8.28. ioctl VIDIOC_G_ENC_INDEX

8.28.1. Name

VIDIOC_G_ENC_INDEX - Get meta data about a compressed video stream

8.28.2. Synopsis

int ioctl(int fd, VIDIOC_G_ENC_INDEX, struct v4l2_enc_idx *argp)

8.28.3. Arguments

fd

File descriptor returned by open().

argp

8.28.4. Description

The VIDIOC_G_ENC_INDEX ioctl provides meta data about a compressed video stream the same or another application currently reads from the driver, which is useful for random access into the stream without decoding it.

To read the data applications must call VIDIOC_G_ENC_INDEX with a pointer to a struct v4l2_enc_idx. On success the driver fills the entry array, stores the number of elements written in the entries field, and initializes the entries_cap field.

Each element of the entry array contains meta data about one picture. A VIDIOC_G_ENC_INDEX call reads up to V4L2_ENC_IDX_ENTRIES entries from a driver buffer, which can hold up to entries_cap entries. This number can be lower or higher than V4L2_ENC_IDX_ENTRIES, but not zero. When the application fails to read the meta data in time the oldest entries will be lost. When the buffer is empty or no capturing/encoding is in progress, entries will be zero.

Currently this ioctl is only defined for MPEG-2 program streams and video elementary streams.

v4l2_enc_idx

struct v4l2_enc_idx

__u32	entries	The number of entries the driver stored in the entry array.
__u32	entries_cap	The number of entries the driver can buffer. Must be greater than zero.
__u32	reserved[4]	Reserved for future extensions. Drivers must set the array to zero.
struct v4l2_enc_idx_entry	entry[V4L2_ENC_IDX_ENTRIES]	Meta data about a compressed video stream. Each element of the array corresponds to one picture, sorted in ascending order by their offset.

v4l2_enc_idx_entry

struct v4l2_enc_idx_entry

__u64	offset	The offset in bytes from the beginning of the compressed video stream to the beginning of this picture, that is a PES packet header as defined in ISO 13818-1 or a picture header as defined in ISO 13818-2. When the encoder is stopped, the driver resets the offset to zero.
__u64	pts	The 33 bit Presentation Time Stamp of this picture as defined in ISO 13818-1.
__u32	length	The length of this picture in bytes.
__u32	flags	Flags containing the coding type of this picture, see Index Entry Flags.
__u32	reserved[2]	Reserved for future extensions. Drivers must set the array to zero.

Index Entry Flags

V4L2_ENC_IDX_FRAME_I	0x00	This is an Intra-coded picture.
V4L2_ENC_IDX_FRAME_P	0x01	This is a Predictive-coded picture.
V4L2_ENC_IDX_FRAME_B	0x02	This is a Bidirectionally predictive-coded picture.
V4L2_ENC_IDX_FRAME_MASK	0x0F	AND the flags field with this mask to obtain the picture coding type.

8.28.5. 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.