VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD — Execute an encoder command
int ioctl( | int fd, |
int request, | |
struct v4l2_encoder_cmd *argp) ; |
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.
Table A.24. struct v4l2_encoder_cmd
__u32 | cmd | The encoder command, see Table A.25, “Encoder Commands”. |
__u32 | flags | Flags to go with the command, see Table A.26, “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. |
Table A.25. 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. |
Table A.26. 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. |
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.
The cmd
field is invalid.
The application sent a PAUSE or RESUME command when the encoder was not running.