Input and output devices support the
write() function, respectively, when the
V4L2_CAP_READWRITE flag in the
capabilities field of struct
v4l2_capability returned by the
ioctl VIDIOC_QUERYCAP ioctl is set.
Drivers may need the CPU to copy the data, but they may also support DMA to or from user memory, so this I/O method is not necessarily less efficient than other methods merely exchanging buffer pointers. It is considered inferior though because no meta-information like frame counters or timestamps are passed. This information is necessary to recognize frame dropping and to synchronize with other data streams. However this is also the simplest I/O method, requiring little or no setup to exchange data. It permits command line stunts like this (the vidctrl tool is fictitious):
$ vidctrl /dev/video --input=0 --format=YUYV --size=352x288 $ dd if=/dev/video of=myimage.422 bs=202752 count=1
To read from the device applications use the
function, to write the
write() function. Drivers
must implement one I/O method if they exchange data with applications,
but it need not be this. 1 When reading or writing is supported, the
driver must also support the
poll() function. 2
It would be desirable if applications could depend on drivers supporting all I/O interfaces, but as much as the complex memory mapping I/O can be inadequate for some devices we have no reason to require this interface, which is most useful for simple applications capturing still images.
At the driver level
poll()are the same, and
select()is too important to be optional.