1 .\" $NetBSD: poll.2,v 1.3 1996/09/07 21:53:08 mycroft Exp $
2 .\" $FreeBSD: src/lib/libc/sys/poll.2,v 1.4.2.3 2001/12/14 18:34:01 ru Exp $
4 .\" Copyright (c) 1996 Charles M. Hannum. All rights reserved.
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36 .Nd synchronous I/O multiplexing
43 .Fn poll "struct pollfd *fds" "unsigned int nfds" "int timeout"
46 examines a set of file descriptors to see if some of them are ready for
50 argument is a pointer to an array of pollfd structures as defined in
54 argument determines the size of the
59 int fd; /* file descriptor */
60 short events; /* events to look for */
61 short revents; /* events returned */
68 .Bl -tag -width XXXrevents
70 File descriptor to poll. If fd is equal to -1 then
72 is cleared (set to zero), and that pollfd is not checked.
74 Events to poll for. (See below.)
76 Events which may occur. (See below.)
83 have the following bits:
84 .Bl -tag -width XXXPOLLWRNORM
86 Data other than high priority data may be read without blocking.
88 Normal data may be read without blocking.
90 Data with a non-zero priority may be read without blocking.
92 High priority data may be read without blocking.
95 Normal data may be written without blocking.
97 Data with a non-zero priority may be written without blocking.
99 An exceptional condition has occurred on the device or socket. This
100 flag is always checked, even if not present in the
104 The device or socket has been disconnected. This flag is always
105 checked, even if not present in the
111 should never be present in the
113 bitmask at the same time.
115 The file descriptor is not open. This flag is always checked, even
116 if not present in the
123 is neither zero nor INFTIM (-1), it specifies a maximum interval to
124 wait for any file descriptor to become ready, in milliseconds. If
126 is INFTIM (-1), the poll blocks indefinitely. If
130 will return without blocking.
133 returns the number of descriptors that are ready for I/O, or -1 if an
134 error occured. If the time limit expires,
139 returns with an error,
140 including one due to an interrupted call,
143 array will be unmodified.
145 This implementation differs from the historical one in that a given
146 file descriptor may not cause
148 to return with an error. In cases where this would have happened in
149 the historical implementation (e.g. trying to poll a
151 descriptor), this implementation instead copies the
155 bitmask. Attempting to perform I/O on this descriptor will then
156 return an error. This behaviour is believed to be more useful.
164 points outside the process's allocated address space.
166 A signal was delivered before the time limit expired and
167 before any of the selected events occurred.
169 The specified time limit is negative.
180 The distinction between some of the fields in the
184 bitmasks is really not useful without STREAMS. The fields are
185 defined for compatibility with existing software.
189 function call appeared in
191 This manual page and the core of the implementation was taken from