipes. On systems derived from BSD, asynchronous I/O works only with terminals and networks.

One limitation of asynchronous I/O is that there is only one signal per process. If we enable more than one descriptor for asynchronous I/O, we cannot tell which descriptor the signal corresponds to when the signal is delivered.

The Single UNIX Specification includes an optional generic asynchronous I/O mechanism, adopted from the real-time draft standard. It is unrelated to the mechanisms we describe here. This mechanism solves a lot of the limitations that exist with these older asynchronous I/O mechanisms, but we will not discuss it further.

14.6.1. System V Asynchronous I/O

In System V, asynchronous I/O is part of the STREAMS system and works only with STREAMS devices and STREAMS pipes. The System V asynchronous I/O signal is SIGPOLL.

Figure 10.1 that the default action for SIGPOLL is to terminate the process, so we should establish the signal handler before calling ioctl.

14.6.2. BSD Asynchronous I/O

Asynchronous I/O in BSD-derived systems is a combination of two signals: SIGIO and SIGURG. The former is the general asynchronous I/O signal, and the latter is used only to notify the process that out-of-band data has arrived on a network connection.

To receive the SIGIO signal, we need to perform three steps.

Step 3 can be performed only on descriptors that refer to terminals or networks, which is a fundamental limitation of the BSD asynchronous I/O facility.

For the SIGURG signal, we need perform only steps 1 and 2. SIGURG is generated only for descriptors that refer to network connections that support out-of-band data.