int ioctl(int fd,
 int request,
 struct v4l2_decoder_cmd *argp);



File descriptor returned by open().





These ioctls control an audio/video (usually MPEG-) decoder. VIDIOC_DECODER_CMD sends a command to the decoder, VIDIOC_TRY_DECODER_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_decoder_cmd and call VIDIOC_DECODER_CMD or VIDIOC_TRY_DECODER_CMD with a pointer to this structure.

The cmd field must contain the command code. Some commands use the flags field for additional information.

A write() or VIDIOC_STREAMON call sends an implicit START command to the decoder if it has not been started yet.

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

These ioctls are optional, not all drivers may support them. They were introduced in Linux 3.3.

Table A.10. struct v4l2_decoder_cmd

__u32cmd  The decoder command, see Table A.11, “Decoder Commands”.
__u32flags  Flags to go with the command. If no flags are defined for this command, drivers and applications must set this field to zero.
 structstart Structure containing additional data for the V4L2_DEC_CMD_START command.
  __s32speedPlayback speed and direction. The playback speed is defined as speed/1000 of the normal speed. So 1000 is normal playback. Negative numbers denote reverse playback, so -1000 does reverse playback at normal speed. Speeds -1, 0 and 1 have special meanings: speed 0 is shorthand for 1000 (normal playback). A speed of 1 steps just one frame forward, a speed of -1 steps just one frame back.
  __u32formatFormat restrictions. This field is set by the driver, not the application. Possible values are V4L2_DEC_START_FMT_NONE if there are no format restrictions or V4L2_DEC_START_FMT_GOP if the decoder operates on full GOPs (Group Of Pictures). This is usually the case for reverse playback: the decoder needs full GOPs, which it can then play in reverse order. So to implement reverse playback the application must feed the decoder the last GOP in the video file, then the GOP before that, etc. etc.
 structstop Structure containing additional data for the V4L2_DEC_CMD_STOP command.
  __u64ptsStop playback at this pts or immediately if the playback is already past that timestamp. Leave to 0 if you want to stop after the last frame was decoded.
  __u32data[16]Reserved for future extensions. Drivers and applications must set the array to zero.

Table A.11. Decoder Commands

V4L2_DEC_CMD_START0Start the decoder. When the decoder is already running or paused, this command will just change the playback speed. That means that calling V4L2_DEC_CMD_START when the decoder was paused will not resume the decoder. You have to explicitly call V4L2_DEC_CMD_RESUME for that. This command has one flag: V4L2_DEC_CMD_START_MUTE_AUDIO. If set, then audio will be muted when playing back at a non-standard speed.
V4L2_DEC_CMD_STOP1Stop the decoder. When the decoder is already stopped, this command does nothing. This command has two flags: if V4L2_DEC_CMD_STOP_TO_BLACK is set, then the decoder will set the picture to black after it stopped decoding. Otherwise the last image will repeat. mem2mem decoders will stop producing new frames altogether. They will send a V4L2_EVENT_EOS event when the last frame has been decoded 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. If V4L2_DEC_CMD_STOP_IMMEDIATELY is set, then the decoder stops immediately (ignoring the pts value), otherwise it will keep decoding until timestamp >= pts or until the last of the pending data from its internal buffers was decoded.
V4L2_DEC_CMD_PAUSE2Pause the decoder. When the decoder has not been started yet, the driver will return an EPERM error code. When the decoder is already paused, this command does nothing. This command has one flag: if V4L2_DEC_CMD_PAUSE_TO_BLACK is set, then set the decoder output to black when paused.
V4L2_DEC_CMD_RESUME3Resume decoding after a PAUSE command. When the decoder has not been started yet, the driver will return an EPERM error code. When the decoder is already running, this command does nothing. No flags are defined for this command.

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.


The cmd field is invalid.


The application sent a PAUSE or RESUME command when the decoder was not running.