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| >Video for Linux Two API Specification: Revision 0.24</TH |
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| ><H1 |
| ><A |
| NAME="FUNC-SELECT" |
| ></A |
| >V4L2 select()</H1 |
| ><DIV |
| CLASS="REFNAMEDIV" |
| ><A |
| NAME="AEN14394" |
| ></A |
| ><H2 |
| >Name</H2 |
| >v4l2-select -- Synchronous I/O multiplexing</DIV |
| ><DIV |
| CLASS="REFSYNOPSISDIV" |
| ><A |
| NAME="AEN14397" |
| ></A |
| ><H2 |
| >Synopsis</H2 |
| ><DIV |
| CLASS="FUNCSYNOPSIS" |
| ><P |
| ></P |
| ><A |
| NAME="AEN14398" |
| ></A |
| ><PRE |
| CLASS="FUNCSYNOPSISINFO" |
| >#include <sys/time.h> |
| #include <sys/types.h> |
| #include <unistd.h></PRE |
| ><P |
| ><CODE |
| ><CODE |
| CLASS="FUNCDEF" |
| >int select</CODE |
| >(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);</CODE |
| ></P |
| ><P |
| ></P |
| ></DIV |
| ></DIV |
| ><DIV |
| CLASS="REFSECT1" |
| ><A |
| NAME="AEN14413" |
| ></A |
| ><H2 |
| >Description</H2 |
| ><P |
| >With the <CODE |
| CLASS="FUNCTION" |
| >select()</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" |
| >select()</CODE |
| > returns the total |
| number of bits set in the <CODE |
| CLASS="STRUCTNAME" |
| >fd_set</CODE |
| >s. 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" |
| >select()</CODE |
| > function succeeds, setting the bit of |
| the file descriptor in <CODE |
| CLASS="PARAMETER" |
| >readfds</CODE |
| > or |
| <CODE |
| CLASS="PARAMETER" |
| >writefds</CODE |
| >, but subsequent <A |
| HREF="r12878.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_DQBUF</CODE |
| ></A |
| > calls |
| will fail.<A |
| NAME="AEN14434" |
| HREF="r14390.htm#FTN.AEN14434" |
| ><SPAN |
| CLASS="footnote" |
| >[1]</SPAN |
| ></A |
| ></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" |
| >select()</CODE |
| > function starts capturing. When that |
| fails, <CODE |
| CLASS="FUNCTION" |
| >select()</CODE |
| > returns successful and a |
| subsequent <CODE |
| CLASS="FUNCTION" |
| >read()</CODE |
| > call, which also attempts to |
| start capturing, will return an appropriate error code. When the |
| driver captures continuously (as opposed to, for example, still |
| images) and data is already available the |
| <CODE |
| CLASS="FUNCTION" |
| >select()</CODE |
| > function returns immediately.</P |
| ><P |
| >When use of the <CODE |
| CLASS="FUNCTION" |
| >write()</CODE |
| > function has |
| been negotiated the <CODE |
| CLASS="FUNCTION" |
| >select()</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" |
| >select()</CODE |
| > function.</P |
| ><P |
| >For more details see the <CODE |
| CLASS="FUNCTION" |
| >select()</CODE |
| > |
| manual page.</P |
| ></DIV |
| ><DIV |
| CLASS="REFSECT1" |
| ><A |
| NAME="AEN14457" |
| ></A |
| ><H2 |
| >Return Value</H2 |
| ><P |
| >On success, <CODE |
| CLASS="FUNCTION" |
| >select()</CODE |
| > returns the number |
| of descriptors contained in the three returned descriptor sets, which |
| will be zero if the timeout expired. On error |
| <SPAN |
| CLASS="RETURNVALUE" |
| >-1</SPAN |
| > is returned, and the |
| <CODE |
| CLASS="VARNAME" |
| >errno</CODE |
| > variable is set appropriately; the sets and |
| <CODE |
| CLASS="PARAMETER" |
| >timeout</CODE |
| > are undefined. Possible error codes |
| are:</P |
| ><P |
| ></P |
| ><DIV |
| CLASS="VARIABLELIST" |
| ><DL |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EBADF</SPAN |
| ></DT |
| ><DD |
| ><P |
| >One or more of the file descriptor sets specified a |
| file descriptor that is not open.</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 |
| >The <CODE |
| CLASS="PARAMETER" |
| >readfds</CODE |
| >, |
| <CODE |
| CLASS="PARAMETER" |
| >writefds</CODE |
| >, <CODE |
| CLASS="PARAMETER" |
| >exceptfds</CODE |
| > or |
| <CODE |
| CLASS="PARAMETER" |
| >timeout</CODE |
| > pointer 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 less than |
| zero or greater than <CODE |
| CLASS="CONSTANT" |
| >FD_SETSIZE</CODE |
| >.</P |
| ></DD |
| ></DL |
| ></DIV |
| ></DIV |
| ><H2 |
| CLASS="FOOTNOTES" |
| >Notes</H2 |
| ><TABLE |
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| ><A |
| NAME="FTN.AEN14434" |
| HREF="r14390.htm#AEN14434" |
| ><SPAN |
| CLASS="footnote" |
| >[1]</SPAN |
| ></A |
| ></TD |
| ><TD |
| ALIGN="LEFT" |
| VALIGN="TOP" |
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| ><P |
| >The Linux kernel implements |
| <CODE |
| CLASS="FUNCTION" |
| >select()</CODE |
| > like the <A |
| HREF="r14169.htm" |
| ><CODE |
| CLASS="FUNCTION" |
| >poll()</CODE |
| ></A |
| > function, but |
| <CODE |
| CLASS="FUNCTION" |
| >select()</CODE |
| > cannot return a |
| <CODE |
| CLASS="CONSTANT" |
| >POLLERR</CODE |
| >.</P |
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