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.

To enable asynchronous I/O for a STREAMS device, we have to call ioctl with a second argument (request) of I_SETSIG. The third argument is an integer value formed from one or more of the constants in Figure 14.26. These constants are defined in <stropts.h>.

Figure 14.26. Conditions for generating SIGPOLL signal

Constant	Description
S_INPUT	A message other than a high-priority message has arrived.
S_RDNORM	An ordinary message has arrived.
S_RDBAND	A message with a nonzero priority band has arrived.
S_BANDURG	If this constant is specified with S_RDBAND, the SIGURG signal is generated instead of SIGPOLL when a nonzero priority band message has arrived.
S_HIPRI	A high-priority message has arrived.
S_OUTPUT	The write queue is no longer full.
S_WRNORM	Same as S_OUTPUT.
S_WRBAND	We can send a nonzero priority band message.
S_MSG	A STREAMS signal message that contains the SIGPOLL signal has arrived.
S_ERROR	An M_ERROR message has arrived.
S_HANGUP	An M_HANGUP message has arrived.

In Figure 14.26, whenever we say "has arrived," we mean "has arrived at the stream head's read queue."

In addition to calling ioctl to specify the conditions that should generate the SIGPOLL signal, we also have to establish a signal handler for this signal. Recall from 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.