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fea681da | 1 | .\" Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl) |
c11b1abf | 2 | .\" and Copyright (C) 2006, Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da | 3 | .\" |
93015253 | 4 | .\" %%%LICENSE_START(VERBATIM) |
fea681da MK |
5 | .\" Permission is granted to make and distribute verbatim copies of this |
6 | .\" manual provided the copyright notice and this permission notice are | |
7 | .\" preserved on all copies. | |
8 | .\" | |
9 | .\" Permission is granted to copy and distribute modified versions of this | |
10 | .\" manual under the conditions for verbatim copying, provided that the | |
11 | .\" entire resulting derived work is distributed under the terms of a | |
12 | .\" permission notice identical to this one. | |
c13182ef | 13 | .\" |
fea681da MK |
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15 | .\" manual page may be incorrect or out-of-date.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 | |
16 | .\" responsibility for errors or omissions, or for damages resulting from | |
17 | .\" the use of the information contained herein. The author(s) may not | |
18 | .\" have taken the same level of care in the production of this manual, | |
19 | .\" which is licensed free of charge, as they might when working | |
20 | .\" professionally. | |
c13182ef | 21 | .\" |
fea681da MK |
22 | .\" Formatted or processed versions of this manual, if unaccompanied by |
23 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 24 | .\" %%%LICENSE_END |
fea681da MK |
25 | .\" |
26 | .\" Additions from Richard Gooch <rgooch@atnf.CSIRO.AU> and aeb, 971207 | |
e755a587 | 27 | .\" 2006-03-13, mtk, Added ppoll() + various other rewordings |
23cb3cb5 MK |
28 | .\" 2006-07-01, mtk, Added POLLRDHUP + various other wording and |
29 | .\" formatting changes. | |
4b9d4c1a | 30 | .\" |
40df3d00 | 31 | .TH POLL 2 2014-01-31 "Linux" "Linux Programmer's Manual" |
fea681da | 32 | .SH NAME |
e755a587 | 33 | poll, ppoll \- wait for some event on a file descriptor |
fea681da | 34 | .SH SYNOPSIS |
e755a587 | 35 | .nf |
23cb3cb5 | 36 | .B #include <poll.h> |
fea681da | 37 | .sp |
e755a587 MK |
38 | .BI "int poll(struct pollfd *" fds ", nfds_t " nfds ", int " timeout ); |
39 | .sp | |
b80f966b | 40 | .BR "#define _GNU_SOURCE" " /* See feature_test_macros(7) */" |
23cb3cb5 | 41 | .B #include <poll.h> |
22ca838f | 42 | .sp |
e755a587 | 43 | .BI "int ppoll(struct pollfd *" fds ", nfds_t " nfds ", " |
b8c599cb | 44 | .BI " const struct timespec *" timeout_ts ", const sigset_t *" sigmask ); |
e755a587 | 45 | .fi |
fea681da | 46 | .SH DESCRIPTION |
96296ef0 | 47 | .BR poll () |
e755a587 MK |
48 | performs a similar task to |
49 | .BR select (2): | |
50 | it waits for one of a set of file descriptors to become ready | |
51 | to perform I/O. | |
52 | ||
53 | The set of file descriptors to be monitored is specified in the | |
54 | .I fds | |
d8dfbff9 | 55 | argument, which is an array of structures of the following form: |
088a639b | 56 | .in +4n |
fea681da | 57 | .nf |
96296ef0 | 58 | |
a08ea57c MK |
59 | struct pollfd { |
60 | int fd; /* file descriptor */ | |
61 | short events; /* requested events */ | |
62 | short revents; /* returned events */ | |
63 | }; | |
a08ea57c | 64 | .in |
d8dfbff9 MK |
65 | .fi |
66 | .PP | |
67 | The caller should specify the number of items in the | |
68 | .I fds | |
69 | array in | |
70 | .IR nfds . | |
71 | ||
fea681da MK |
72 | The field |
73 | .I fd | |
74 | contains a file descriptor for an open file. | |
096ca08a MK |
75 | If this field is negative, then the corresponding |
76 | .I events | |
77 | field is ignored and the | |
78 | .I revents | |
79 | field returns zero. | |
80 | (This provides an easy way of ignoring a | |
81 | file descriptor for a single | |
82 | .BR poll () | |
83 | call: simply negate the | |
84 | .I fd | |
85 | field.) | |
23cb3cb5 | 86 | |
fea681da MK |
87 | The field |
88 | .I events | |
10f5f294 | 89 | is an input parameter, a bit mask specifying the events the application |
b857fda5 MK |
90 | is interested in for the file descriptor |
91 | .IR fd . | |
210cded6 MK |
92 | This field may be specified as zero, |
93 | in which case the only events that can be returned in | |
b857fda5 | 94 | .I revents |
210cded6 MK |
95 | are |
96 | .BR POLLHUP , | |
97 | .BR POLLERR , | |
98 | and | |
99 | .B POLLNVAL | |
100 | (see below). | |
23cb3cb5 | 101 | |
fea681da MK |
102 | The field |
103 | .I revents | |
104 | is an output parameter, filled by the kernel with the events that | |
e755a587 MK |
105 | actually occurred. |
106 | The bits returned in | |
107 | .I revents | |
c13182ef | 108 | can include any of those specified in |
e755a587 MK |
109 | .IR events , |
110 | or one of the values | |
111 | .BR POLLERR , | |
112 | .BR POLLHUP , | |
fea681da MK |
113 | or |
114 | .BR POLLNVAL . | |
115 | (These three bits are meaningless in the | |
116 | .I events | |
117 | field, and will be set in the | |
118 | .I revents | |
119 | field whenever the corresponding condition is true.) | |
23cb3cb5 | 120 | |
fea681da | 121 | If none of the events requested (and no error) has occurred for any |
e755a587 MK |
122 | of the file descriptors, then |
123 | .BR poll () | |
124 | blocks until one of the events occurs. | |
96296ef0 | 125 | |
c13182ef | 126 | The |
e755a587 | 127 | .I timeout |
0d9101c4 | 128 | argument specifies the number of milliseconds that |
e755a587 | 129 | .BR poll () |
0d9101c4 | 130 | should block waiting for a file descriptor to become ready. |
40df3d00 MK |
131 | The call will block until either: |
132 | .IP * 3 | |
133 | a file descriptor becomes ready; | |
134 | .IP * | |
135 | the call is interrupted by a signal handler; or | |
136 | .IP * | |
216b9e61 | 137 | the timeout expires. |
40df3d00 MK |
138 | .PP |
139 | Note that the | |
140 | .I timeout | |
141 | interval will be rounded up to the system clock granularity, | |
19257d8f | 142 | and kernel scheduling delays mean that the blocking interval |
0d9101c4 | 143 | may overrun by a small amount. |
e755a587 MK |
144 | Specifying a negative value in |
145 | .I timeout | |
146 | means an infinite timeout. | |
77e74cf1 MK |
147 | Specifying a |
148 | .I timeout | |
149 | of zero causes | |
150 | .BR poll () | |
01d8b73f | 151 | to return immediately, even if no file descriptors are ready. |
23cb3cb5 MK |
152 | |
153 | The bits that may be set/returned in | |
154 | .I events | |
155 | and | |
156 | .I revents | |
c84371c6 | 157 | are defined in \fI<poll.h>\fP: |
23cb3cb5 MK |
158 | .RS |
159 | .TP | |
160 | .B POLLIN | |
161 | There is data to read. | |
162 | .TP | |
163 | .B POLLPRI | |
c13182ef | 164 | There is urgent data to read (e.g., out-of-band data on TCP socket; |
b218b023 | 165 | pseudoterminal master in packet mode has seen state change in slave). |
23cb3cb5 MK |
166 | .TP |
167 | .B POLLOUT | |
168 | Writing now will not block. | |
169 | .TP | |
170 | .BR POLLRDHUP " (since Linux 2.6.17)" | |
c13182ef | 171 | Stream socket peer closed connection, |
23cb3cb5 | 172 | or shut down writing half of connection. |
c13182ef | 173 | The |
23cb3cb5 | 174 | .B _GNU_SOURCE |
e417acb0 MK |
175 | feature test macro must be defined |
176 | (before including | |
177 | .I any | |
178 | header files) | |
179 | in order to obtain this definition. | |
23cb3cb5 MK |
180 | .TP |
181 | .B POLLERR | |
182 | Error condition (output only). | |
183 | .TP | |
184 | .B POLLHUP | |
185 | Hang up (output only). | |
186 | .TP | |
187 | .B POLLNVAL | |
c13182ef | 188 | Invalid request: |
23cb3cb5 MK |
189 | .I fd |
190 | not open (output only). | |
191 | .RE | |
192 | .PP | |
c13182ef | 193 | When compiling with |
23cb3cb5 | 194 | .B _XOPEN_SOURCE |
c13182ef | 195 | defined, one also has the following, |
23cb3cb5 MK |
196 | which convey no further information beyond the bits listed above: |
197 | .RS | |
198 | .TP | |
199 | .B POLLRDNORM | |
200 | Equivalent to | |
201 | .BR POLLIN . | |
202 | .TP | |
203 | .B POLLRDBAND | |
204 | Priority band data can be read (generally unused on Linux). | |
205 | .\" POLLRDBAND is used in the DECnet protocol. | |
206 | .TP | |
207 | .B POLLWRNORM | |
208 | Equivalent to | |
209 | .BR POLLOUT . | |
210 | .TP | |
211 | .B POLLWRBAND | |
212 | Priority data may be written. | |
213 | .RE | |
214 | .PP | |
c13182ef | 215 | Linux also knows about, but does not use |
23cb3cb5 | 216 | .BR POLLMSG . |
e755a587 | 217 | .SS ppoll() |
c13182ef | 218 | The relationship between |
e755a587 | 219 | .BR poll () |
c13182ef MK |
220 | and |
221 | .BR ppoll () | |
e755a587 | 222 | is analogous to the relationship between |
0bfa087b | 223 | .BR select (2) |
e755a587 | 224 | and |
0bfa087b | 225 | .BR pselect (2): |
e755a587 | 226 | like |
0bfa087b | 227 | .BR pselect (2), |
e755a587 MK |
228 | .BR ppoll () |
229 | allows an application to safely wait until either a file descriptor | |
230 | becomes ready or until a signal is caught. | |
231 | .PP | |
b8c599cb | 232 | Other than the difference in the precision of the |
39c05c26 YK |
233 | .I timeout |
234 | argument, the following | |
e755a587 MK |
235 | .BR ppoll () |
236 | call: | |
237 | .nf | |
238 | ||
b8c599cb | 239 | ready = ppoll(&fds, nfds, timeout_ts, &sigmask); |
e755a587 MK |
240 | |
241 | .fi | |
242 | is equivalent to | |
243 | .I atomically | |
244 | executing the following calls: | |
245 | .nf | |
246 | ||
247 | sigset_t origmask; | |
b8c599cb | 248 | int timeout; |
e755a587 | 249 | |
c3074d70 | 250 | timeout = (timeout_ts == NULL) ? \-1 : |
b8c599cb | 251 | (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000); |
e755a587 | 252 | sigprocmask(SIG_SETMASK, &sigmask, &origmask); |
00877d8f | 253 | ready = poll(&fds, nfds, timeout); |
e755a587 MK |
254 | sigprocmask(SIG_SETMASK, &origmask, NULL); |
255 | .fi | |
256 | .PP | |
257 | See the description of | |
258 | .BR pselect (2) | |
259 | for an explanation of why | |
260 | .BR ppoll () | |
261 | is necessary. | |
262 | ||
3ee89512 MK |
263 | If the |
264 | .I sigmask | |
265 | argument is specified as NULL, then | |
266 | no signal mask manipulation is performed | |
267 | (and thus | |
268 | .BR ppoll () | |
269 | differs from | |
270 | .BR poll () | |
79fd50e6 YK |
271 | only in the precision of the |
272 | .I timeout | |
273 | argument). | |
3ee89512 | 274 | |
e755a587 | 275 | The |
b8c599cb | 276 | .I timeout_ts |
c13182ef | 277 | argument specifies an upper limit on the amount of time that |
e755a587 MK |
278 | .BR ppoll () |
279 | will block. | |
280 | This argument is a pointer to a structure of the following form: | |
088a639b | 281 | .in +4n |
e755a587 MK |
282 | .nf |
283 | ||
284 | struct timespec { | |
285 | long tv_sec; /* seconds */ | |
286 | long tv_nsec; /* nanoseconds */ | |
287 | }; | |
288 | .fi | |
a08ea57c | 289 | .in |
e755a587 MK |
290 | |
291 | If | |
b8c599cb | 292 | .I timeout_ts |
c13182ef | 293 | is specified as NULL, then |
e755a587 MK |
294 | .BR ppoll () |
295 | can block indefinitely. | |
47297adb | 296 | .SH RETURN VALUE |
96296ef0 | 297 | On success, a positive number is returned; this is |
c7094399 | 298 | the number of structures which have nonzero |
fea681da MK |
299 | .I revents |
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 | |
c13182ef MK |
302 | descriptors were ready. |
303 | On error, \-1 is returned, and | |
fea681da MK |
304 | .I errno |
305 | is set appropriately. | |
306 | .SH ERRORS | |
307 | .TP | |
fea681da MK |
308 | .B EFAULT |
309 | The array given as argument was not contained in the calling program's | |
310 | address space. | |
311 | .TP | |
312 | .B EINTR | |
01538d0d MK |
313 | A signal occurred before any requested event; see |
314 | .BR signal (7). | |
fea681da MK |
315 | .TP |
316 | .B EINVAL | |
317 | The | |
318 | .I nfds | |
682edefb MK |
319 | value exceeds the |
320 | .B RLIMIT_NOFILE | |
321 | value. | |
fea681da MK |
322 | .TP |
323 | .B ENOMEM | |
324 | There was no space to allocate file descriptor tables. | |
e755a587 | 325 | .SH VERSIONS |
b5cc2ffb | 326 | The |
c13182ef | 327 | .BR poll () |
b5cc2ffb | 328 | system call was introduced in Linux 2.1.23. |
6f0b2c8c MK |
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) | |
1e321034 | 332 | .BR poll () |
6f0b2c8c MK |
333 | wrapper function provides emulation using |
334 | .BR select (2). | |
e755a587 MK |
335 | |
336 | The | |
337 | .BR ppoll () | |
338 | system call was added to Linux in kernel 2.6.16. | |
339 | The | |
340 | .BR ppoll () | |
341 | library call was added in glibc 2.4. | |
47297adb | 342 | .SH CONFORMING TO |
a1d5f77c MK |
343 | .BR poll () |
344 | conforms to POSIX.1-2001. | |
345 | .BR ppoll () | |
8382f16d | 346 | is Linux-specific. |
a1d5f77c | 347 | .\" NetBSD 3.0 has a pollts() which is like Linux ppoll(). |
e755a587 | 348 | .SH NOTES |
c8f2dd47 | 349 | Some implementations define the nonstandard constant |
c1164764 MK |
350 | .B INFTIM |
351 | with the value \-1 for use as a | |
b8c599cb MK |
352 | .IR timeout |
353 | for | |
ba2e3e9a | 354 | .BR poll (). |
c1164764 | 355 | This constant is not provided in glibc. |
f33050d6 MK |
356 | |
357 | For a discussion of what may happen if a file descriptor being monitored by | |
358 | .BR poll () | |
359 | is closed in another thread, see | |
360 | .BR select (2). | |
c634028a | 361 | .SS Linux notes |
4fb31341 MK |
362 | The Linux |
363 | .BR ppoll () | |
364 | system call modifies its | |
b8c599cb | 365 | .I timeout_ts |
4fb31341 | 366 | argument. |
d9bfdb9c | 367 | However, the glibc wrapper function hides this behavior |
4fb31341 MK |
368 | by using a local variable for the timeout argument that |
369 | is passed to the system call. | |
370 | Thus, the glibc | |
371 | .BR ppoll () | |
372 | function does not modify its | |
b8c599cb | 373 | .I timeout_ts |
4fb31341 | 374 | argument. |
a1d5f77c MK |
375 | .SH BUGS |
376 | See the discussion of spurious readiness notifications under the | |
377 | BUGS section of | |
378 | .BR select (2). | |
47297adb | 379 | .SH SEE ALSO |
d806bc05 | 380 | .BR restart_syscall (2), |
fea681da | 381 | .BR select (2), |
50e5322c | 382 | .BR select_tut (2), |
1d7c4d16 | 383 | .BR time (7) |