Asynchronous Input and Output

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poll(2) enables a user to monitor multiple streams synchronously. poll(2) normally blocks until an event occurs on any of the polled file descriptors. In some applications, however, you want to process incoming data asynchronously. For example, an application can attempt to do some local processing and be interrupted when a pending event occurs. Some time-critical applications must not block, and must have immediate success or failure indication.

The I_SETSIG ioctl(2) (see streamio(7I)) is used to request that a SIGPOLL signal be sent to a user process when a specific event occurs. Table 3–5 lists events for I_SETSIG. These are similar to those described for poll(2).

Table 3-5 I_SETSIGioctl(2) Events

Event	Description
`S_INPUT`	A message other than a high-priority message has arrived on a stream head read queue. This event is maintained for compatibility with the previous releases of the Oracle Solaris operating environment.
`S_RDNORM`	A normal (nonpriority) message has arrived on the stream head read queue.
`S_RDBAND`	A priority message (band > 0) has arrived on the stream head read queue.
`S_HIPRI`	A high-priority message has arrived on the stream head read queue.
`S_OUTPUT`	A write queue for normal data (priority band = 0) is no longer full (not flow controlled). This notifies a user that there is space on the queue for sending or writing normal data downstream.
`S_WRNORM`	The same as `S_OUTPUT`.
`S_WRBAND`	A priority band greater than 0 of a queue downstream exists and is writable. This notifies a user that there is space on the queue for sending or writing priority data downstream.
`S_MSG`	A signal message sent from a module or driver has arrived on the stream head read queue.
`S_ERROR`	An error message reaches the stream head.
`S_HANGUP`	A hangup message sent from a module or driver has arrived at the stream head.
`S_BANDURG`	When used with `S_RDBAND`, `SIGURG` is generated instead of `SIGPOLL` when a priority message reaches the front of the stream head read queue.

S_INPUT, S_RDNORM, S_RDBAND, and S_HIPRI are set even if the message is of zero length. A user process can handle only high-priority messages by setting the arg to S_HIPRI.