This section describes the synchronous polling mechanism and asynchronous event notification in STREAMS.
User processes can efficiently monitor and control multiple streams with two system calls: poll(2) and the I_SETSIG ioctl(2) command. These calls enable a user process to detect events that occur at the stream head on one or more streams, including receipt of data or messages on the read queue and cessation of flow control on the write queue. Note that poll(2) is usable on any character device file descriptor, not just STREAMS.
To monitor streams with poll(2), a user process issues that system call and specifies the streams and other files to be monitored, the events to check, and the amount of time to wait for an event. poll(2) blocks the process until the time expires or until an event occurs. If an event occurs, it returns the type of event and the descriptor on which the event occurred.
Instead of waiting for an event to occur, a user process can monitor one or more streams while processing other data. To do so, issue the I_SETSIG ioctl(2), specifying a stream and events (as with poll(2)). This ioctl(2) does not block the process and force the user process to wait for the event, but returns immediately and issues a signal when an event occurs. The process calls one of sigaction(2), signal(3c), or sigset(3C) to catch the resulting SIGPOLL signal.
If any selected event occurs on any of the selected streams, STREAMS sends SIGPOLL to all associated requesting processes. The processes have no information on what event occurred on what stream. A signaled process can get more information by calling poll(2).