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>
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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16 .\" manual page may be incorrect or out-of-date.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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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 2012-04-15 "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 an upper limit on the time for which
127 will block, in milliseconds.
128 Specifying a negative value in
130 means an infinite timeout.
132 The bits that may be set/returned in
136 are defined in \fI<poll.h>\fP:
140 There is data to read.
143 There is urgent data to read (e.g., out-of-band data on TCP socket;
144 pseudoterminal master in packet mode has seen state change in slave).
147 Writing now will not block.
149 .BR POLLRDHUP " (since Linux 2.6.17)"
150 Stream socket peer closed connection,
151 or shut down writing half of connection.
154 feature test macro must be defined
158 in order to obtain this definition.
161 Error condition (output only).
164 Hang up (output only).
169 not open (output only).
174 defined, one also has the following,
175 which convey no further information beyond the bits listed above:
183 Priority band data can be read (generally unused on Linux).
184 .\" POLLRDBAND is used in the DECnet protocol.
191 Priority data may be written.
194 Linux also knows about, but does not use
197 The relationship between
201 is analogous to the relationship between
208 allows an application to safely wait until either a file descriptor
209 becomes ready or until a signal is caught.
211 Other than the difference in the precision of the
213 argument, the following
218 ready = ppoll(&fds, nfds, timeout_ts, &sigmask);
223 executing the following calls:
229 timeout = (timeout_ts == NULL) ? \-1 :
230 (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
231 sigprocmask(SIG_SETMASK, &sigmask, &origmask);
232 ready = poll(&fds, nfds, timeout);
233 sigprocmask(SIG_SETMASK, &origmask, NULL);
236 See the description of
238 for an explanation of why
244 argument is specified as NULL, then
245 no signal mask manipulation is performed
250 only in the precision of the
256 argument specifies an upper limit on the amount of time that
259 This argument is a pointer to a structure of the following form:
264 long tv_sec; /* seconds */
265 long tv_nsec; /* nanoseconds */
272 is specified as NULL, then
274 can block indefinitely.
276 On success, a positive number is returned; this is
277 the number of structures which have nonzero
279 fields (in other words, those descriptors with events or errors reported).
280 A value of 0 indicates that the call timed out and no file
281 descriptors were ready.
282 On error, \-1 is returned, and
284 is set appropriately.
288 The array given as argument was not contained in the calling program's
292 A signal occurred before any requested event; see
303 There was no space to allocate file descriptor tables.
307 system call was introduced in Linux 2.1.23.
310 library call was introduced in libc 5.4.28
311 (and provides emulation using
313 if your kernel does not
320 system call was added to Linux in kernel 2.6.16.
323 library call was added in glibc 2.4.
326 conforms to POSIX.1-2001.
329 .\" NetBSD 3.0 has a pollts() which is like Linux ppoll().
331 Some implementations define the nonstandard constant
333 with the value \-1 for use as a
337 This constant is not provided in glibc.
341 system call modifies its
344 However, the glibc wrapper function hides this behavior
345 by using a local variable for the timeout argument that
346 is passed to the system call.
349 function does not modify its
353 See the discussion of spurious readiness notifications under the