8.31. ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT¶
VIDIOC_G_FMT - VIDIOC_S_FMT - VIDIOC_TRY_FMT - Get or set the data format, try a format
ioctl(int fd, VIDIOC_G_FMT, struct v4l2_format *argp)¶
ioctl(int fd, VIDIOC_S_FMT, struct v4l2_format *argp)¶
ioctl(int fd, VIDIOC_TRY_FMT, struct v4l2_format *argp)¶
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
v4l2_format to the respective buffer (stream)
type. For example video capture devices use
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
pix or the struct
member. When the requested buffer type is not supported drivers return
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
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 struct
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
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.
||Type of the data stream, see
||Definition of an image format, see Image Formats, used by video capture and output devices.|
||Definition of an image format, see Image Formats, used by video capture and output devices that support the multi-planar version of the API.|
||Definition of an overlaid image, see Video Overlay Interface, used by video overlay devices.|
||Raw VBI capture or output parameters. This is discussed in more detail in Raw VBI Data Interface. Used by raw VBI capture and output devices.|
||Sliced VBI capture or output parameters. See Sliced VBI Data Interface for details. Used by sliced VBI capture and output devices.|
||Definition of a data format, see Image Formats, used by SDR capture and output devices.|
||Place holder for future extensions.|
8.31.5. 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 struct
typefield is invalid or the requested buffer type not supported.
- The device is busy and cannot change the format. This could be because or the device is streaming or buffers are allocated or queued to the driver. Relevant for VIDIOC_S_FMT only.