7.10. ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD¶

7.10.1. Name¶

VIDIOC_ENCODER_CMD - VIDIOC_TRY_ENCODER_CMD - Execute an encoder command

7.10.2. Synopsis¶

int ioctl(int fd, VIDIOC_ENCODER_CMD, struct v4l2_encoder_cmd *argp)¶

int ioctl(int fd, VIDIOC_TRY_ENCODER_CMD, struct v4l2_encoder_cmd *argp)¶

7.10.3. Arguments¶

fd: File descriptor returned by open().
argp: Pointer to struct v4l2_encoder_cmd.

7.10.4. Description¶

These ioctls control an audio/video (usually MPEG-) encoder. VIDIOC_ENCODER_CMD sends a command to the encoder, VIDIOC_TRY_ENCODER_CMD can be used to try a command without actually executing it.

To send a command applications must initialize all fields of a struct v4l2_encoder_cmd and call VIDIOC_ENCODER_CMD or VIDIOC_TRY_ENCODER_CMD with a pointer to this structure.

The cmd field must contain the command code. The flags field is currently only used by the STOP command and contains one bit: If the V4L2_ENC_CMD_STOP_AT_GOP_END flag is set, encoding will continue until the end of the current Group Of Pictures, otherwise it will stop immediately.

A read() or VIDIOC_STREAMON call sends an implicit START command to the encoder if it has not been started yet. After a STOP command, read() calls will read the remaining data buffered by the driver. When the buffer is empty, read() will return zero and the next read() call will restart the encoder.

A close() or VIDIOC_STREAMOFF call of a streaming file descriptor sends an implicit immediate STOP to the encoder, and all buffered data is discarded.

These ioctls are optional, not all drivers may support them. They were introduced in Linux 2.6.21.

v4l2_encoder_cmd¶

struct v4l2_encoder_cmd¶
__u32	`cmd`	The encoder command, see Encoder Commands.
__u32	`flags`	Flags to go with the command, see Encoder Command Flags. If no flags are defined for this command, drivers and applications must set this field to zero.
__u32	`data`[8]	Reserved for future extensions. Drivers and applications must set the array to zero.

Encoder Commands¶
`V4L2_ENC_CMD_START`	0	Start the encoder. When the encoder is already running or paused, this command does nothing. No flags are defined for this command.
`V4L2_ENC_CMD_STOP`	1	Stop the encoder. When the `V4L2_ENC_CMD_STOP_AT_GOP_END` flag is set, encoding will continue until the end of the current Group Of Pictures, otherwise encoding will stop immediately. When the encoder is already stopped, this command does nothing. mem2mem encoders will send a `V4L2_EVENT_EOS` event when the last frame has been encoded and all frames are ready to be dequeued and will set the `V4L2_BUF_FLAG_LAST` buffer flag on the last buffer of the capture queue to indicate there will be no new buffers produced to dequeue. This buffer may be empty, indicated by the driver setting the `bytesused` field to 0. Once the `V4L2_BUF_FLAG_LAST` flag was set, the VIDIOC_DQBUF ioctl will not block anymore, but return an `EPIPE` error code.
`V4L2_ENC_CMD_PAUSE`	2	Pause the encoder. When the encoder has not been started yet, the driver will return an `EPERM` error code. When the encoder is already paused, this command does nothing. No flags are defined for this command.
`V4L2_ENC_CMD_RESUME`	3	Resume encoding after a PAUSE command. When the encoder has not been started yet, the driver will return an `EPERM` error code. When the encoder is already running, this command does nothing. No flags are defined for this command.

Encoder Command Flags¶
`V4L2_ENC_CMD_STOP_AT_GOP_END`	0x0001	Stop encoding at the end of the current Group Of Pictures, rather than immediately.

7.10.5. 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 cmd field is invalid.
EPERM: The application sent a PAUSE or RESUME command when the encoder was not running.