Video for Linux Two API Specification: Revision 0.24
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# ioctl VIDIOC_QUERYCAP

## Name

VIDIOC_QUERYCAP -- Query device capabilities

## Synopsis

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

## Arguments

`fd`

  File descriptor returned by `open()`.

`request`

  VIDIOC_QUERYCAP

`argp`

## 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.

Table 1. 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. 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.

__u8 | `card`[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.
__u8 | `bus_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).
__u32 | `version` | 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);
__u32 | `capabilities` | Device capabilities, see Table 2.
__u32 | `reserved`[4] | Reserved for future extensions. Drivers must set this array to zero.

Table 2. Device Capabilities Flags

`V4L2_CAP_VIDEO_CAPTURE` | 0x00000001 | The device supports the Video Capture interface.
`V4L2_CAP_VIDEO_OUTPUT` | 0x00000002 | The device supports the Video Output 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 | [to be defined]
`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.[a]
`V4L2_CAP_TUNER` | 0x00010000 | The 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_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 Section 1.5.
`V4L2_CAP_RADIO` | 0x00040000 | This is a radio receiver.
`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.
Notes: 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:

EINVAL

  The device is not compatible with this specification.

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