7.14. ioctl VIDIOC_ENUM_FMT

7.14.1. Name

VIDIOC_ENUM_FMT - Enumerate image formats

7.14.2. Synopsis

int ioctl(int fd, VIDIOC_ENUM_FMT, struct v4l2_fmtdesc *argp)

7.14.3. Arguments

fd

File descriptor returned by open().

argp

Pointer to struct v4l2_fmtdesc.

7.14.4. Description

To enumerate image formats applications initialize the type, mbus_code and index fields of struct v4l2_fmtdesc and call the ioctl VIDIOC_ENUM_FMT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code. All formats are enumerable by beginning at index zero and incrementing by one until EINVAL is returned. If applicable, drivers shall return formats in preference order, where preferred formats are returned before (that is, with lower index value) less-preferred formats.

Depending on the V4L2_CAP_IO_MC capability, the mbus_code field is handled differently:

  1. V4L2_CAP_IO_MC is not set (also known as a ‘video-node-centric’ driver)

    Applications shall initialize the mbus_code field to zero and drivers shall ignore the value of the field.

    Drivers shall enumerate all image formats.

    Note

    After switching the input or output the list of enumerated image formats may be different.

  2. V4L2_CAP_IO_MC is set (also known as an ‘MC-centric’ driver)

    If the mbus_code field is zero, then all image formats shall be enumerated.

    If the mbus_code field is initialized to a valid (non-zero) media bus format code, then drivers shall restrict enumeration to only the image formats that can produce (for video output devices) or be produced from (for video capture devices) that media bus code. If the mbus_code is unsupported by the driver, then EINVAL shall be returned.

    Regardless of the value of the mbus_code field, the enumerated image formats shall not depend on the active configuration of the video device or device pipeline.

v4l2_fmtdesc
struct v4l2_fmtdesc

__u32

index

Number of the format in the enumeration, set by the application. This is in no way related to the pixelformat field.

__u32

type

Type of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE, V4L2_BUF_TYPE_VIDEO_OVERLAY, V4L2_BUF_TYPE_SDR_CAPTURE, V4L2_BUF_TYPE_SDR_OUTPUT, V4L2_BUF_TYPE_META_CAPTURE and V4L2_BUF_TYPE_META_OUTPUT. See v4l2_buf_type.

__u32

flags

See Image Format Description Flags

__u8

description[32]

Description of the format, a NUL-terminated ASCII string. This information is intended for the user, for example: “YUV 4:2:2”.

__u32

pixelformat

The image format identifier. This is a four character code as computed by the v4l2_fourcc() macro:

#define v4l2_fourcc(a,b,c,d)

(((__u32)(a)<<0)|((__u32)(b)<<8)|((__u32)(c)<<16)|((__u32)(d)<<24))

Several image formats are already defined by this specification in Image Formats.

Attention

These codes are not the same as those used in the Windows world.

__u32

mbus_code

Media bus code restricting the enumerated formats, set by the application. Only applicable to drivers that advertise the V4L2_CAP_IO_MC capability, shall be 0 otherwise.

__u32

reserved[3]

Reserved for future extensions. Drivers must set the array to zero.

Image Format Description Flags

V4L2_FMT_FLAG_COMPRESSED

0x0001

This is a compressed format.

V4L2_FMT_FLAG_EMULATED

0x0002

This format is not native to the device but emulated through software (usually libv4l2), where possible try to use a native format instead for better performance.

V4L2_FMT_FLAG_CONTINUOUS_BYTESTREAM

0x0004

The hardware decoder for this compressed bytestream format (aka coded format) is capable of parsing a continuous bytestream. Applications do not need to parse the bytestream themselves to find the boundaries between frames/fields.

This flag can only be used in combination with the V4L2_FMT_FLAG_COMPRESSED flag, since this applies to compressed formats only. This flag is valid for stateful decoders only.

V4L2_FMT_FLAG_DYN_RESOLUTION

0x0008

Dynamic resolution switching is supported by the device for this compressed bytestream format (aka coded format). It will notify the user via the event V4L2_EVENT_SOURCE_CHANGE when changes in the video parameters are detected.

This flag can only be used in combination with the V4L2_FMT_FLAG_COMPRESSED flag, since this applies to compressed formats only. This flag is valid for stateful codecs only.

V4L2_FMT_FLAG_ENC_CAP_FRAME_INTERVAL

0x0010

The hardware encoder supports setting the CAPTURE coded frame interval separately from the OUTPUT raw frame interval. Setting the OUTPUT raw frame interval with VIDIOC_S_PARM also sets the CAPTURE coded frame interval to the same value. If this flag is set, then the CAPTURE coded frame interval can be set to a different value afterwards. This is typically used for offline encoding where the OUTPUT raw frame interval is used as a hint for reserving hardware encoder resources and the CAPTURE coded frame interval is the actual frame rate embedded in the encoded video stream.

This flag can only be used in combination with the V4L2_FMT_FLAG_COMPRESSED flag, since this applies to compressed formats only. This flag is valid for stateful encoders only.

7.14.5. 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.

EINVAL

The struct v4l2_fmtdesc type is not supported or the index is out of bounds.

If V4L2_CAP_IO_MC is set and the specified mbus_code is unsupported, then also return this error code.