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| >Video for Linux Two API Specification: Revision 0.24</TH |
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| ><H1 |
| ><A |
| NAME="FUNC-MMAP" |
| ></A |
| >V4L2 mmap()</H1 |
| ><DIV |
| CLASS="REFNAMEDIV" |
| ><A |
| NAME="AEN13893" |
| ></A |
| ><H2 |
| >Name</H2 |
| >v4l2-mmap -- Map device memory into application address space</DIV |
| ><DIV |
| CLASS="REFSYNOPSISDIV" |
| ><A |
| NAME="AEN13896" |
| ></A |
| ><H2 |
| >Synopsis</H2 |
| ><DIV |
| CLASS="FUNCSYNOPSIS" |
| ><P |
| ></P |
| ><A |
| NAME="AEN13897" |
| ></A |
| ><PRE |
| CLASS="FUNCSYNOPSISINFO" |
| >#include <unistd.h> |
| #include <sys/mman.h></PRE |
| ><P |
| ><CODE |
| ><CODE |
| CLASS="FUNCDEF" |
| >void *mmap</CODE |
| >(void *start, size_t length, int prot, int flags, int fd, off_t offset);</CODE |
| ></P |
| ><P |
| ></P |
| ></DIV |
| ></DIV |
| ><DIV |
| CLASS="REFSECT1" |
| ><A |
| NAME="AEN13914" |
| ></A |
| ><H2 |
| >Arguments</H2 |
| ><P |
| ></P |
| ><DIV |
| CLASS="VARIABLELIST" |
| ><DL |
| ><DT |
| ><CODE |
| CLASS="PARAMETER" |
| >start</CODE |
| ></DT |
| ><DD |
| ><P |
| >Map the buffer to this address in the |
| application's address space. When the <CODE |
| CLASS="CONSTANT" |
| >MAP_FIXED</CODE |
| > |
| flag is specified, <CODE |
| CLASS="PARAMETER" |
| >start</CODE |
| > must be a multiple of the |
| pagesize and mmap will fail when the specified address |
| cannot be used. Use of this option is discouraged; applications should |
| just specify a <CODE |
| CLASS="CONSTANT" |
| >NULL</CODE |
| > pointer here.</P |
| ></DD |
| ><DT |
| ><CODE |
| CLASS="PARAMETER" |
| >length</CODE |
| ></DT |
| ><DD |
| ><P |
| >Length of the memory area to map. This must be the |
| same value as returned by the driver in the struct <A |
| HREF="x5953.htm#V4L2-BUFFER" |
| >v4l2_buffer</A |
| > |
| <CODE |
| CLASS="STRUCTFIELD" |
| >length</CODE |
| > field.</P |
| ></DD |
| ><DT |
| ><CODE |
| CLASS="PARAMETER" |
| >prot</CODE |
| ></DT |
| ><DD |
| ><P |
| >The <CODE |
| CLASS="PARAMETER" |
| >prot</CODE |
| > argument describes the |
| desired memory protection. Regardless of the device type and the |
| direction of data exchange it should be set to |
| <CODE |
| CLASS="CONSTANT" |
| >PROT_READ</CODE |
| > | <CODE |
| CLASS="CONSTANT" |
| >PROT_WRITE</CODE |
| >, |
| permitting read and write access to image buffers. Drivers should |
| support at least this combination of flags. Note the Linux |
| <TT |
| CLASS="FILENAME" |
| >video-buf</TT |
| > kernel module, which is used by the |
| bttv, saa7134, saa7146, cx88 and vivi driver supports only |
| <CODE |
| CLASS="CONSTANT" |
| >PROT_READ</CODE |
| > | <CODE |
| CLASS="CONSTANT" |
| >PROT_WRITE</CODE |
| >. When |
| the driver does not support the desired protection the |
| <CODE |
| CLASS="FUNCTION" |
| >mmap()</CODE |
| > function fails.</P |
| ><P |
| >Note device memory accesses (e. g. the memory on a |
| graphics card with video capturing hardware) may incur a performance |
| penalty compared to main memory accesses, or reads may be |
| significantly slower than writes or vice versa. Other I/O methods may |
| be more efficient in this case.</P |
| ></DD |
| ><DT |
| ><CODE |
| CLASS="PARAMETER" |
| >flags</CODE |
| ></DT |
| ><DD |
| ><P |
| >The <CODE |
| CLASS="PARAMETER" |
| >flags</CODE |
| > parameter |
| specifies the type of the mapped object, mapping options and whether |
| modifications made to the mapped copy of the page are private to the |
| process or are to be shared with other references.</P |
| ><P |
| ><CODE |
| CLASS="CONSTANT" |
| >MAP_FIXED</CODE |
| > requests that the |
| driver selects no other address than the one specified. If the |
| specified address cannot be used, <CODE |
| CLASS="FUNCTION" |
| >mmap()</CODE |
| > will fail. If |
| <CODE |
| CLASS="CONSTANT" |
| >MAP_FIXED</CODE |
| > is specified, |
| <CODE |
| CLASS="PARAMETER" |
| >start</CODE |
| > must be a multiple of the pagesize. Use |
| of this option is discouraged.</P |
| ><P |
| >One of the <CODE |
| CLASS="CONSTANT" |
| >MAP_SHARED</CODE |
| > or |
| <CODE |
| CLASS="CONSTANT" |
| >MAP_PRIVATE</CODE |
| > flags must be set. |
| <CODE |
| CLASS="CONSTANT" |
| >MAP_SHARED</CODE |
| > allows applications to share the |
| mapped memory with other (e. g. child-) processes. Note the Linux |
| <TT |
| CLASS="FILENAME" |
| >video-buf</TT |
| > module which is used by the bttv, |
| saa7134, saa7146, cx88 and vivi driver supports only |
| <CODE |
| CLASS="CONSTANT" |
| >MAP_SHARED</CODE |
| >. <CODE |
| CLASS="CONSTANT" |
| >MAP_PRIVATE</CODE |
| > |
| requests copy-on-write semantics. V4L2 applications should not set the |
| <CODE |
| CLASS="CONSTANT" |
| >MAP_PRIVATE</CODE |
| >, <CODE |
| CLASS="CONSTANT" |
| >MAP_DENYWRITE</CODE |
| >, |
| <CODE |
| CLASS="CONSTANT" |
| >MAP_EXECUTABLE</CODE |
| > or <CODE |
| CLASS="CONSTANT" |
| >MAP_ANON</CODE |
| > |
| flag.</P |
| ></DD |
| ><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" |
| >offset</CODE |
| ></DT |
| ><DD |
| ><P |
| >Offset of the buffer in device memory. This must be the |
| same value as returned by the driver in the struct <A |
| HREF="x5953.htm#V4L2-BUFFER" |
| >v4l2_buffer</A |
| > |
| <CODE |
| CLASS="STRUCTFIELD" |
| >m</CODE |
| > union <CODE |
| CLASS="STRUCTFIELD" |
| >offset</CODE |
| > field.</P |
| ></DD |
| ></DL |
| ></DIV |
| ></DIV |
| ><DIV |
| CLASS="REFSECT1" |
| ><A |
| NAME="AEN13982" |
| ></A |
| ><H2 |
| >Description</H2 |
| ><P |
| >The <CODE |
| CLASS="FUNCTION" |
| >mmap()</CODE |
| > function asks to map |
| <CODE |
| CLASS="PARAMETER" |
| >length</CODE |
| > bytes starting at |
| <CODE |
| CLASS="PARAMETER" |
| >offset</CODE |
| > in the memory of the device specified by |
| <CODE |
| CLASS="PARAMETER" |
| >fd</CODE |
| > into the application address space, |
| preferably at address <CODE |
| CLASS="PARAMETER" |
| >start</CODE |
| >. This latter |
| address is a hint only, and is usually specified as 0.</P |
| ><P |
| >Suitable length and offset parameters are queried with the |
| <A |
| HREF="r13022.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_QUERYBUF</CODE |
| ></A |
| > ioctl. Buffers must be allocated with the |
| <A |
| HREF="r13696.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_REQBUFS</CODE |
| ></A |
| > ioctl before they can be queried.</P |
| ><P |
| >To unmap buffers the <A |
| HREF="r14037.htm" |
| ><CODE |
| CLASS="FUNCTION" |
| >munmap()</CODE |
| ></A |
| > function is used.</P |
| ></DIV |
| ><DIV |
| CLASS="REFSECT1" |
| ><A |
| NAME="AEN13998" |
| ></A |
| ><H2 |
| >Return Value</H2 |
| ><P |
| >On success <CODE |
| CLASS="FUNCTION" |
| >mmap()</CODE |
| > returns a pointer to |
| the mapped buffer. On error <CODE |
| CLASS="CONSTANT" |
| >MAP_FAILED</CODE |
| > (-1) is |
| returned, and the <CODE |
| CLASS="VARNAME" |
| >errno</CODE |
| > variable is set |
| appropriately. Possible error codes are:</P |
| ><P |
| ></P |
| ><DIV |
| CLASS="VARIABLELIST" |
| ><DL |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EBADF</SPAN |
| ></DT |
| ><DD |
| ><P |
| ><CODE |
| CLASS="PARAMETER" |
| >fd</CODE |
| > is not a valid file |
| descriptor.</P |
| ></DD |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EACCES</SPAN |
| ></DT |
| ><DD |
| ><P |
| ><CODE |
| CLASS="PARAMETER" |
| >fd</CODE |
| > is |
| not open for reading and writing.</P |
| ></DD |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >EINVAL</SPAN |
| ></DT |
| ><DD |
| ><P |
| >The <CODE |
| CLASS="PARAMETER" |
| >start</CODE |
| > or |
| <CODE |
| CLASS="PARAMETER" |
| >length</CODE |
| > or <CODE |
| CLASS="PARAMETER" |
| >offset</CODE |
| > are not |
| suitable. (E. g. they are too large, or not aligned on a |
| <CODE |
| CLASS="CONSTANT" |
| >PAGESIZE</CODE |
| > boundary.)</P |
| ><P |
| >The <CODE |
| CLASS="PARAMETER" |
| >flags</CODE |
| > or |
| <CODE |
| CLASS="PARAMETER" |
| >prot</CODE |
| > value is not supported.</P |
| ><P |
| >No buffers have been allocated with the |
| <A |
| HREF="r13696.htm" |
| ><CODE |
| CLASS="CONSTANT" |
| >VIDIOC_REQBUFS</CODE |
| ></A |
| > ioctl.</P |
| ></DD |
| ><DT |
| ><SPAN |
| CLASS="ERRORCODE" |
| >ENOMEM</SPAN |
| ></DT |
| ><DD |
| ><P |
| >Not enough physical or virtual memory was available to |
| complete the request.</P |
| ></DD |
| ></DL |
| ></DIV |
| ></DIV |
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