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| >Video for Linux Two API Specification: Revision 0.24</TH |
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| >V4L2 poll()</H1 |
| ><DIV |
| CLASS="REFNAMEDIV" |
| ><A |
| NAME="AEN14173" |
| ></A |
| ><H2 |
| >Name</H2 |
| >v4l2-poll -- Wait for some event on a file descriptor</DIV |
| ><DIV |
| CLASS="REFSYNOPSISDIV" |
| ><A |
| NAME="AEN14176" |
| ></A |
| ><H2 |
| >Synopsis</H2 |
| ><DIV |
| CLASS="FUNCSYNOPSIS" |
| ><P |
| ></P |
| ><A |
| NAME="AEN14177" |
| ></A |
| ><PRE |
| CLASS="FUNCSYNOPSISINFO" |
| >#include <sys/poll.h></PRE |
| ><P |
| ><CODE |
| ><CODE |
| CLASS="FUNCDEF" |
| >int poll</CODE |
| >(struct pollfd *ufds, unsigned int nfds, int timeout);</CODE |
| ></P |
| ><P |
| ></P |
| ></DIV |
| ></DIV |
| ><DIV |
| CLASS="REFSECT1" |
| ><A |
| NAME="AEN14188" |
| ></A |
| ><H2 |
| >Description</H2 |
| ><P |
| >With the <CODE |
| CLASS="FUNCTION" |
| >poll()</CODE |
| > function applications |
| can suspend execution until the driver has captured data or is ready |
| to accept data for output.</P |
| ><P |
| >When streaming I/O has been negotiated this function waits |
| until a buffer has been filled or displayed and can be dequeued with |
| the <A |
| HREF="r12878.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_DQBUF</CODE |
| ></A |
| > ioctl. When buffers are already in the outgoing |
| queue of the driver the function returns immediately.</P |
| ><P |
| >On success <CODE |
| CLASS="FUNCTION" |
| >poll()</CODE |
| > returns the number of |
| file descriptors that have been selected (that is, file descriptors |
| for which the <CODE |
| CLASS="STRUCTFIELD" |
| >revents</CODE |
| > field of the |
| respective <CODE |
| CLASS="STRUCTNAME" |
| >pollfd</CODE |
| > structure is non-zero). |
| Capture devices set the <CODE |
| CLASS="CONSTANT" |
| >POLLIN</CODE |
| > and |
| <CODE |
| CLASS="CONSTANT" |
| >POLLRDNORM</CODE |
| > flags in the |
| <CODE |
| CLASS="STRUCTFIELD" |
| >revents</CODE |
| > field, output devices the |
| <CODE |
| CLASS="CONSTANT" |
| >POLLOUT</CODE |
| > and <CODE |
| CLASS="CONSTANT" |
| >POLLWRNORM</CODE |
| > |
| flags. When the function timed out it returns a value of zero, on |
| failure it returns <SPAN |
| CLASS="RETURNVALUE" |
| >-1</SPAN |
| > and the |
| <CODE |
| CLASS="VARNAME" |
| >errno</CODE |
| > variable is set appropriately. When the |
| application did not call <A |
| HREF="r12878.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_QBUF</CODE |
| ></A |
| > or <A |
| HREF="r13817.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_STREAMON</CODE |
| ></A |
| > yet the |
| <CODE |
| CLASS="FUNCTION" |
| >poll()</CODE |
| > function succeeds, but sets the |
| <CODE |
| CLASS="CONSTANT" |
| >POLLERR</CODE |
| > flag in the |
| <CODE |
| CLASS="STRUCTFIELD" |
| >revents</CODE |
| > field.</P |
| ><P |
| >When use of the <CODE |
| CLASS="FUNCTION" |
| >read()</CODE |
| > function has |
| been negotiated and the driver does not capture yet, the |
| <CODE |
| CLASS="FUNCTION" |
| >poll</CODE |
| > function starts capturing. When that fails |
| it returns a <CODE |
| CLASS="CONSTANT" |
| >POLLERR</CODE |
| > as above. Otherwise it waits |
| until data has been captured and can be read. When the driver captures |
| continuously (as opposed to, for example, still images) the function |
| may return immediately.</P |
| ><P |
| >When use of the <CODE |
| CLASS="FUNCTION" |
| >write()</CODE |
| > function has |
| been negotiated the <CODE |
| CLASS="FUNCTION" |
| >poll</CODE |
| > function just waits |
| until the driver is ready for a non-blocking |
| <CODE |
| CLASS="FUNCTION" |
| >write()</CODE |
| > call.</P |
| ><P |
| >All drivers implementing the <CODE |
| CLASS="FUNCTION" |
| >read()</CODE |
| > or |
| <CODE |
| CLASS="FUNCTION" |
| >write()</CODE |
| > function or streaming I/O must also |
| support the <CODE |
| CLASS="FUNCTION" |
| >poll()</CODE |
| > function.</P |
| ><P |
| >For more details see the |
| <CODE |
| CLASS="FUNCTION" |
| >poll()</CODE |
| > manual page.</P |
| ></DIV |
| ><DIV |
| CLASS="REFSECT1" |
| ><A |
| NAME="AEN14227" |
| ></A |
| ><H2 |
| >Return Value</H2 |
| ><P |
| >On success, <CODE |
| CLASS="FUNCTION" |
| >poll()</CODE |
| > returns the number |
| structures which have non-zero <CODE |
| CLASS="STRUCTFIELD" |
| >revents</CODE |
| > |
| fields, or zero if the call timed out. On error |
| <SPAN |
| CLASS="RETURNVALUE" |
| >-1</SPAN |
| > is returned, and the |
| <CODE |
| CLASS="VARNAME" |
| >errno</CODE |
| > variable is set appropriately:</P |
| ><P |
| ></P |
| ><DIV |
| CLASS="VARIABLELIST" |
| ><DL |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EBADF</SPAN |
| ></DT |
| ><DD |
| ><P |
| >One or more of the <CODE |
| CLASS="PARAMETER" |
| >ufds</CODE |
| > members |
| specify an invalid file descriptor.</P |
| ></DD |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EBUSY</SPAN |
| ></DT |
| ><DD |
| ><P |
| >The driver does not support multiple read or write |
| streams and the device is already in use.</P |
| ></DD |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EFAULT</SPAN |
| ></DT |
| ><DD |
| ><P |
| ><CODE |
| CLASS="PARAMETER" |
| >ufds</CODE |
| > references an inaccessible |
| memory area.</P |
| ></DD |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EINTR</SPAN |
| ></DT |
| ><DD |
| ><P |
| >The call was interrupted by a signal.</P |
| ></DD |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EINVAL</SPAN |
| ></DT |
| ><DD |
| ><P |
| >The <CODE |
| CLASS="PARAMETER" |
| >nfds</CODE |
| > argument is greater |
| than <CODE |
| CLASS="CONSTANT" |
| >OPEN_MAX</CODE |
| >.</P |
| ></DD |
| ></DL |
| ></DIV |
| ></DIV |
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