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.