7.48. ioctl VIDIOC_QUERYCAP

7.48.1. Name

VIDIOC_QUERYCAP - Query device capabilities

7.48.2. Synopsis

int ioctl(int fd, VIDIOC_QUERYCAP, struct v4l2_capability *argp)

7.48.3. Arguments

fd
File descriptor returned by open().
argp
Pointer to struct v4l2_capability.

7.48.4. Description

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.

v4l2_capability
struct v4l2_capability
__u8 driver[16]

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.

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.
__u8 card[32] Name of the device, a NUL-terminated UTF-8 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.
__u8 bus_info[32] Location of the device in the system, a NUL-terminated ASCII string. For example: “PCI:0000:05:06.0”. This information is intended for users, to distinguish multiple identical devices. If no such information is available the field must simply count the devices controlled by the driver (“platform:vivi-000”). The bus_info must start with “PCI:” for PCI boards, “PCIe:” for PCI Express boards, “usb-” for USB devices, “I2C:” for i2c devices, “ISA:” for ISA devices, “parport” for parallel port devices and “platform:” for platform devices.
__u32 version

Version number of the driver.

Starting with kernel 3.1, the version reported is provided by the V4L2 subsystem following the kernel numbering scheme. However, it may not always return the same version as the kernel if, for example, a stable or distribution-modified kernel uses the V4L2 stack from a newer kernel.

The version number is formatted using the KERNEL_VERSION() macro. For example if the media stack corresponds to the V4L2 version shipped with Kernel 4.14, it would be equivalent to:

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

__u32 version = KERNEL_VERSION(4, 14, 0);

printf ("Version: %u.%u.%u\\n",

(version >> 16) & 0xFF, (version >> 8) & 0xFF, version & 0xFF);
__u32 capabilities Available capabilities of the physical device as a whole, see Device Capabilities Flags. The same physical device can export multiple devices in /dev (e.g. /dev/videoX, /dev/vbiY and /dev/radioZ). The capabilities field should contain a union of all capabilities available around the several V4L2 devices exported to userspace. For all those devices the capabilities field returns the same set of capabilities. This allows applications to open just one of the devices (typically the video device) and discover whether video, vbi and/or radio are also supported.
__u32 device_caps Device capabilities of the opened device, see Device Capabilities Flags. Should contain the available capabilities of that specific device node. So, for example, device_caps of a radio device will only contain radio related capabilities and no video or vbi capabilities. This field is only set if the capabilities field contains the V4L2_CAP_DEVICE_CAPS capability. Only the capabilities field can have the V4L2_CAP_DEVICE_CAPS capability, device_caps will never set V4L2_CAP_DEVICE_CAPS.
__u32 reserved[3] Reserved for future extensions. Drivers must set this array to zero.
Device Capabilities Flags
V4L2_CAP_VIDEO_CAPTURE 0x00000001 The device supports the single-planar API through the Video Capture interface.
V4L2_CAP_VIDEO_CAPTURE_MPLANE 0x00001000 The device supports the multi-planar API through the Video Capture interface.
V4L2_CAP_VIDEO_OUTPUT 0x00000002 The device supports the single-planar API through the Video Output interface.
V4L2_CAP_VIDEO_OUTPUT_MPLANE 0x00002000 The device supports the multi-planar API through the Video Output interface.
V4L2_CAP_VIDEO_M2M 0x00004000 The device supports the single-planar API through the Video Memory-To-Memory interface.
V4L2_CAP_VIDEO_M2M_MPLANE 0x00008000 The device supports the multi-planar API through the Video Memory-To-Memory interface.
V4L2_CAP_VIDEO_OVERLAY 0x00000004 The 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_CAPTURE 0x00000010 The device supports the Raw VBI Capture interface, providing Teletext and Closed Caption data.
V4L2_CAP_VBI_OUTPUT 0x00000020 The device supports the Raw VBI Output interface.
V4L2_CAP_SLICED_VBI_CAPTURE 0x00000040 The device supports the Sliced VBI Capture interface.
V4L2_CAP_SLICED_VBI_OUTPUT 0x00000080 The device supports the Sliced VBI Output interface.
V4L2_CAP_RDS_CAPTURE 0x00000100 The device supports the RDS capture interface.
V4L2_CAP_VIDEO_OUTPUT_OVERLAY 0x00000200 The 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. [1]
V4L2_CAP_HW_FREQ_SEEK 0x00000400 The device supports the ioctl VIDIOC_S_HW_FREQ_SEEK ioctl for hardware frequency seeking.
V4L2_CAP_RDS_OUTPUT 0x00000800 The device supports the RDS output interface.
V4L2_CAP_TUNER 0x00010000 The device has some sort of tuner to receive RF-modulated video signals. For more information about tuner programming see Tuners and Modulators.
V4L2_CAP_AUDIO 0x00020000 The 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 Audio Inputs and Outputs.
V4L2_CAP_RADIO 0x00040000 This is a radio receiver.
V4L2_CAP_MODULATOR 0x00080000 The device has some sort of modulator to emit RF-modulated video/audio signals. For more information about modulator programming see Tuners and Modulators.
V4L2_CAP_SDR_CAPTURE 0x00100000 The device supports the SDR Capture interface.
V4L2_CAP_EXT_PIX_FORMAT 0x00200000 The device supports the struct v4l2_pix_format extended fields.
V4L2_CAP_SDR_OUTPUT 0x00400000 The device supports the SDR Output interface.
V4L2_CAP_META_CAPTURE 0x00800000 The device supports the Metadata Interface capture interface.
V4L2_CAP_READWRITE 0x01000000 The device supports the read() and/or write() I/O methods.
V4L2_CAP_ASYNCIO 0x02000000 The device supports the asynchronous I/O methods.
V4L2_CAP_STREAMING 0x04000000 The device supports the streaming I/O method.
V4L2_CAP_TOUCH 0x10000000 This is a touch device.
V4L2_CAP_DEVICE_CAPS 0x80000000 The driver fills the device_caps field. This capability can only appear in the capabilities field and never in the device_caps field.

7.48.5. Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.

[1]The struct v4l2_framebuffer lacks an enum v4l2_buf_type field, therefore the type of overlay is implied by the driver capabilities.