VIDIOC_QUERYCAP -- Query device capabilities


int ioctl(int fd, int request, struct v4l2_capability *argp);



File descriptor returned by open().





All V4L2 devices support the VIDIOC_QUERYCAP ioctl. It is used to identify kernel devices compatible with this specification and to obtain information about driver and hardware capabilities. The ioctl takes a pointer to a struct v4l2_capability which is filled by the driver. When the driver is not compatible with this specification the ioctl returns an EINVAL error code.

Table 1. struct v4l2_capability


Name of the driver, a unique NUL-terminated ASCII string. For example: "bttv". Driver specific applications can use this information to verify the driver identity. It is also useful to work around known bugs, or to identify drivers in error reports. The driver version is stored in the version field.

Storing strings in fixed sized arrays is bad practice but unavoidable here. Drivers and applications should take precautions to never read or write beyond the end of the array and to make sure the strings are properly NUL-terminated.

__u8card[32]Name of the device, a NUL-terminated ASCII string. For example: "Yoyodyne TV/FM". One driver may support different brands or models of video hardware. This information is intended for users, for example in a menu of available devices. Since multiple TV cards of the same brand may be installed which are supported by the same driver, this name should be combined with the character device file name (e. g. /dev/video2) or the bus_info string to avoid ambiguities.
__u8bus_info[32]Location of the device in the system, a NUL-terminated ASCII string. For example: "PCI Slot 4". This information is intended for users, to distinguish multiple identical devices. If no such information is available the field may simply count the devices controlled by the driver, or contain the empty string (bus_info[0] = 0).

Version number of the driver. Together with the driver field this identifies a particular driver. The version number is formatted using the KERNEL_VERSION() macro:

#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))

__u32 version = KERNEL_VERSION(0, 8, 1);

printf ("Version: %u.%u.%u\n",
        (version >> 16) & 0xFF,
        (version >> 8) & 0xFF,
         version & 0xFF);

__u32capabilitiesDevice capabilities, see Table 2.
__u32reserved[4]Reserved for future extensions. Drivers must set this array to zero.

Table 2. Device Capabilities Flags

V4L2_CAP_VIDEO_CAPTURE0x00000001The device supports the Video Capture interface.
V4L2_CAP_VIDEO_OUTPUT0x00000002The device supports the Video Output interface.
V4L2_CAP_VIDEO_OVERLAY0x00000004The device supports the Video Overlay interface. A video overlay device typically stores captured images directly in the video memory of a graphics card, with hardware clipping and scaling.
V4L2_CAP_VBI_CAPTURE0x00000010The device supports the Raw VBI Capture interface, providing Teletext and Closed Caption data.
V4L2_CAP_VBI_OUTPUT0x00000020The device supports the Raw VBI Output interface.
V4L2_CAP_SLICED_VBI_CAPTURE0x00000040The device supports the Sliced VBI Capture interface.
V4L2_CAP_SLICED_VBI_OUTPUT0x00000080The device supports the Sliced VBI Output interface.
V4L2_CAP_RDS_CAPTURE0x00000100[to be defined]
V4L2_CAP_VIDEO_OUTPUT_OVERLAY0x00000200The device supports the Video Output Overlay (OSD) interface. Unlike the Video Overlay interface, this is a secondary function of video output devices and overlays an image onto an outgoing video signal. When the driver sets this flag, it must clear the V4L2_CAP_VIDEO_OVERLAY flag and vice versa.[a]
V4L2_CAP_TUNER0x00010000The device has some sort of tuner or modulator to receive or emit RF-modulated video signals. For more information about tuner and modulator programming see Section 1.6.
V4L2_CAP_AUDIO0x00020000The device has audio inputs or outputs. It may or may not support audio recording or playback, in PCM or compressed formats. PCM audio support must be implemented as ALSA or OSS interface. For more information on audio inputs and outputs see Section 1.5.
V4L2_CAP_RADIO0x00040000This is a radio receiver.
V4L2_CAP_READWRITE0x01000000The device supports the read() and/or write() I/O methods.
V4L2_CAP_ASYNCIO0x02000000The device supports the asynchronous I/O methods.
V4L2_CAP_STREAMING0x04000000The device supports the streaming I/O method.
a. The struct v4l2_framebuffer lacks an enum v4l2_buf_type field, therefore the type of overlay is implied by the driver capabilities.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:


The device is not compatible with this specification.