1 .\" $NetBSD: poll.2,v 1.3 1996/09/07 21:53:08 mycroft Exp $
4 .\" Copyright (c) 1996 Charles M. Hannum. All rights reserved.
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36 .Nd synchronous I/O multiplexing
42 .Fn poll "struct pollfd fds[]" "nfds_t nfds" "int timeout"
45 .Fa "struct pollfd fds[]"
47 .Fa "const struct timespec * restrict timeout"
48 .Fa "const sigset_t * restrict newsigmask"
54 examines a set of file descriptors to see if some of them are ready for
58 argument is a pointer to an array of pollfd structures as defined in
63 argument determines the size of the
68 int fd; /* file descriptor */
69 short events; /* events to look for */
70 short revents; /* events returned */
77 .Bl -tag -width XXXrevents
79 File descriptor to poll.
80 If fd is equal to -1 then
82 is cleared (set to zero), and that pollfd is not checked.
87 Events which may occur.
95 have the following bits:
96 .Bl -tag -width XXXPOLLWRNORM
98 Data other than high priority data may be read without blocking.
100 Normal data may be read without blocking.
102 Data with a non-zero priority may be read without blocking.
104 High priority data may be read without blocking.
107 Normal data may be written without blocking.
109 Data with a non-zero priority may be written without blocking.
111 An exceptional condition has occurred on the device or socket.
113 flag is always checked, even if not present in the
117 The device or socket has been disconnected.
119 checked, even if not present in the
126 should never be present in the
128 bitmask at the same time.
130 The file descriptor is not open.
131 This flag is always checked, even
132 if not present in the
139 is neither zero nor INFTIM (-1), it specifies a maximum interval to
140 wait for any file descriptor to become ready, in milliseconds.
143 is INFTIM (-1), the poll blocks indefinitely.
148 will return without blocking.
154 is used to safely wait until either a set of file descriptors becomes
155 ready or until a signal is caught.
160 arguments are identical to the analogous arguments of
167 .Vt "const struct timespec"
170 (shown below) rather than the
174 A null pointer may be passed to indicate that
176 should wait indefinitely.
179 specifies a signal mask which is set while waiting for input.
182 returns, the original signal mask is restored.
185 time_t tv_sec; /* seconds */
186 long tv_nsec; /* and nanoseconds */
193 returns the number of descriptors that are ready for I/O, or -1 if an
195 If the time limit expires,
200 returns with an error,
201 including one due to an interrupted system call,
204 array will be unmodified.
206 This implementation differs from the historical one in that a given
207 file descriptor may not cause
209 to return with an error.
210 In cases where this would have happened in
211 the historical implementation (e.g.\& trying to poll a
213 descriptor), this implementation instead copies the
218 Attempting to perform I/O on this descriptor will then
220 This behaviour is believed to be more useful.
230 points outside the process's allocated address space.
232 A signal was delivered before the time limit expired and
233 before any of the selected events occurred.
235 The specified time limit is invalid. One of its components is negative or too large.
254 is not specified by POSIX.
260 This manual page and the core of the implementation was taken from
264 function first appeared in
267 The distinction between some of the fields in the
271 bitmasks is really not useful without STREAMS.
273 defined for compatibility with existing software.