Arguments

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 or V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE. 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 either the struct v4l2_pix_format pix or the struct v4l2_pix_format_mplane pix_mp 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, Interfaces. 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 type field is invalid, 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.

The format as returned by VIDIOC_TRY_FMT must be identical to what VIDIOC_S_FMT returns for the same input or output.

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.