Video for Linux Two API Specification: Revision 0.24
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# ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT

## Name

VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT -- Get or set the data format, try a format

## Synopsis

``int ioctl`(int fd, int request, struct v4l2_format *argp);`

## Arguments

`fd`

  File descriptor returned by `open()`.

`request`

  VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT

`argp`

## Description

These ioctls are used to negotiate the format of data (typically image format) exchanged between driver and application.

To query the current parameters applications set the `type` field of a struct `v4l2_format` to the respective buffer (stream) type. For example video capture devices use `V4L2_BUF_TYPE_VIDEO_CAPTURE`. When the application calls the `VIDIOC_G_FMT` ioctl with a pointer to this structure the driver fills the respective member of the `fmt` union. In case of video capture devices that is the struct v4l2_pix_format `pix` member. When the requested buffer type is not supported drivers return an EINVAL error code.

To change the current format parameters applications initialize the `type` field and all fields of the respective `fmt` union member. For details see the documentation of the various devices types in Chapter 4. Good practice is to query the current parameters first, and to modify only those parameters not suitable for the application. When the application calls the `VIDIOC_S_FMT` ioctl with a pointer to a `v4l2_format` structure the driver checks and adjusts the parameters against hardware abilities. Drivers should not return an error code unless the input is ambiguous, this is a mechanism to fathom device capabilities and to approach parameters acceptable for both the application and driver. On success the driver may program the hardware, allocate resources and generally prepare for data exchange. Finally the `VIDIOC_S_FMT` ioctl returns the current format parameters as `VIDIOC_G_FMT` does. Very simple, inflexible devices may even ignore all input and always return the default parameters. However all V4L2 devices exchanging data with the application must implement the `VIDIOC_G_FMT` and `VIDIOC_S_FMT` ioctl. When the requested buffer type is not supported drivers return an EINVAL error code on a `VIDIOC_S_FMT` attempt. When I/O is already in progress or the resource is not available for other reasons drivers return the EBUSY error code.

The `VIDIOC_TRY_FMT` ioctl is equivalent to `VIDIOC_S_FMT` with one exception: it does not change driver state. It can also be called at any time, never returning EBUSY. This function is provided to negotiate parameters, to learn about hardware limitations, without disabling I/O or possibly time consuming hardware preparations. Although strongly recommended drivers are not required to implement this ioctl.

Table 1. struct `v4l2_format`

enum v4l2_buf_type | `type` |  | Type of the data stream, see Table 3-2.
union | `fmt` |  |
| struct v4l2_pix_format | `pix` | Definition of an image format, see Chapter 2, used by video capture and output devices.
| struct v4l2_window | `win` | Definition of an overlaid image, see Section 4.2, used by video overlay devices.
| struct v4l2_vbi_format | `vbi` | Raw VBI capture or output parameters. This is discussed in more detail in Section 4.7. Used by raw VBI capture and output devices.
| struct v4l2_sliced_vbi_format | `sliced` | Sliced VBI capture or output parameters. See Section 4.8 for details. Used by sliced VBI capture and output devices.
| __u8 | `raw_data`[200] | Place holder for future extensions and custom (driver defined) formats with `type` `V4L2_BUF_TYPE_PRIVATE` and higher.

## Return Value

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

EBUSY

  The data format cannot be changed at this time, for example because I/O is already in progress.

EINVAL

  The struct v4l2_format `type` field is invalid, the requested buffer type not supported, or `VIDIOC_TRY_FMT` was called and is not supported with this buffer type.

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