.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. c:namespace:: V4L
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.. _VIDIOC_DECODER_CMD:
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************************************************
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ioctl VIDIOC_DECODER_CMD, VIDIOC_TRY_DECODER_CMD
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************************************************
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Name
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====
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VIDIOC_DECODER_CMD - VIDIOC_TRY_DECODER_CMD - Execute an decoder command
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Synopsis
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========
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.. c:macro:: VIDIOC_DECODER_CMD
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``int ioctl(int fd, VIDIOC_DECODER_CMD, struct v4l2_decoder_cmd *argp)``
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.. c:macro:: VIDIOC_TRY_DECODER_CMD
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``int ioctl(int fd, VIDIOC_TRY_DECODER_CMD, struct v4l2_decoder_cmd *argp)``
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Arguments
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=========
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``fd``
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File descriptor returned by :c:func:`open()`.
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``argp``
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pointer to struct :c:type:`v4l2_decoder_cmd`.
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Description
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===========
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These ioctls control an audio/video (usually MPEG-) decoder.
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``VIDIOC_DECODER_CMD`` sends a command to the decoder,
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``VIDIOC_TRY_DECODER_CMD`` can be used to try a command without actually
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executing it. To send a command applications must initialize all fields
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of a struct :c:type:`v4l2_decoder_cmd` and call
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``VIDIOC_DECODER_CMD`` or ``VIDIOC_TRY_DECODER_CMD`` with a pointer to
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this structure.
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The ``cmd`` field must contain the command code. Some commands use the
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``flags`` field for additional information.
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A :c:func:`write()` or :ref:`VIDIOC_STREAMON`
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call sends an implicit START command to the decoder if it has not been
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started yet. Applies to both queues of mem2mem decoders.
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A :c:func:`close()` or :ref:`VIDIOC_STREAMOFF <VIDIOC_STREAMON>`
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call of a streaming file descriptor sends an implicit immediate STOP
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command to the decoder, and all buffered data is discarded. Applies to both
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queues of mem2mem decoders.
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In principle, these ioctls are optional, not all drivers may support them. They were
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introduced in Linux 3.3. They are, however, mandatory for stateful mem2mem decoders
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(as further documented in :ref:`decoder`).
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.. tabularcolumns:: |p{1.1cm}|p{2.4cm}|p{1.2cm}|p{1.6cm}|p{10.6cm}|
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.. c:type:: v4l2_decoder_cmd
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.. cssclass:: longtable
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.. flat-table:: struct v4l2_decoder_cmd
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:header-rows: 0
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:stub-columns: 0
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:widths: 1 1 1 3
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* - __u32
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- ``cmd``
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-
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- The decoder command, see :ref:`decoder-cmds`.
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* - __u32
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- ``flags``
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-
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- Flags to go with the command. If no flags are defined for this
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command, drivers and applications must set this field to zero.
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* - union {
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- (anonymous)
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* - struct
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- ``start``
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-
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- Structure containing additional data for the
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``V4L2_DEC_CMD_START`` command.
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* -
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- __s32
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- ``speed``
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- Playback speed and direction. The playback speed is defined as
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``speed``/1000 of the normal speed. So 1000 is normal playback.
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Negative numbers denote reverse playback, so -1000 does reverse
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playback at normal speed. Speeds -1, 0 and 1 have special
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meanings: speed 0 is shorthand for 1000 (normal playback). A speed
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of 1 steps just one frame forward, a speed of -1 steps just one
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frame back.
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* -
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- __u32
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- ``format``
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- Format restrictions. This field is set by the driver, not the
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application. Possible values are ``V4L2_DEC_START_FMT_NONE`` if
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there are no format restrictions or ``V4L2_DEC_START_FMT_GOP`` if
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the decoder operates on full GOPs (*Group Of Pictures*). This is
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usually the case for reverse playback: the decoder needs full
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GOPs, which it can then play in reverse order. So to implement
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reverse playback the application must feed the decoder the last
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GOP in the video file, then the GOP before that, etc. etc.
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* - struct
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- ``stop``
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-
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- Structure containing additional data for the ``V4L2_DEC_CMD_STOP``
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command.
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* -
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- __u64
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- ``pts``
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- Stop playback at this ``pts`` or immediately if the playback is
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already past that timestamp. Leave to 0 if you want to stop after
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the last frame was decoded.
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* - struct
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- ``raw``
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* -
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- __u32
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- ``data``\ [16]
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- Reserved for future extensions. Drivers and applications must set
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the array to zero.
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* - }
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-
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.. tabularcolumns:: |p{5.6cm}|p{0.6cm}|p{11.3cm}|
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.. _decoder-cmds:
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.. flat-table:: Decoder Commands
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:header-rows: 0
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:stub-columns: 0
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:widths: 56 6 113
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* - ``V4L2_DEC_CMD_START``
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- 0
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- Start the decoder. When the decoder is already running or paused,
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this command will just change the playback speed. That means that
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calling ``V4L2_DEC_CMD_START`` when the decoder was paused will
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*not* resume the decoder. You have to explicitly call
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``V4L2_DEC_CMD_RESUME`` for that. This command has one flag:
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``V4L2_DEC_CMD_START_MUTE_AUDIO``. If set, then audio will be
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muted when playing back at a non-standard speed.
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For a device implementing the :ref:`decoder`, once the drain sequence
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is initiated with the ``V4L2_DEC_CMD_STOP`` command, it must be driven
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to completion before this command can be invoked. Any attempt to
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invoke the command while the drain sequence is in progress will trigger
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an ``EBUSY`` error code. The command may be also used to restart the
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decoder in case of an implicit stop initiated by the decoder itself,
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without the ``V4L2_DEC_CMD_STOP`` being called explicitly. See
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:ref:`decoder` for more details.
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* - ``V4L2_DEC_CMD_STOP``
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- 1
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- Stop the decoder. When the decoder is already stopped, this
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command does nothing. This command has two flags: if
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``V4L2_DEC_CMD_STOP_TO_BLACK`` is set, then the decoder will set
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the picture to black after it stopped decoding. Otherwise the last
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image will repeat. If
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``V4L2_DEC_CMD_STOP_IMMEDIATELY`` is set, then the decoder stops
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immediately (ignoring the ``pts`` value), otherwise it will keep
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decoding until timestamp >= pts or until the last of the pending
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data from its internal buffers was decoded.
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For a device implementing the :ref:`decoder`, the command will initiate
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the drain sequence as documented in :ref:`decoder`. No flags or other
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arguments are accepted in this case. Any attempt to invoke the command
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again before the sequence completes will trigger an ``EBUSY`` error
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code.
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* - ``V4L2_DEC_CMD_PAUSE``
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- 2
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- Pause the decoder. When the decoder has not been started yet, the
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driver will return an ``EPERM`` error code. When the decoder is
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already paused, this command does nothing. This command has one
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flag: if ``V4L2_DEC_CMD_PAUSE_TO_BLACK`` is set, then set the
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decoder output to black when paused.
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* - ``V4L2_DEC_CMD_RESUME``
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- 3
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- Resume decoding after a PAUSE command. When the decoder has not
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been started yet, the driver will return an ``EPERM`` error code. When
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the decoder is already running, this command does nothing. No
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flags are defined for this command.
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* - ``V4L2_DEC_CMD_FLUSH``
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- 4
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- Flush any held capture buffers. Only valid for stateless decoders.
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This command is typically used when the application reached the
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end of the stream and the last output buffer had the
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``V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF`` flag set. This would prevent
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dequeueing the capture buffer containing the last decoded frame.
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So this command can be used to explicitly flush that final decoded
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frame. This command does nothing if there are no held capture buffers.
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Return Value
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============
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On success 0 is returned, on error -1 and the ``errno`` variable is set
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appropriately. The generic error codes are described at the
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:ref:`Generic Error Codes <gen-errors>` chapter.
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EBUSY
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A drain sequence of a device implementing the :ref:`decoder` is still in
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progress. It is not allowed to issue another decoder command until it
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completes.
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EINVAL
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The ``cmd`` field is invalid.
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EPERM
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The application sent a PAUSE or RESUME command when the decoder was
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not running.
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