1 .\" Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl)
2 .\" and Copyright (C) 2006, Michael Kerrisk <mtk.manpages@gmail.com>
4 .\" %%%LICENSE_START(VERBATIM)
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14 .\" Since the Linux kernel and libraries are constantly changing, this
15 .\" manual page may be incorrect or out-of-date. The author(s) assume no
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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 2012-08-17 "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.
75 If this field is negative, then the corresponding
77 field is ignored and the
80 (This provides an easy way of ignoring a
81 file descriptor for a single
83 call: simply negate the
89 is an input parameter, a bit mask specifying the events the application
90 is interested in for the file descriptor
92 If this field is specified as zero,
93 then all events are ignored for
101 is an output parameter, filled by the kernel with the events that
105 can include any of those specified in
112 (These three bits are meaningless in the
114 field, and will be set in the
116 field whenever the corresponding condition is true.)
118 If none of the events requested (and no error) has occurred for any
119 of the file descriptors, then
121 blocks until one of the events occurs.
125 argument specifies the minimum number of milliseconds that
128 (This interval will be rounded up to the system clock granularity,
129 and kernel scheduling delays mean that the blocking interval
130 may overrun by a small amount.)
131 Specifying a negative value in
133 means an infinite timeout.
138 to return immediately, even if no file descriptors are ready.
140 The bits that may be set/returned in
144 are defined in \fI<poll.h>\fP:
148 There is data to read.
151 There is urgent data to read (e.g., out-of-band data on TCP socket;
152 pseudoterminal master in packet mode has seen state change in slave).
155 Writing now will not block.
157 .BR POLLRDHUP " (since Linux 2.6.17)"
158 Stream socket peer closed connection,
159 or shut down writing half of connection.
162 feature test macro must be defined
166 in order to obtain this definition.
169 Error condition (output only).
172 Hang up (output only).
177 not open (output only).
182 defined, one also has the following,
183 which convey no further information beyond the bits listed above:
191 Priority band data can be read (generally unused on Linux).
192 .\" POLLRDBAND is used in the DECnet protocol.
199 Priority data may be written.
202 Linux also knows about, but does not use
205 The relationship between
209 is analogous to the relationship between
216 allows an application to safely wait until either a file descriptor
217 becomes ready or until a signal is caught.
219 Other than the difference in the precision of the
221 argument, the following
226 ready = ppoll(&fds, nfds, timeout_ts, &sigmask);
231 executing the following calls:
237 timeout = (timeout_ts == NULL) ? \-1 :
238 (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
239 sigprocmask(SIG_SETMASK, &sigmask, &origmask);
240 ready = poll(&fds, nfds, timeout);
241 sigprocmask(SIG_SETMASK, &origmask, NULL);
244 See the description of
246 for an explanation of why
252 argument is specified as NULL, then
253 no signal mask manipulation is performed
258 only in the precision of the
264 argument specifies an upper limit on the amount of time that
267 This argument is a pointer to a structure of the following form:
272 long tv_sec; /* seconds */
273 long tv_nsec; /* nanoseconds */
280 is specified as NULL, then
282 can block indefinitely.
284 On success, a positive number is returned; this is
285 the number of structures which have nonzero
287 fields (in other words, those descriptors with events or errors reported).
288 A value of 0 indicates that the call timed out and no file
289 descriptors were ready.
290 On error, \-1 is returned, and
292 is set appropriately.
296 The array given as argument was not contained in the calling program's
300 A signal occurred before any requested event; see
311 There was no space to allocate file descriptor tables.
315 system call was introduced in Linux 2.1.23.
316 On older kernels that lack this system call,
317 .\" library call was introduced in libc 5.4.28
318 the glibc (and the old Linux libc)
320 wrapper function provides emulation using
325 system call was added to Linux in kernel 2.6.16.
328 library call was added in glibc 2.4.
331 conforms to POSIX.1-2001.
334 .\" NetBSD 3.0 has a pollts() which is like Linux ppoll().
336 Some implementations define the nonstandard constant
338 with the value \-1 for use as a
342 This constant is not provided in glibc.
344 For a discussion of what may happen if a file descriptor being monitored by
346 is closed in another thread, see
351 system call modifies its
354 However, the glibc wrapper function hides this behavior
355 by using a local variable for the timeout argument that
356 is passed to the system call.
359 function does not modify its
363 See the discussion of spurious readiness notifications under the