1 '\" te
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6 .TH POLL 7D "April 9, 2016"
7 .SH NAME
8 poll \- driver for fast poll on many file descriptors
9 .SH SYNOPSIS
10 .LP
11 .nf
12 \fB#include <sys/devpoll.h>
13 int fd = open("/dev/poll", O_RDWR);
14 ssize_t n = write(int fd, struct pollfd buf[], int bufsize);
15 int n = ioctl(int fd, DP_POLL, struct dvpoll* arg);
16 int n = ioctl(int fd, DP_ISPOLLED, struct pollfd* pfd);\fR
17 .fi
18
19 .SH PARAMETERS
20 .ne 2
21 .na
22 \fB\fIfd\fR \fR
23 .ad
24 .RS 12n
25 Open file descriptor that refers to the \fB/dev/poll\fR driver.
26 .RE
27
28 .sp
29 .ne 2
30 .na
31 \fB\fIpath\fR \fR
32 .ad
33 .RS 12n
34 \fB/dev/poll\fR
35 .RE
36
37 .sp
38 .ne 2
39 .na
40 \fB\fIbuf\fR \fR
41 .ad
42 .RS 12n
43 Array of \fBpollfd\fR structures.
44 .RE
45
46 .sp
47 .ne 2
48 .na
49 \fB\fIbufsize\fR \fR
50 .ad
51 .RS 12n
52 Size of \fIbuf\fR in bytes.
53 .RE
54
55 .sp
56 .ne 2
57 .na
58 \fB\fIarg\fR \fR
59 .ad
60 .RS 12n
61 Pointer to \fBpollcall\fR structure.
62 .RE
63
64 .sp
65 .ne 2
66 .na
67 \fB\fIpfd\fR \fR
68 .ad
69 .RS 12n
70 Pointer to \fBpollfd\fR structure.
71 .RE
72
73 .SH DESCRIPTION
74 .LP
75 The \fB/dev/poll\fR driver is a special driver that enables you to monitor
76 multiple sets of polled file descriptors. By using the \fB/dev/poll\fR
77 driver, you can efficiently poll large numbers of file descriptors. Access to
78 the \fB/dev/poll\fR driver is provided through \fBopen\fR(2), \fBwrite\fR(2),
79 and \fBioctl(2)\fR system calls.
80 .sp
81 .LP
82 Writing an array of \fBpollfd\fR struct to the \fB/dev/poll\fR driver has the
83 effect of adding these file descriptors to the monitored \fBpoll\fR file
84 descriptor set represented by the \fIfd\fR. To monitor multiple file
85 descriptor sets, open the \fB/dev/poll\fR driver multiple times. Each \fBfd\fR
86 corresponds to one set. For each \fBpollfd\fR struct entry (defined in
87 \fBsys/poll.h\fR):
88 .sp
89 .in +2
90 .nf
91 struct pollfd {
92 int fd;
93 short events;
94 short revents;
95 }
96 .fi
97 .in -2
98
99 .sp
100 .LP
101 The \fBfd\fR field specifies the file descriptor being polled. The
102 \fBevents\fR field indicates the interested \fBpoll\fR \fBevents\fR on the file
103 descriptor. If a \fBpollfd\fR array contains multiple \fBpollfd\fR entries with
104 the same \fBfd\fR field, the "events" field in each \fBpollfd\fR entry is
105 OR'ed. A special \fBPOLLREMOVE\fR event in the \fBevents\fR field of the
106 \fBpollfd\fR structure removes the \fBfd\fR from the monitored set. The
107 \fBrevents\fR field is not used. Write returns the number of bytes written
108 successfully or \fB-1\fR when write fails.
109 .sp
110 .LP
111 The \fBDP_POLL\fR ioctl is used to retrieve returned \fBpoll\fR \fBevents\fR
112 occurred on the polled file descriptors in the monitored set represented by
113 \fIfd\fR. \fIarg\fR \fIis\fR \fIa\fR pointer to the devpoll structures which
114 are defined as follows:
115 .sp
116 .in +2
117 .nf
118 struct dvpoll {
119 struct pollfd* dp_fds;
120 int dp_nfds;
121 int dp_timeout;
122 }
123 .fi
124 .in -2
125
126 .sp
127 .LP
128 The \fBdp_fds\fR points to a buffer that holds an array of returned
129 \fBpollfd\fR structures. The \fBdp_nfds\fR field specifies the size of the
130 buffer in terms of the number of \fBpollfd\fR entries it contains. The
131 \fBdp_nfds\fR field also indicates the maximum number of file descriptors from
132 which poll information can be obtained. If there is no interested \fBevents\fR
133 on any of the polled file descriptors, the \fBDP_POLL\fR ioctl call will wait
134 \fBdp_timeout\fR milliseconds before returning. If \fBdp_timeout\fR is
135 \fB0\fR, the ioctl call returns immediately. If \fBdp_timeout\fR is \fB-1\fR,
136 the call blocks until an interested \fBpoll\fR \fBevents\fR is available or the
137 call is interrupted. Upon return, if the ioctl call has failed, \fB-1\fR is
138 returned. The memory content pointed by \fBdp_fds\fR is not modified. A return
139 value \fB0\fR means the ioctl is timed out. In this case, the memory content
140 pointed by \fBdp_fds\fR is not modified. If the call is successful, it returns
141 the number of valid \fBpollfd\fR entries in the array pointed by \fBdp_fds\fR;
142 the contents of the rest of the buffer is undefined. For each valid
143 \fBpollfd\fR entry, the \fBfd\fR field indicates the file desciptor on which
144 the polled \fBevents\fR happened. The \fBevents\fR field is the user specified
145 \fBpoll\fR \fBevents\fR. The \fBrevents\fR field contains the \fBevents\fR
146 occurred. \fB-1\fR is returned if the call fails.
147 .sp
148 .LP
149 \fBDP_ISPOLLED\fR ioctl allows you to query if a file descriptor is already in
150 the monitored set represented by \fBfd\fR. The \fBfd\fR field of the
151 \fBpollfd\fR structure indicates the file descriptor of interest. The
152 \fBDP_ISPOLLED\fR ioctl returns \fB1\fR if the file descriptor is in the set.
153 The \fBevents\fR field contains \fB0\fR. The \fBrevents\fR field contains the
154 currently polled \fBevents\fR. The ioctl returns \fB0\fR if the file
155 descriptor is not in the set. The \fBpollfd\fR structure pointed by \fIpfd\fR
156 is not modified. The ioctl returns a \fB-1\fR if the call fails.
157 .SH EXAMPLES
158 .LP
159 The following example shows how \fB/dev/poll\fR may be used.
160 .sp
161 .in +2
162 .nf
163 {
164 ...
165 /*
166 * open the driver
167 */
168 if ((wfd = open("/dev/poll", O_RDWR)) < 0) {
169 exit(-1);
170 }
171 pollfd = (struct pollfd* )malloc(sizeof(struct pollfd) * MAXBUF);
172 if (pollfd == NULL) {
173 close(wfd);
174 exit(-1);
175 }
176 /*
177 * initialize buffer
178 */
179 for (i = 0; i < MAXBUF; i++) {
180 pollfd[i].fd = fds[i];
181 pollfd[i].events = POLLIN;
182 pollfd[i].revents = 0;
183 }
184 if (write(wfd, &pollfd[0], sizeof(struct pollfd) * MAXBUF) !=
185 sizeof(struct pollfd) * MAXBUF) {
186 perror("failed to write all pollfds");
187 close (wfd);
188 free(pollfd);
189 exit(-1);
190 }
191 /*
192 * read from the devpoll driver
193 */
194 dopoll.dp_timeout = -1;
195 dopoll.dp_nfds = MAXBUF;
196 dopoll.dp_fds = pollfd;
197 result = ioctl(wfd, DP_POLL, &dopoll);
198 if (result < 0) {
199 perror("/dev/poll ioctl DP_POLL failed");
200 close (wfd);
201 free(pollfd);
202 exit(-1);
203 }
204 for (i = 0; i < result; i++) {
205 read(dopoll.dp_fds[i].fd, rbuf, STRLEN);
206 }
207 ...
208 }
209 .fi
210 .in -2
211
212 .sp
213 .LP
214 The following example is part of a test program which shows how
215 \fBDP_ISPOLLED()\fR ioctl may be used.
216 .sp
217 .in +2
218 .nf
219 {
220 ...
221
222 loopcnt = 0;
223 while (loopcnt < ITERATION) {
224 rn = random();
225 rn %= RANGE;
226 if (write(fds[rn], TESTSTRING, strlen(TESTSTRING)) !=
227 strlen(TESTSTRING)) {
228 perror("write to fifo failed.");
229 close (wfd);
230 free(pollfd);
231 error = 1;
232 goto out1;
233 }
234 dpfd.fd = fds[rn];
235 dpfd.events = 0;
236 dpfd.revents = 0;
237 result = ioctl(wfd, DP_ISPOLLED, &dpfd);
238 if (result < 0) {
239 perror("/dev/poll ioctl DP_ISPOLLED failed");
240 printf("errno = %d\en", errno);
241 close (wfd);
242 free(pollfd);
243 error = 1;
244 goto out1;
245 }
246 if (result != 1) {
247 printf("DP_ISPOLLED returned incorrect result: %d.\en",
248 result);
249 close (wfd);
250 free(pollfd);
251 error = 1;
252 goto out1;
253 }
254 if (dpfd.fd != fds[rn]) {
255 printf("DP_ISPOLLED returned wrong fd %d, expect %d\en",
256 dpfd.fd, fds[rn]);
257 close (wfd);
258 free(pollfd);
259 error = 1;
260 goto out1;
261 }
262 if (dpfd.revents != POLLIN) {
263 printf("DP_ISPOLLED returned unexpected revents %d\en",
264 dpfd.revents);
265 close (wfd);
266 free(pollfd);
267 error = 1;
268 goto out1;
269 }
270 if (read(dpfd.fd, rbuf, strlen(TESTSTRING)) !=
271 strlen(TESTSTRING)) {
272 perror("read from fifo failed");
273 close (wfd);
274 free(pollfd);
275 error = 1;
276 goto out1;
277 }
278 loopcnt++;
279 }
280
281 .fi
282 .in -2
283
284 .SH ERRORS
285 .ne 2
286 .na
287 \fB\fBEACCES\fR \fR
288 .ad
289 .RS 11n
290 A process does not have permission to access the content cached in
291 \fB/dev/poll\fR.
292 .RE
293
294 .sp
295 .ne 2
296 .na
297 \fB\fBEINTR\fR \fR
298 .ad
299 .RS 11n
300 A signal was caught during the execution of the \fBioctl\fR(2) function.
301 .RE
302
303 .sp
304 .ne 2
305 .na
306 \fB\fBEFAULT\fR \fR
307 .ad
308 .RS 11n
309 The request argument requires a data transfer to or from a buffer pointed to by
310 \fIarg\fR, but \fIarg\fR points to an illegal address.
311 .RE
312
313 .sp
314 .ne 2
315 .na
316 \fB\fBEINVAL\fR \fR
317 .ad
318 .RS 11n
319 The request or \fIarg\fR parameter is not valid for this device, or field of
320 the dvpoll struct pointed by \fIarg\fR is not valid (for example, when using
321 write/pwrite dp_nfds is greater than {OPEN_MAX}, or when using the DPPOLL ioctl
322 dp_nfds is greater than or equal to {OPEN_MAX}}.
323 .RE
324
325 .sp
326 .ne 2
327 .na
328 \fB\fBENXIO\fR \fR
329 .ad
330 .RS 11n
331 The \fBO_NONBLOCK\fR flag is set, the named file is a FIFO, the \fBO_WRONLY\fR
332 flag is set, and no process has the file open for reading; or the named file is
333 a character special or block special file and the device associated with this
334 special file does not exist.
335 .RE
336
337 .SH ATTRIBUTES
338 .LP
339 See \fBattributes\fR(5) for a description of the following attributes:
340 .sp
341
342 .sp
343 .TS
344 box;
345 l l
346 l l .
347 ATTRIBUTE TYPE ATTRIBUTE VALUE
348 Architecture SPARC, x86
349 Interface Stability Obsolete
350 MT-Level Safe
351 .TE
352
353 .SH SEE ALSO
354 .LP
355 \fBopen\fR(2), \fBpoll\fR(2), \fBwrite\fR(2), \fBattributes\fR(5)
356 .SH NOTES
357 .LP
358 The \fB/dev/poll\fR API is particularly beneficial to applications that poll a
359 large number of file descriptors repeatedly. Applications will exhibit the
360 best performance gain if the polled file descriptor list rarely change.
361 .sp
362 .LP
363 When using the \fB/dev/poll\fR driver, you should remove a closed file
364 descriptor from a monitored poll set. Failure to do so may result in a
365 \fBPOLLNVAL\fR \fBrevents\fR being returned for the closed file descriptor.
366 When a file descriptor is closed but not removed from the monitored set, and is
367 reused in subsequent open of a different device, you will be polling the device
368 associated with the reused file descriptor. In a multithreaded application,
369 careful coordination among threads doing close and \fBDP_POLL\fR ioctl is
370 recommended for consistent results.
371 .sp
372 .LP
373 The \fB/dev/poll\fR driver caches a list of polled file descriptors, which are
374 specific to a process. Therefore, the \fB/dev/poll\fR file descriptor of a
375 process will be inherited by its child process, just like any other file
376 descriptors. But the child process will have very limited access through this
377 inherited \fB/dev/poll\fR file descriptor. Any attempt to write or do ioctl by
378 the child process will result in an \fBEACCES\fR error. The child process
379 should close the inherited \fB/dev/poll\fR file descriptor and open its own if
380 desired.
381 .sp
382 .LP
383 The \fB/dev/poll\fR driver does not yet support polling. Polling on a
384 \fB/dev/poll\fR file descriptor will result in \fBPOLLERR\fR being returned in
385 the \fBrevents\fR field of \fBpollfd\fR structure.