7.67. V4L2 read()¶
v4l2-read - Read from a V4L2 device
- File descriptor returned by open().
- Buffer to be filled
- Max number of bytes to read
read() attempts to read up to
count bytes from file
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
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
O_NONBLOCK flag was given to the open()
function it returns immediately with an
EAGAIN error code when no data
is available. The select() or
poll() functions can always be used to suspend
execution until data becomes available. All drivers supporting the
read() function must also support select() and
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 Interfaces.
7.67.5. 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
- Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available for reading.
fdis not a valid file descriptor or is not open for reading, or the process already has the maximum number of files open.
- The driver does not support multiple read streams and the device is already in use.
bufreferences an inaccessible memory area.
- The call was interrupted by a signal before any data was read.
- I/O error. This indicates some hardware problem or a failure to communicate with a remote device (USB camera etc.).
- The read() function is not supported by this driver, not on this device, or generally not on this type of device.