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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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 2014-01-31 "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 This field may be specified as zero,
93 in which case the only events that can be returned in
104 is an output parameter, filled by the kernel with the events that
108 can include any of those specified in
115 (These three bits are meaningless in the
117 field, and will be set in the
119 field whenever the corresponding condition is true.)
121 If none of the events requested (and no error) has occurred for any
122 of the file descriptors, then
124 blocks until one of the events occurs.
128 argument specifies the number of milliseconds that
130 should block waiting for a file descriptor to become ready.
131 The call will block until either:
133 a file descriptor becomes ready;
135 the call is interrupted by a signal handler; or
141 interval will be rounded up to the system clock granularity,
142 and kernel scheduling delays mean that the blocking interval
143 may overrun by a small amount.
144 Specifying a negative value in
146 means an infinite timeout.
151 to return immediately, even if no file descriptors are ready.
153 The bits that may be set/returned in
157 are defined in \fI<poll.h>\fP:
161 There is data to read.
164 There is urgent data to read (e.g., out-of-band data on TCP socket;
165 pseudoterminal master in packet mode has seen state change in slave).
168 Writing now will not block.
170 .BR POLLRDHUP " (since Linux 2.6.17)"
171 Stream socket peer closed connection,
172 or shut down writing half of connection.
175 feature test macro must be defined
179 in order to obtain this definition.
182 Error condition (output only).
185 Hang up (output only).
190 not open (output only).
195 defined, one also has the following,
196 which convey no further information beyond the bits listed above:
204 Priority band data can be read (generally unused on Linux).
205 .\" POLLRDBAND is used in the DECnet protocol.
212 Priority data may be written.
215 Linux also knows about, but does not use
218 The relationship between
222 is analogous to the relationship between
229 allows an application to safely wait until either a file descriptor
230 becomes ready or until a signal is caught.
232 Other than the difference in the precision of the
234 argument, the following
239 ready = ppoll(&fds, nfds, timeout_ts, &sigmask);
244 executing the following calls:
250 timeout = (timeout_ts == NULL) ? \-1 :
251 (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
252 sigprocmask(SIG_SETMASK, &sigmask, &origmask);
253 ready = poll(&fds, nfds, timeout);
254 sigprocmask(SIG_SETMASK, &origmask, NULL);
257 See the description of
259 for an explanation of why
265 argument is specified as NULL, then
266 no signal mask manipulation is performed
271 only in the precision of the
277 argument specifies an upper limit on the amount of time that
280 This argument is a pointer to a structure of the following form:
285 long tv_sec; /* seconds */
286 long tv_nsec; /* nanoseconds */
293 is specified as NULL, then
295 can block indefinitely.
297 On success, a positive number is returned; this is
298 the number of structures which have nonzero
300 fields (in other words, those descriptors with events or errors reported).
301 A value of 0 indicates that the call timed out and no file
302 descriptors were ready.
303 On error, \-1 is returned, and
305 is set appropriately.
309 The array given as argument was not contained in the calling program's
313 A signal occurred before any requested event; see
324 There was no space to allocate file descriptor tables.
328 system call was introduced in Linux 2.1.23.
329 On older kernels that lack this system call,
330 .\" library call was introduced in libc 5.4.28
331 the glibc (and the old Linux libc)
333 wrapper function provides emulation using
338 system call was added to Linux in kernel 2.6.16.
341 library call was added in glibc 2.4.
344 conforms to POSIX.1-2001.
347 .\" NetBSD 3.0 has a pollts() which is like Linux ppoll().
349 Some implementations define the nonstandard constant
351 with the value \-1 for use as a
355 This constant is not provided in glibc.
357 For a discussion of what may happen if a file descriptor being monitored by
359 is closed in another thread, see
364 system call modifies its
367 However, the glibc wrapper function hides this behavior
368 by using a local variable for the timeout argument that
369 is passed to the system call.
372 function does not modify its
376 See the discussion of spurious readiness notifications under the
380 .BR restart_syscall (2),
385 This page is part of release 3.65 of the Linux
388 A description of the project,
389 and information about reporting bugs,
391 \%http://www.kernel.org/doc/man\-pages/.