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 2014-09-06 "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
87 Note, however, that this technique can't be used to ignore file descriptor 0.)
91 is an input parameter, a bit mask specifying the events the application
92 is interested in for the file descriptor
94 This field may be specified as zero,
95 in which case the only events that can be returned in
106 is an output parameter, filled by the kernel with the events that
110 can include any of those specified in
117 (These three bits are meaningless in the
119 field, and will be set in the
121 field whenever the corresponding condition is true.)
123 If none of the events requested (and no error) has occurred for any
124 of the file descriptors, then
126 blocks until one of the events occurs.
130 argument specifies the number of milliseconds that
132 should block waiting for a file descriptor to become ready.
133 The call will block until either:
135 a file descriptor becomes ready;
137 the call is interrupted by a signal handler; or
143 interval will be rounded up to the system clock granularity,
144 and kernel scheduling delays mean that the blocking interval
145 may overrun by a small amount.
146 Specifying a negative value in
148 means an infinite timeout.
153 to return immediately, even if no file descriptors are ready.
155 The bits that may be set/returned in
159 are defined in \fI<poll.h>\fP:
163 There is data to read.
166 There is urgent data to read (e.g., out-of-band data on TCP socket;
167 pseudoterminal master in packet mode has seen state change in slave).
170 Writing is now possible, though a write larger that the available space
171 in a socket or pipe will still block (unless
175 .BR POLLRDHUP " (since Linux 2.6.17)"
176 Stream socket peer closed connection,
177 or shut down writing half of connection.
180 feature test macro must be defined
184 in order to obtain this definition.
187 Error condition (output only).
190 Hang up (output only).
195 not open (output only).
200 defined, one also has the following,
201 which convey no further information beyond the bits listed above:
209 Priority band data can be read (generally unused on Linux).
210 .\" POLLRDBAND is used in the DECnet protocol.
217 Priority data may be written.
220 Linux also knows about, but does not use
223 The relationship between
227 is analogous to the relationship between
234 allows an application to safely wait until either a file descriptor
235 becomes ready or until a signal is caught.
237 Other than the difference in the precision of the
239 argument, the following
244 ready = ppoll(&fds, nfds, timeout_ts, &sigmask);
249 executing the following calls:
255 timeout = (timeout_ts == NULL) ? \-1 :
256 (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);
257 sigprocmask(SIG_SETMASK, &sigmask, &origmask);
258 ready = poll(&fds, nfds, timeout);
259 sigprocmask(SIG_SETMASK, &origmask, NULL);
262 See the description of
264 for an explanation of why
270 argument is specified as NULL, then
271 no signal mask manipulation is performed
276 only in the precision of the
282 argument specifies an upper limit on the amount of time that
285 This argument is a pointer to a structure of the following form:
290 long tv_sec; /* seconds */
291 long tv_nsec; /* nanoseconds */
298 is specified as NULL, then
300 can block indefinitely.
302 On success, a positive number is returned; this is
303 the number of structures which have nonzero
305 fields (in other words, those descriptors with events or errors reported).
306 A value of 0 indicates that the call timed out and no file
307 descriptors were ready.
308 On error, \-1 is returned, and
310 is set appropriately.
314 The array given as argument was not contained in the calling program's
318 A signal occurred before any requested event; see
329 There was no space to allocate file descriptor tables.
333 system call was introduced in Linux 2.1.23.
334 On older kernels that lack this system call,
335 .\" library call was introduced in libc 5.4.28
336 the glibc (and the old Linux libc)
338 wrapper function provides emulation using
343 system call was added to Linux in kernel 2.6.16.
346 library call was added in glibc 2.4.
349 conforms to POSIX.1-2001.
352 .\" NetBSD 3.0 has a pollts() which is like Linux ppoll().
354 Some implementations define the nonstandard constant
356 with the value \-1 for use as a
360 This constant is not provided in glibc.
362 For a discussion of what may happen if a file descriptor being monitored by
364 is closed in another thread, see
366 .SS C library/kernel ABI differences
369 system call modifies its
372 However, the glibc wrapper function hides this behavior
373 by using a local variable for the timeout argument that
374 is passed to the system call.
377 function does not modify its
383 system call has a fifth argument,
384 .IR "size_t sigsetsize" ,
385 which specifies the size in bytes of the
390 wrapper function specifies this argument as a fixed value
392 .IR sizeof(sigset_t) ).
394 See the discussion of spurious readiness notifications under the
398 .BR restart_syscall (2),
403 This page is part of release 3.78 of the Linux
406 A description of the project,
407 information about reporting bugs,
408 and the latest version of this page,
410 \%http://www.kernel.org/doc/man\-pages/.