1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN""http://www.w3.org/TR/html4/loose.dtd"> 2 <HTML 3 ><HEAD 4 ><TITLE 5 >V4L2 poll()</TITLE 6 ><META 7 NAME="GENERATOR" 8 CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK 9 REL="HOME" 10 TITLE="Video for Linux Two API Specification" 11 HREF="book1.htm"><LINK 12 REL="UP" 13 TITLE="Function Reference" 14 HREF="r7624.htm"><LINK 15 REL="PREVIOUS" 16 TITLE="V4L2 open()" 17 HREF="r14090.htm"><LINK 18 REL="NEXT" 19 TITLE="V4L2 read()" 20 HREF="r14264.htm"></HEAD 21 ><BODY 22 CLASS="REFENTRY" 23 BGCOLOR="#FFFFFF" 24 TEXT="#000000" 25 LINK="#0000FF" 26 VLINK="#840084" 27 ALINK="#0000FF" 28 ><DIV 29 CLASS="NAVHEADER" 30 ><TABLE 31 SUMMARY="Header navigation table" 32 WIDTH="100%" 33 BORDER="0" 34 CELLPADDING="0" 35 CELLSPACING="0" 36 ><TR 37 ><TH 38 COLSPAN="3" 39 ALIGN="center" 40 >Video for Linux Two API Specification: Revision 0.24</TH 41 ></TR 42 ><TR 43 ><TD 44 WIDTH="10%" 45 ALIGN="left" 46 VALIGN="bottom" 47 ><A 48 HREF="r14090.htm" 49 ACCESSKEY="P" 50 >Prev</A 51 ></TD 52 ><TD 53 WIDTH="80%" 54 ALIGN="center" 55 VALIGN="bottom" 56 ></TD 57 ><TD 58 WIDTH="10%" 59 ALIGN="right" 60 VALIGN="bottom" 61 ><A 62 HREF="r14264.htm" 63 ACCESSKEY="N" 64 >Next</A 65 ></TD 66 ></TR 67 ></TABLE 68 ><HR 69 ALIGN="LEFT" 70 WIDTH="100%"></DIV 71 ><H1 72 ><A 73 NAME="FUNC-POLL" 74 ></A 75 >V4L2 poll()</H1 76 ><DIV 77 CLASS="REFNAMEDIV" 78 ><A 79 NAME="AEN14173" 80 ></A 81 ><H2 82 >Name</H2 83 >v4l2-poll -- Wait for some event on a file descriptor</DIV 84 ><DIV 85 CLASS="REFSYNOPSISDIV" 86 ><A 87 NAME="AEN14176" 88 ></A 89 ><H2 90 >Synopsis</H2 91 ><DIV 92 CLASS="FUNCSYNOPSIS" 93 ><P 94 ></P 95 ><A 96 NAME="AEN14177" 97 ></A 98 ><PRE 99 CLASS="FUNCSYNOPSISINFO" 100 >#include <sys/poll.h></PRE 101 ><P 102 ><CODE 103 ><CODE 104 CLASS="FUNCDEF" 105 >int poll</CODE 106 >(struct pollfd *ufds, unsigned int nfds, int timeout);</CODE 107 ></P 108 ><P 109 ></P 110 ></DIV 111 ></DIV 112 ><DIV 113 CLASS="REFSECT1" 114 ><A 115 NAME="AEN14188" 116 ></A 117 ><H2 118 >Description</H2 119 ><P 120 >With the <CODE 121 CLASS="FUNCTION" 122 >poll()</CODE 123 > function applications 124 can suspend execution until the driver has captured data or is ready 125 to accept data for output.</P 126 ><P 127 >When streaming I/O has been negotiated this function waits 128 until a buffer has been filled or displayed and can be dequeued with 129 the <A 130 HREF="r12878.htm" 131 ><CODE 132 CLASS="CONSTANT" 133 >VIDIOC_DQBUF</CODE 134 ></A 135 > ioctl. When buffers are already in the outgoing 136 queue of the driver the function returns immediately.</P 137 ><P 138 >On success <CODE 139 CLASS="FUNCTION" 140 >poll()</CODE 141 > returns the number of 142 file descriptors that have been selected (that is, file descriptors 143 for which the <CODE 144 CLASS="STRUCTFIELD" 145 >revents</CODE 146 > field of the 147 respective <CODE 148 CLASS="STRUCTNAME" 149 >pollfd</CODE 150 > structure is non-zero). 151 Capture devices set the <CODE 152 CLASS="CONSTANT" 153 >POLLIN</CODE 154 > and 155 <CODE 156 CLASS="CONSTANT" 157 >POLLRDNORM</CODE 158 > flags in the 159 <CODE 160 CLASS="STRUCTFIELD" 161 >revents</CODE 162 > field, output devices the 163 <CODE 164 CLASS="CONSTANT" 165 >POLLOUT</CODE 166 > and <CODE 167 CLASS="CONSTANT" 168 >POLLWRNORM</CODE 169 > 170 flags. When the function timed out it returns a value of zero, on 171 failure it returns <SPAN 172 CLASS="RETURNVALUE" 173 >-1</SPAN 174 > and the 175 <CODE 176 CLASS="VARNAME" 177 >errno</CODE 178 > variable is set appropriately. When the 179 application did not call <A 180 HREF="r12878.htm" 181 ><CODE 182 CLASS="CONSTANT" 183 >VIDIOC_QBUF</CODE 184 ></A 185 > or <A 186 HREF="r13817.htm" 187 ><CODE 188 CLASS="CONSTANT" 189 >VIDIOC_STREAMON</CODE 190 ></A 191 > yet the 192 <CODE 193 CLASS="FUNCTION" 194 >poll()</CODE 195 > function succeeds, but sets the 196 <CODE 197 CLASS="CONSTANT" 198 >POLLERR</CODE 199 > flag in the 200 <CODE 201 CLASS="STRUCTFIELD" 202 >revents</CODE 203 > field.</P 204 ><P 205 >When use of the <CODE 206 CLASS="FUNCTION" 207 >read()</CODE 208 > function has 209 been negotiated and the driver does not capture yet, the 210 <CODE 211 CLASS="FUNCTION" 212 >poll</CODE 213 > function starts capturing. When that fails 214 it returns a <CODE 215 CLASS="CONSTANT" 216 >POLLERR</CODE 217 > as above. Otherwise it waits 218 until data has been captured and can be read. When the driver captures 219 continuously (as opposed to, for example, still images) the function 220 may return immediately.</P 221 ><P 222 >When use of the <CODE 223 CLASS="FUNCTION" 224 >write()</CODE 225 > function has 226 been negotiated the <CODE 227 CLASS="FUNCTION" 228 >poll</CODE 229 > function just waits 230 until the driver is ready for a non-blocking 231 <CODE 232 CLASS="FUNCTION" 233 >write()</CODE 234 > call.</P 235 ><P 236 >All drivers implementing the <CODE 237 CLASS="FUNCTION" 238 >read()</CODE 239 > or 240 <CODE 241 CLASS="FUNCTION" 242 >write()</CODE 243 > function or streaming I/O must also 244 support the <CODE 245 CLASS="FUNCTION" 246 >poll()</CODE 247 > function.</P 248 ><P 249 >For more details see the 250 <CODE 251 CLASS="FUNCTION" 252 >poll()</CODE 253 > manual page.</P 254 ></DIV 255 ><DIV 256 CLASS="REFSECT1" 257 ><A 258 NAME="AEN14227" 259 ></A 260 ><H2 261 >Return Value</H2 262 ><P 263 >On success, <CODE 264 CLASS="FUNCTION" 265 >poll()</CODE 266 > returns the number 267 structures which have non-zero <CODE 268 CLASS="STRUCTFIELD" 269 >revents</CODE 270 > 271 fields, or zero if the call timed out. On error 272 <SPAN 273 CLASS="RETURNVALUE" 274 >-1</SPAN 275 > is returned, and the 276 <CODE 277 CLASS="VARNAME" 278 >errno</CODE 279 > variable is set appropriately:</P 280 ><P 281 ></P 282 ><DIV 283 CLASS="VARIABLELIST" 284 ><DL 285 ><DT 286 ><SPAN 287 CLASS="ERRORCODE" 288 >EBADF</SPAN 289 ></DT 290 ><DD 291 ><P 292 >One or more of the <CODE 293 CLASS="PARAMETER" 294 >ufds</CODE 295 > members 296 specify an invalid file descriptor.</P 297 ></DD 298 ><DT 299 ><SPAN 300 CLASS="ERRORCODE" 301 >EBUSY</SPAN 302 ></DT 303 ><DD 304 ><P 305 >The driver does not support multiple read or write 306 streams and the device is already in use.</P 307 ></DD 308 ><DT 309 ><SPAN 310 CLASS="ERRORCODE" 311 >EFAULT</SPAN 312 ></DT 313 ><DD 314 ><P 315 ><CODE 316 CLASS="PARAMETER" 317 >ufds</CODE 318 > references an inaccessible 319 memory area.</P 320 ></DD 321 ><DT 322 ><SPAN 323 CLASS="ERRORCODE" 324 >EINTR</SPAN 325 ></DT 326 ><DD 327 ><P 328 >The call was interrupted by a signal.</P 329 ></DD 330 ><DT 331 ><SPAN 332 CLASS="ERRORCODE" 333 >EINVAL</SPAN 334 ></DT 335 ><DD 336 ><P 337 >The <CODE 338 CLASS="PARAMETER" 339 >nfds</CODE 340 > argument is greater 341 than <CODE 342 CLASS="CONSTANT" 343 >OPEN_MAX</CODE 344 >.</P 345 ></DD 346 ></DL 347 ></DIV 348 ></DIV 349 ><DIV 350 CLASS="NAVFOOTER" 351 ><HR 352 ALIGN="LEFT" 353 WIDTH="100%"><TABLE 354 SUMMARY="Footer navigation table" 355 WIDTH="100%" 356 BORDER="0" 357 CELLPADDING="0" 358 CELLSPACING="0" 359 ><TR 360 ><TD 361 WIDTH="33%" 362 ALIGN="left" 363 VALIGN="top" 364 ><A 365 HREF="r14090.htm" 366 ACCESSKEY="P" 367 >Prev</A 368 ></TD 369 ><TD 370 WIDTH="34%" 371 ALIGN="center" 372 VALIGN="top" 373 ><A 374 HREF="book1.htm" 375 ACCESSKEY="H" 376 >Home</A 377 ></TD 378 ><TD 379 WIDTH="33%" 380 ALIGN="right" 381 VALIGN="top" 382 ><A 383 HREF="r14264.htm" 384 ACCESSKEY="N" 385 >Next</A 386 ></TD 387 ></TR 388 ><TR 389 ><TD 390 WIDTH="33%" 391 ALIGN="left" 392 VALIGN="top" 393 >V4L2 open()</TD 394 ><TD 395 WIDTH="34%" 396 ALIGN="center" 397 VALIGN="top" 398 ><A 399 HREF="r7624.htm" 400 ACCESSKEY="U" 401 >Up</A 402 ></TD 403 ><TD 404 WIDTH="33%" 405 ALIGN="right" 406 VALIGN="top" 407 >V4L2 read()</TD 408 ></TR 409 ></TABLE 410 ></DIV 411 ></BODY 412 ></HTML 413 > 414
