From 849369d6c66d3054688672f97d31fceb8e8230fb Mon Sep 17 00:00:00 2001 From: root Date: Fri, 25 Dec 2015 04:40:36 +0000 Subject: initial_commit --- Documentation/DocBook/v4l/func-read.xml | 189 ++++++++++++++++++++++++++++++++ 1 file changed, 189 insertions(+) create mode 100644 Documentation/DocBook/v4l/func-read.xml (limited to 'Documentation/DocBook/v4l/func-read.xml') diff --git a/Documentation/DocBook/v4l/func-read.xml b/Documentation/DocBook/v4l/func-read.xml new file mode 100644 index 00000000..a5089bf8 --- /dev/null +++ b/Documentation/DocBook/v4l/func-read.xml @@ -0,0 +1,189 @@ + + + V4L2 read() + &manvol; + + + + v4l2-read + Read from a V4L2 device + + + + + #include <unistd.h> + + ssize_t read + int fd + void *buf + size_t count + + + + + + Arguments + + + + fd + + &fd; + + + + buf + + + + + + count + + + + + + + + + Description + + read() attempts to read up to +count bytes from file descriptor +fd into the buffer starting at +buf. The layout of the data in the buffer is +discussed in the respective device interface section, see ##. If count is zero, +read() returns zero and has no other results. If +count is greater than +SSIZE_MAX, the result is unspecified. Regardless +of the count value each +read() call will provide at most one frame (two +fields) worth of data. + + By default read() blocks until data +becomes available. When the O_NONBLOCK flag was +given to the &func-open; function it +returns immediately with an &EAGAIN; when no data is available. The +&func-select; or &func-poll; functions +can always be used to suspend execution until data becomes available. All +drivers supporting the read() function must also +support select() and +poll(). + + Drivers can implement read functionality in different +ways, using a single or multiple buffers and discarding the oldest or +newest frames once the internal buffers are filled. + + read() never returns a "snapshot" of a +buffer being filled. Using a single buffer the driver will stop +capturing when the application starts reading the buffer until the +read is finished. Thus only the period of the vertical blanking +interval is available for reading, or the capture rate must fall below +the nominal frame rate of the video standard. + +The behavior of +read() when called during the active picture +period or the vertical blanking separating the top and bottom field +depends on the discarding policy. A driver discarding the oldest +frames keeps capturing into an internal buffer, continuously +overwriting the previously, not read frame, and returns the frame +being received at the time of the read() call as +soon as it is complete. + + A driver discarding the newest frames stops capturing until +the next read() call. The frame being received at +read() time is discarded, returning the following +frame instead. Again this implies a reduction of the capture rate to +one half or less of the nominal frame rate. An example of this model +is the video read mode of the bttv driver, initiating a DMA to user +memory when read() is called and returning when +the DMA finished. + + In the multiple buffer model drivers maintain a ring of +internal buffers, automatically advancing to the next free buffer. +This allows continuous capturing when the application can empty the +buffers fast enough. Again, the behavior when the driver runs out of +free buffers depends on the discarding policy. + + Applications can get and set the number of buffers used +internally by the driver with the &VIDIOC-G-PARM; and &VIDIOC-S-PARM; +ioctls. They are optional, however. The discarding policy is not +reported and cannot be changed. For minimum requirements see . + + + + Return Value + + On success, the number of bytes read is returned. It is not +an error if this number is smaller than the number of bytes requested, +or the amount of data required for one frame. This may happen for +example because read() was interrupted by a +signal. On error, -1 is returned, and the errno +variable is set appropriately. In this case the next read will start +at the beginning of a new frame. Possible error codes are: + + + + EAGAIN + + Non-blocking I/O has been selected using +O_NONBLOCK and no data was immediately available for reading. + + + + EBADF + + fd is not a valid file +descriptor or is not open for reading, or the process already has the +maximum number of files open. + + + + EBUSY + + The driver does not support multiple read streams and the +device is already in use. + + + + EFAULT + + buf references an inaccessible +memory area. + + + + EINTR + + The call was interrupted by a signal before any +data was read. + + + + EIO + + I/O error. This indicates some hardware problem or a +failure to communicate with a remote device (USB camera etc.). + + + + EINVAL + + The read() function is not +supported by this driver, not on this device, or generally not on this +type of device. + + + + + + + -- cgit v1.2.3