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| >Video for Linux Two API Specification: Revision 0.24</TH |
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| ><H1 |
| ><A |
| NAME="VIDIOC-QBUF" |
| ></A |
| >ioctl VIDIOC_QBUF, VIDIOC_DQBUF</H1 |
| ><DIV |
| CLASS="REFNAMEDIV" |
| ><A |
| NAME="AEN12882" |
| ></A |
| ><H2 |
| >Name</H2 |
| >VIDIOC_QBUF, VIDIOC_DQBUF -- Exchange a buffer with the driver</DIV |
| ><DIV |
| CLASS="REFSYNOPSISDIV" |
| ><A |
| NAME="AEN12886" |
| ></A |
| ><H2 |
| >Synopsis</H2 |
| ><DIV |
| CLASS="FUNCSYNOPSIS" |
| ><P |
| ></P |
| ><A |
| NAME="AEN12887" |
| ></A |
| ><P |
| ><CODE |
| ><CODE |
| CLASS="FUNCDEF" |
| >int ioctl</CODE |
| >(int fd, int request, struct v4l2_buffer *argp);</CODE |
| ></P |
| ><P |
| ></P |
| ></DIV |
| ></DIV |
| ><DIV |
| CLASS="REFSECT1" |
| ><A |
| NAME="AEN12897" |
| ></A |
| ><H2 |
| >Arguments</H2 |
| ><P |
| ></P |
| ><DIV |
| CLASS="VARIABLELIST" |
| ><DL |
| ><DT |
| ><CODE |
| CLASS="PARAMETER" |
| >fd</CODE |
| ></DT |
| ><DD |
| ><P |
| >File descriptor returned by <A |
| HREF="r14090.htm" |
| ><CODE |
| CLASS="FUNCTION" |
| >open()</CODE |
| ></A |
| >.</P |
| ></DD |
| ><DT |
| ><CODE |
| CLASS="PARAMETER" |
| >request</CODE |
| ></DT |
| ><DD |
| ><P |
| >VIDIOC_QBUF, VIDIOC_DQBUF</P |
| ></DD |
| ><DT |
| ><CODE |
| CLASS="PARAMETER" |
| >argp</CODE |
| ></DT |
| ><DD |
| ><P |
| ></P |
| ></DD |
| ></DL |
| ></DIV |
| ></DIV |
| ><DIV |
| CLASS="REFSECT1" |
| ><A |
| NAME="AEN12917" |
| ></A |
| ><H2 |
| >Description</H2 |
| ><P |
| >Applications call the <CODE |
| CLASS="CONSTANT" |
| >VIDIOC_QBUF</CODE |
| > ioctl |
| to enqueue an empty (capturing) or filled (output) buffer in the |
| driver's incoming queue. The semantics depend on the selected I/O |
| method.</P |
| ><P |
| >To enqueue a <A |
| HREF="x5791.htm" |
| >memory mapped</A |
| > |
| buffer applications set the <CODE |
| CLASS="STRUCTFIELD" |
| >type</CODE |
| > field of a |
| struct <A |
| HREF="x5953.htm#V4L2-BUFFER" |
| >v4l2_buffer</A |
| > to the same buffer type as previously struct <A |
| HREF="r10944.htm#V4L2-FORMAT" |
| >v4l2_format</A |
| > |
| <CODE |
| CLASS="STRUCTFIELD" |
| >type</CODE |
| > and struct <A |
| HREF="r13696.htm#V4L2-REQUESTBUFFERS" |
| >v4l2_requestbuffers</A |
| > |
| <CODE |
| CLASS="STRUCTFIELD" |
| >type</CODE |
| >, the <CODE |
| CLASS="STRUCTFIELD" |
| >memory</CODE |
| > |
| field to <CODE |
| CLASS="CONSTANT" |
| >V4L2_MEMORY_MMAP</CODE |
| > and the |
| <CODE |
| CLASS="STRUCTFIELD" |
| >index</CODE |
| > field. Valid index numbers range from |
| zero to the number of buffers allocated with <A |
| HREF="r13696.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_REQBUFS</CODE |
| ></A |
| > |
| (struct <A |
| HREF="r13696.htm#V4L2-REQUESTBUFFERS" |
| >v4l2_requestbuffers</A |
| > <CODE |
| CLASS="STRUCTFIELD" |
| >count</CODE |
| >) minus one. The |
| contents of the struct <CODE |
| CLASS="STRUCTNAME" |
| >v4l2_buffer</CODE |
| > returned |
| by a <A |
| HREF="r13022.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_QUERYBUF</CODE |
| ></A |
| > ioctl will do as well. When the buffer is |
| intended for output (<CODE |
| CLASS="STRUCTFIELD" |
| >type</CODE |
| > is |
| <CODE |
| CLASS="CONSTANT" |
| >V4L2_BUF_TYPE_VIDEO_OUTPUT</CODE |
| > or |
| <CODE |
| CLASS="CONSTANT" |
| >V4L2_BUF_TYPE_VBI_OUTPUT</CODE |
| >) applications must also |
| initialize the <CODE |
| CLASS="STRUCTFIELD" |
| >bytesused</CODE |
| >, |
| <CODE |
| CLASS="STRUCTFIELD" |
| >field</CODE |
| > and |
| <CODE |
| CLASS="STRUCTFIELD" |
| >timestamp</CODE |
| > fields. See <A |
| HREF="x5953.htm" |
| >Section 3.5</A |
| > for details. When |
| <CODE |
| CLASS="CONSTANT" |
| >VIDIOC_QBUF</CODE |
| > is called with a pointer to this |
| structure the driver sets the |
| <CODE |
| CLASS="CONSTANT" |
| >V4L2_BUF_FLAG_MAPPED</CODE |
| > and |
| <CODE |
| CLASS="CONSTANT" |
| >V4L2_BUF_FLAG_QUEUED</CODE |
| > flags and clears the |
| <CODE |
| CLASS="CONSTANT" |
| >V4L2_BUF_FLAG_DONE</CODE |
| > flag in the |
| <CODE |
| CLASS="STRUCTFIELD" |
| >flags</CODE |
| > field, or it returns an |
| <SPAN |
| CLASS="ERRORCODE" |
| >EINVAL</SPAN |
| > error code.</P |
| ><P |
| >To enqueue a <A |
| HREF="x5884.htm" |
| >user pointer</A |
| > |
| buffer applications set the <CODE |
| CLASS="STRUCTFIELD" |
| >type</CODE |
| > field of a |
| struct <A |
| HREF="x5953.htm#V4L2-BUFFER" |
| >v4l2_buffer</A |
| > to the same buffer type as previously struct <A |
| HREF="r10944.htm#V4L2-FORMAT" |
| >v4l2_format</A |
| > |
| <CODE |
| CLASS="STRUCTFIELD" |
| >type</CODE |
| > and struct <A |
| HREF="r13696.htm#V4L2-REQUESTBUFFERS" |
| >v4l2_requestbuffers</A |
| > |
| <CODE |
| CLASS="STRUCTFIELD" |
| >type</CODE |
| >, the <CODE |
| CLASS="STRUCTFIELD" |
| >memory</CODE |
| > |
| field to <CODE |
| CLASS="CONSTANT" |
| >V4L2_MEMORY_USERPTR</CODE |
| > and the |
| <CODE |
| CLASS="STRUCTFIELD" |
| >m.userptr</CODE |
| > field to the address of the |
| buffer and <CODE |
| CLASS="STRUCTFIELD" |
| >length</CODE |
| > to its size. When the |
| buffer is intended for output additional fields must be set as above. |
| When <CODE |
| CLASS="CONSTANT" |
| >VIDIOC_QBUF</CODE |
| > is called with a pointer to this |
| structure the driver sets the <CODE |
| CLASS="CONSTANT" |
| >V4L2_BUF_FLAG_QUEUED</CODE |
| > |
| flag and clears the <CODE |
| CLASS="CONSTANT" |
| >V4L2_BUF_FLAG_MAPPED</CODE |
| > and |
| <CODE |
| CLASS="CONSTANT" |
| >V4L2_BUF_FLAG_DONE</CODE |
| > flags in the |
| <CODE |
| CLASS="STRUCTFIELD" |
| >flags</CODE |
| > field, or it returns an error code. |
| This ioctl locks the memory pages of the buffer in physical memory, |
| they cannot be swapped out to disk. Buffers remain locked until |
| dequeued, until the <A |
| HREF="r13817.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_STREAMOFF</CODE |
| ></A |
| > or <A |
| HREF="r13696.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_REQBUFS</CODE |
| ></A |
| > ioctl are |
| called, or until the device is closed.</P |
| ><P |
| >Applications call the <CODE |
| CLASS="CONSTANT" |
| >VIDIOC_DQBUF</CODE |
| > |
| ioctl to dequeue a filled (capturing) or displayed (output) buffer |
| from the driver's outgoing queue. They just set the |
| <CODE |
| CLASS="STRUCTFIELD" |
| >type</CODE |
| > and <CODE |
| CLASS="STRUCTFIELD" |
| >memory</CODE |
| > |
| fields of a struct <A |
| HREF="x5953.htm#V4L2-BUFFER" |
| >v4l2_buffer</A |
| > as above, when <CODE |
| CLASS="CONSTANT" |
| >VIDIOC_DQBUF</CODE |
| > |
| is called with a pointer to this structure the driver fills the |
| remaining fields or returns an error code.</P |
| ><P |
| >By default <CODE |
| CLASS="CONSTANT" |
| >VIDIOC_DQBUF</CODE |
| > blocks when no |
| buffer is in the outgoing queue. When the |
| <CODE |
| CLASS="CONSTANT" |
| >O_NONBLOCK</CODE |
| > flag was given to the <A |
| HREF="r14090.htm" |
| ><CODE |
| CLASS="FUNCTION" |
| >open()</CODE |
| ></A |
| > |
| function, <CODE |
| CLASS="CONSTANT" |
| >VIDIOC_DQBUF</CODE |
| > returns immediately |
| with an <SPAN |
| CLASS="ERRORCODE" |
| >EAGAIN</SPAN |
| > error code when no buffer is available.</P |
| ><P |
| >The <CODE |
| CLASS="STRUCTNAME" |
| >v4l2_buffer</CODE |
| > structure is |
| specified in <A |
| HREF="x5953.htm" |
| >Section 3.5</A |
| >.</P |
| ></DIV |
| ><DIV |
| CLASS="REFSECT1" |
| ><A |
| NAME="AEN12989" |
| ></A |
| ><H2 |
| >Return Value</H2 |
| ><P |
| >On success <SPAN |
| CLASS="RETURNVALUE" |
| >0</SPAN |
| > is returned, on error <SPAN |
| CLASS="RETURNVALUE" |
| >-1</SPAN |
| > and the <CODE |
| CLASS="VARNAME" |
| >errno</CODE |
| > variable is set appropriately:</P |
| ><P |
| ></P |
| ><DIV |
| CLASS="VARIABLELIST" |
| ><DL |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EAGAIN</SPAN |
| ></DT |
| ><DD |
| ><P |
| >Non-blocking I/O has been selected using |
| <CODE |
| CLASS="CONSTANT" |
| >O_NONBLOCK</CODE |
| > and no buffer was in the outgoing |
| queue.</P |
| ></DD |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EINVAL</SPAN |
| ></DT |
| ><DD |
| ><P |
| >The buffer <CODE |
| CLASS="STRUCTFIELD" |
| >type</CODE |
| > is not |
| supported, or the <CODE |
| CLASS="STRUCTFIELD" |
| >index</CODE |
| > is out of bounds, |
| or no buffers have been allocated yet, or the |
| <CODE |
| CLASS="STRUCTFIELD" |
| >userptr</CODE |
| > or |
| <CODE |
| CLASS="STRUCTFIELD" |
| >length</CODE |
| > are invalid.</P |
| ></DD |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >ENOMEM</SPAN |
| ></DT |
| ><DD |
| ><P |
| >Not enough physical or virtual memory was available to |
| enqueue a user pointer buffer.</P |
| ></DD |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EIO</SPAN |
| ></DT |
| ><DD |
| ><P |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_DQBUF</CODE |
| > failed due to an |
| internal error. Can also indicate temporary problems like signal |
| loss. Note the driver might dequeue an (empty) buffer despite |
| returning an error, or even stop capturing.</P |
| ></DD |
| ></DL |
| ></DIV |
| ></DIV |
| ><DIV |
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