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 2014-05-10 "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) */"
41 .B #include <signal.h>
44 .BI "int ppoll(struct pollfd *" fds ", nfds_t " nfds ", "
45 .BI " const struct timespec *" timeout_ts ", const sigset_t *" sigmask );
49 performs a similar task to
51 it waits for one of a set of file descriptors to become ready
54 The set of file descriptors to be monitored is specified in the
56 argument, which is an array of structures of the following form:
61 int fd; /* file descriptor */
62 short events; /* requested events */
63 short revents; /* returned events */
68 The caller should specify the number of items in the
75 contains a file descriptor for an open file.
76 If this field is negative, then the corresponding
78 field is ignored and the
81 (This provides an easy way of ignoring a
82 file descriptor for a single
84 call: simply negate the
90 is an input parameter, a bit mask specifying the events the application
91 is interested in for the file descriptor
93 This field may be specified as zero,
94 in which case the only events that can be returned in
105 is an output parameter, filled by the kernel with the events that
109 can include any of those specified in
116 (These three bits are meaningless in the
118 field, and will be set in the
120 field whenever the corresponding condition is true.)
122 If none of the events requested (and no error) has occurred for any
123 of the file descriptors, then
125 blocks until one of the events occurs.
129 argument specifies the number of milliseconds that
131 should block waiting for a file descriptor to become ready.
132 The call will block until either:
134 a file descriptor becomes ready;
136 the call is interrupted by a signal handler; or
142 interval will be rounded up to the system clock granularity,
143 and kernel scheduling delays mean that the blocking interval
144 may overrun by a small amount.
145 Specifying a negative value in
147 means an infinite timeout.
152 to return immediately, even if no file descriptors are ready.
154 The bits that may be set/returned in
158 are defined in \fI<poll.h>\fP:
162 There is data to read.
165 There is urgent data to read (e.g., out-of-band data on TCP socket;
166 pseudoterminal master in packet mode has seen state change in slave).
169 Writing now will not block.
171 .BR POLLRDHUP " (since Linux 2.6.17)"
172 Stream socket peer closed connection,
173 or shut down writing half of connection.
176 feature test macro must be defined
180 in order to obtain this definition.
183 Error condition (output only).
186 Hang up (output only).
191 not open (output only).
196 defined, one also has the following,
197 which convey no further information beyond the bits listed above:
205 Priority band data can be read (generally unused on Linux).
206 .\" POLLRDBAND is used in the DECnet protocol.
213 Priority data may be written.
216 Linux also knows about, but does not use
219 The relationship between
223 is analogous to the relationship between
230 allows an application to safely wait until either a file descriptor
231 becomes ready or until a signal is caught.
233 Other than the difference in the precision of the
235 argument, the following
240 ready = ppoll(&fds, nfds, timeout_ts, &sigmask);
245 executing the following calls:
251 timeout = (timeout_ts == NULL) ? \-1 :
252 (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
253 sigprocmask(SIG_SETMASK, &sigmask, &origmask);
254 ready = poll(&fds, nfds, timeout);
255 sigprocmask(SIG_SETMASK, &origmask, NULL);
258 See the description of
260 for an explanation of why
266 argument is specified as NULL, then
267 no signal mask manipulation is performed
272 only in the precision of the
278 argument specifies an upper limit on the amount of time that
281 This argument is a pointer to a structure of the following form:
286 long tv_sec; /* seconds */
287 long tv_nsec; /* nanoseconds */
294 is specified as NULL, then
296 can block indefinitely.
298 On success, a positive number is returned; this is
299 the number of structures which have nonzero
301 fields (in other words, those descriptors with events or errors reported).
302 A value of 0 indicates that the call timed out and no file
303 descriptors were ready.
304 On error, \-1 is returned, and
306 is set appropriately.
310 The array given as argument was not contained in the calling program's
314 A signal occurred before any requested event; see
325 There was no space to allocate file descriptor tables.
329 system call was introduced in Linux 2.1.23.
330 On older kernels that lack this system call,
331 .\" library call was introduced in libc 5.4.28
332 the glibc (and the old Linux libc)
334 wrapper function provides emulation using
339 system call was added to Linux in kernel 2.6.16.
342 library call was added in glibc 2.4.
345 conforms to POSIX.1-2001.
348 .\" NetBSD 3.0 has a pollts() which is like Linux ppoll().
350 Some implementations define the nonstandard constant
352 with the value \-1 for use as a
356 This constant is not provided in glibc.
358 For a discussion of what may happen if a file descriptor being monitored by
360 is closed in another thread, see
365 system call modifies its
368 However, the glibc wrapper function hides this behavior
369 by using a local variable for the timeout argument that
370 is passed to the system call.
373 function does not modify its
377 See the discussion of spurious readiness notifications under the
381 .BR restart_syscall (2),
386 This page is part of release 3.68 of the Linux
389 A description of the project,
390 information about reporting bugs,
391 and the latest version of this page,
393 \%http://www.kernel.org/doc/man\-pages/.