1 .\" Hey Emacs! This file is -*- nroff -*- source.
3 .\" Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl)
4 .\" and Copyright (C) 2006, Michael Kerrisk <mtk.manpages@gmail.com>
6 .\" Permission is granted to make and distribute verbatim copies of this
7 .\" manual provided the copyright notice and this permission notice are
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15 .\" Since the Linux kernel and libraries are constantly changing, this
16 .\" manual page may be incorrect or out-of-date. The author(s) assume no
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20 .\" which is licensed free of charge, as they might when working
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26 .\" Additions from Richard Gooch <rgooch@atnf.CSIRO.AU> and aeb, 971207
27 .\" 2006-03-13, mtk, Added ppoll() + various other rewordings
28 .\" 2006-07-01, mtk, Added POLLRDHUP + various other wording and
29 .\" formatting changes.
31 .TH POLL 2 2010-09-20 "Linux" "Linux Programmer's Manual"
33 poll, ppoll \- wait for some event on a file descriptor
38 .BI "int poll(struct pollfd *" fds ", nfds_t " nfds ", int " timeout );
40 .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */"
43 .BI "int ppoll(struct pollfd *" fds ", nfds_t " nfds ", "
44 .BI " const struct timespec *" timeout_ts ", const sigset_t *" sigmask );
48 performs a similar task to
50 it waits for one of a set of file descriptors to become ready
53 The set of file descriptors to be monitored is specified in the
55 argument, which is an array of structures of the following form:
60 int fd; /* file descriptor */
61 short events; /* requested events */
62 short revents; /* returned events */
67 The caller should specify the number of items in the
74 contains a file descriptor for an open file.
78 is an input parameter, a bit mask specifying the events the application
83 is an output parameter, filled by the kernel with the events that
87 can include any of those specified in
94 (These three bits are meaningless in the
96 field, and will be set in the
98 field whenever the corresponding condition is true.)
100 If none of the events requested (and no error) has occurred for any
101 of the file descriptors, then
103 blocks until one of the events occurs.
107 argument specifies an upper limit on the time for which
109 will block, in milliseconds.
110 Specifying a negative value in
112 means an infinite timeout.
114 The bits that may be set/returned in
118 are defined in \fI<poll.h>\fP:
122 There is data to read.
125 There is urgent data to read (e.g., out-of-band data on TCP socket;
126 pseudoterminal master in packet mode has seen state change in slave).
129 Writing now will not block.
131 .BR POLLRDHUP " (since Linux 2.6.17)"
132 Stream socket peer closed connection,
133 or shut down writing half of connection.
136 feature test macro must be defined
140 in order to obtain this definition.
143 Error condition (output only).
146 Hang up (output only).
151 not open (output only).
156 defined, one also has the following,
157 which convey no further information beyond the bits listed above:
165 Priority band data can be read (generally unused on Linux).
166 .\" POLLRDBAND is used in the DECnet protocol.
173 Priority data may be written.
176 Linux also knows about, but does not use
179 The relationship between
183 is analogous to the relationship between
190 allows an application to safely wait until either a file descriptor
191 becomes ready or until a signal is caught.
193 Other than the difference in the precision of the
195 argument, the following
200 ready = ppoll(&fds, nfds, timeout_ts, &sigmask);
205 executing the following calls:
211 timeout = (timeout_ts == NULL) ? \-1 :
212 (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
213 sigprocmask(SIG_SETMASK, &sigmask, &origmask);
214 ready = poll(&fds, nfds, timeout);
215 sigprocmask(SIG_SETMASK, &origmask, NULL);
218 See the description of
220 for an explanation of why
226 argument is specified as NULL, then
227 no signal mask manipulation is performed
232 only in the precision of the
238 argument specifies an upper limit on the amount of time that
241 This argument is a pointer to a structure of the following form:
246 long tv_sec; /* seconds */
247 long tv_nsec; /* nanoseconds */
254 is specified as NULL, then
256 can block indefinitely.
258 On success, a positive number is returned; this is
259 the number of structures which have nonzero
261 fields (in other words, those descriptors with events or errors reported).
262 A value of 0 indicates that the call timed out and no file
263 descriptors were ready.
264 On error, \-1 is returned, and
266 is set appropriately.
270 The array given as argument was not contained in the calling program's
274 A signal occurred before any requested event; see
285 There was no space to allocate file descriptor tables.
289 system call was introduced in Linux 2.1.23.
292 library call was introduced in libc 5.4.28
293 (and provides emulation using
295 if your kernel does not
302 system call was added to Linux in kernel 2.6.16.
305 library call was added in glibc 2.4.
308 conforms to POSIX.1-2001.
311 .\" NetBSD 3.0 has a pollts() which is like Linux ppoll().
313 Some implementations define the nonstandard constant
315 with the value \-1 for use as a
319 This constant is not provided in glibc.
323 system call modifies its
326 However, the glibc wrapper function hides this behavior
327 by using a local variable for the timeout argument that
328 is passed to the system call.
331 function does not modify its
335 See the discussion of spurious readiness notifications under the