1POLL(3P) POSIX Programmer's Manual POLL(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
13 poll — input/output multiplexing
14
16 #include <poll.h>
17 int poll(struct pollfd fds[], nfds_t nfds, int timeout);
19
21 The poll() function provides applications with a mechanism for multi‐
22 plexing input/output over a set of file descriptors. For each member of
23 the array pointed to by fds, poll() shall examine the given file
24 descriptor for the event(s) specified in events. The number of pollfd
25 structures in the fds array is specified by nfds. The poll() function
26 shall identify those file descriptors on which an application can read
27 or write data, or on which certain events have occurred.
28 The fds argument specifies the file descriptors to be examined and the
30 events of interest for each file descriptor. It is a pointer to an
31 array with one member for each open file descriptor of interest. The
32 array's members are pollfd structures within which fd specifies an open
33 file descriptor and events and revents are bitmasks constructed by
34 OR'ing a combination of the following event flags:
35 POLLIN Data other than high-priority data may be read without
37 blocking.
38 For STREAMS, this flag is set in revents even if the mes‐
40 sage is of zero length. This flag shall be equivalent to
41 POLLRDNORM | POLLRDBAND.
42 POLLRDNORM Normal data may be read without blocking.
44 For STREAMS, data on priority band 0 may be read without
46 blocking. This flag is set in revents even if the message
47 is of zero length.
48 POLLRDBAND Priority data may be read without blocking.
50 For STREAMS, data on priority bands greater than 0 may be
52 read without blocking. This flag is set in revents even if
53 the message is of zero length.
54 POLLPRI High-priority data may be read without blocking.
56 For STREAMS, this flag is set in revents even if the mes‐
58 sage is of zero length.
59 POLLOUT Normal data may be written without blocking.
61 For STREAMS, data on priority band 0 may be written without
63 blocking.
64 POLLWRNORM Equivalent to POLLOUT.
66 POLLWRBAND Priority data may be written.
68 For STREAMS, data on priority bands greater than 0 may be
70 written without blocking. If any priority band has been
71 written to on this STREAM, this event only examines bands
72 that have been written to at least once.
73 POLLERR An error has occurred on the device or stream. This flag is
75 only valid in the revents bitmask; it shall be ignored in
76 the events member.
77 POLLHUP A device has been disconnected, or a pipe or FIFO has been
79 closed by the last process that had it open for writing.
80 Once set, the hangup state of a FIFO shall persist until
81 some process opens the FIFO for writing or until all read-
82 only file descriptors for the FIFO are closed. This event
83 and POLLOUT are mutually-exclusive; a stream can never be
84 writable if a hangup has occurred. However, this event and
85 POLLIN, POLLRDNORM, POLLRDBAND, or POLLPRI are not mutu‐
86 ally-exclusive. This flag is only valid in the revents bit‐
87 mask; it shall be ignored in the events member.
88 POLLNVAL The specified fd value is invalid. This flag is only valid
90 in the revents member; it shall ignored in the events mem‐
91 ber.
92 The significance and semantics of normal, priority, and high-priority
94 data are file and device-specific.
95 If the value of fd is less than 0, events shall be ignored, and revents
97 shall be set to 0 in that entry on return from poll().
98 In each pollfd structure, poll() shall clear the revents member, except
100 that where the application requested a report on a condition by setting
101 one of the bits of events listed above, poll() shall set the corre‐
102 sponding bit in revents if the requested condition is true. In addi‐
103 tion, poll() shall set the POLLHUP, POLLERR, and POLLNVAL flag in
104 revents if the condition is true, even if the application did not set
105 the corresponding bit in events.
106 If none of the defined events have occurred on any selected file
108 descriptor, poll() shall wait at least timeout milliseconds for an
109 event to occur on any of the selected file descriptors. If the value of
110 timeout is 0, poll() shall return immediately. If the value of timeout
111 is −1, poll() shall block until a requested event occurs or until the
112 call is interrupted.
113 Implementations may place limitations on the granularity of timeout
115 intervals. If the requested timeout interval requires a finer granular‐
116 ity than the implementation supports, the actual timeout interval shall
117 be rounded up to the next supported value.
118 The poll() function shall not be affected by the O_NONBLOCK flag.
120 The poll() function shall support regular files, terminal and pseudo-
122 terminal devices, FIFOs, pipes, sockets and STREAMS-based files. The
123 behavior of poll() on elements of fds that refer to other types of file
124 is unspecified.
125 Regular files shall always poll TRUE for reading and writing.
127 A file descriptor for a socket that is listening for connections shall
129 indicate that it is ready for reading, once connections are available.
130 A file descriptor for a socket that is connecting asynchronously shall
131 indicate that it is ready for writing, once a connection has been
132 established.
133
135 Upon successful completion, poll() shall return a non-negative value. A
136 positive value indicates the total number of file descriptors that have
137 been selected (that is, file descriptors for which the revents member
138 is non-zero). A value of 0 indicates that the call timed out and no
139 file descriptors have been selected. Upon failure, poll() shall return
140 −1 and set errno to indicate the error.
141
143 The poll() function shall fail if:
144 EAGAIN The allocation of internal data structures failed but a subse‐
146 quent request may succeed.
147 EINTR A signal was caught during poll().
149 EINVAL The nfds argument is greater than {OPEN_MAX}, or one of the fd
151 members refers to a STREAM or multiplexer that is linked
152 (directly or indirectly) downstream from a multiplexer.
153 The following sections are informative.
155
157 Checking for Events on a Stream
158 The following example opens a pair of STREAMS devices and then waits
159 for either one to become writable. This example proceeds as follows:
160 1. Sets the timeout parameter to 500 milliseconds.
162 2. Opens the STREAMS devices /dev/dev0 and /dev/dev1, and then polls
164 them, specifying POLLOUT and POLLWRBAND as the events of interest.
165 The STREAMS device names /dev/dev0 and /dev/dev1 are only examples
167 of how STREAMS devices can be named; STREAMS naming conventions may
168 vary among systems conforming to the POSIX.1‐2008.
169 3. Uses the ret variable to determine whether an event has occurred on
171 either of the two STREAMS. The poll() function is given 500 mil‐
172 liseconds to wait for an event to occur (if it has not occurred
173 prior to the poll() call).
174 4. Checks the returned value of ret. If a positive value is returned,
176 one of the following can be done:
177 a. Priority data can be written to the open STREAM on priority
179 bands greater than 0, because the POLLWRBAND event occurred on
180 the open STREAM (fds[0] or fds[1]).
181 b. Data can be written to the open STREAM on priority-band 0,
183 because the POLLOUT event occurred on the open STREAM (fds[0]
184 or fds[1]).
185 5. If the returned value is not a positive value, permission to write
187 data to the open STREAM (on any priority band) is denied.
188 6. If the POLLHUP event occurs on the open STREAM (fds[0] or fds[1]),
190 the device on the open STREAM has disconnected.
191 #include <stropts.h>
193 #include <poll.h>
194 ...
195 struct pollfd fds[2];
196 int timeout_msecs = 500;
197 int ret;
198 int i;
199 /* Open STREAMS device. */
201 fds[0].fd = open("/dev/dev0", ...);
202 fds[1].fd = open("/dev/dev1", ...);
203 fds[0].events = POLLOUT | POLLWRBAND;
204 fds[1].events = POLLOUT | POLLWRBAND;
205 ret = poll(fds, 2, timeout_msecs);
207 if (ret > 0) {
209 /* An event on one of the fds has occurred. */
210 for (i=0; i<2; i++) {
211 if (fds[i].revents & POLLWRBAND) {
212 /* Priority data may be written on device number i. */
213 ...
214 }
215 if (fds[i].revents & POLLOUT) {
216 /* Data may be written on device number i. */
217 ...
218 }
219 if (fds[i].revents & POLLHUP) {
220 /* A hangup has occurred on device number i. */
221 ...
222 }
223 }
224 }
225
227 None.
228
230 The POLLHUP event does not occur for FIFOs just because the FIFO is not
231 open for writing. It only occurs when the FIFO is closed by the last
232 writer and persists until some process opens the FIFO for writing or
233 until all read-only file descriptors for the FIFO are closed.
234
236 None.
237
239 Section 2.6, STREAMS, getmsg(), pselect(), putmsg(), read(), write()
240 The Base Definitions volume of POSIX.1‐2008, <poll.h>, <stropts.h>
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244 Portions of this text are reprinted and reproduced in electronic form
245 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
246 -- Portable Operating System Interface (POSIX), The Open Group Base
247 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
248 cal and Electronics Engineers, Inc and The Open Group. (This is
249 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
250 event of any discrepancy between this version and the original IEEE and
251 The Open Group Standard, the original IEEE and The Open Group Standard
252 is the referee document. The original Standard can be obtained online
253 at http://www.unix.org/online.html .
254 Any typographical or formatting errors that appear in this page are
256 most likely to have been introduced during the conversion of the source
257 files to man page format. To report such errors, see https://www.ker‐
258 nel.org/doc/man-pages/reporting_bugs.html .
259IEEE/The Open Group 2013 POLL(3P)