1.1. Opening and Closing Devices¶
1.1.1. Device Naming¶
V4L2 drivers are implemented as kernel modules, loaded manually by the system administrator or automatically when a device is first discovered. The driver modules plug into the “videodev” kernel module. It provides helper functions and a common application interface specified in this document.
Each driver thus loaded registers one or more device nodes with major number 81 and a minor number between 0 and 255. Minor numbers are allocated dynamically unless the kernel is compiled with the kernel option CONFIG_VIDEO_FIXED_MINOR_RANGES. In that case minor numbers are allocated in ranges depending on the device node type (video, radio, etc.).
Many drivers support “video_nr”, “radio_nr” or “vbi_nr” module options to select specific video/radio/vbi node numbers. This allows the user to request that the device node is named e.g. /dev/video5 instead of leaving it to chance. When the driver supports multiple devices of the same type more than one device node number can be assigned, separated by commas:
# modprobe mydriver video_nr=0,1 radio_nr=0,1
/etc/modules.conf this may be written as:
options mydriver video_nr=0,1 radio_nr=0,1
When no device node number is given as module option the driver supplies a default.
Normally udev will create the device nodes in /dev automatically for you. If udev is not installed, then you need to enable the CONFIG_VIDEO_FIXED_MINOR_RANGES kernel option in order to be able to correctly relate a minor number to a device node number. I.e., you need to be certain that minor number 5 maps to device node name video5. With this kernel option different device types have different minor number ranges. These ranges are listed in Interfaces.
The creation of character special files (with mknod) is a privileged operation and devices cannot be opened by major and minor number. That means applications cannot reliable scan for loaded or installed drivers. The user must enter a device name, or the application can try the conventional device names.
1.1.3. Multiple Opens¶
V4L2 devices can be opened more than once.  When this is supported by the driver, users can for example start a “panel” application to change controls like brightness or audio volume, while another application captures video and audio. In other words, panel applications are comparable to an ALSA audio mixer application. Just opening a V4L2 device should not change the state of the device. 
Once an application has allocated the memory buffers needed for streaming data (by calling the ioctl VIDIOC_REQBUFS or ioctl VIDIOC_CREATE_BUFS ioctls, or implicitly by calling the read() or write() functions) that application (filehandle) becomes the owner of the device. It is no longer allowed to make changes that would affect the buffer sizes (e.g. by calling the VIDIOC_S_FMT ioctl) and other applications are no longer allowed to allocate buffers or start or stop streaming. The EBUSY error code will be returned instead.
Merely opening a V4L2 device does not grant exclusive access.  Initiating data exchange however assigns the right to read or write the requested type of data, and to change related properties, to this file descriptor. Applications can request additional access privileges using the priority mechanism described in Application Priority.
|There are still some old and obscure drivers that have not been
updated to allow for multiple opens. This implies that for such
drivers open() can return an
EBUSY error code
when the device is already in use.
|Unfortunately, opening a radio device often switches the state of the device to radio mode in many drivers. This behavior should be fixed eventually as it violates the V4L2 specification.
|Drivers could recognize the
O_EXCL open flag. Presently this is
not required, so applications cannot know if it really works.