Video for Linux Two API Specification: Revision 0.24
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# ioctl VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD

## Name

VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD -- Execute an encoder command

## Synopsis

``int ioctl`(int fd, int request, struct v4l2_encoder_cmd *argp);`

## Arguments

`fd`

  File descriptor returned by `open()`.

`request`

  VIDIOC_ENCODER_CMD, VIDIOC_TRY_ENCODER_CMD

`argp`

## Description

> Experimental: This is an experimental interface and may change in the future.

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`() call sends a 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`() call sends an 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 1. struct `v4l2_encoder_cmd`

__u32 | `cmd` | The encoder command, see Table 2.
__u32 | `flags` | Flags to go with the command, see Table 3. 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 2. 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.
`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 3. 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.

## Return Value

On success 0 is returned, on error -1 and the `errno` variable is set appropriately:

EINVAL

  The driver does not support this ioctl, or the `cmd` field is invalid.

EPERM

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

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