7.16. ioctl VIDIOC_ENUM_FRAMEINTERVALS

7.16.1. Name

VIDIOC_ENUM_FRAMEINTERVALS - Enumerate frame intervals

7.16.2. Synopsis

VIDIOC_ENUM_FRAMEINTERVALS

int ioctl(int fd, VIDIOC_ENUM_FRAMEINTERVALS, struct v4l2_frmivalenum *argp)

7.16.3. Arguments

fd
File descriptor returned by open().
argp
Pointer to struct v4l2_frmivalenum that contains a pixel format and size and receives a frame interval.

7.16.4. Description

This ioctl allows applications to enumerate all frame intervals that the device supports for the given pixel format and frame size.

The supported pixel formats and frame sizes can be obtained by using the ioctl VIDIOC_ENUM_FMT and ioctl VIDIOC_ENUM_FRAMESIZES functions.

The return value and the content of the v4l2_frmivalenum.type field depend on the type of frame intervals the device supports. Here are the semantics of the function for the different cases:

  • Discrete: The function returns success if the given index value (zero-based) is valid. The application should increase the index by one for each call until EINVAL is returned. The v4l2_frmivalenum.type field is set to V4L2_FRMIVAL_TYPE_DISCRETE by the driver. Of the union only the discrete member is valid.
  • Step-wise: The function returns success if the given index value is zero and EINVAL for any other index value. The v4l2_frmivalenum.type field is set to V4L2_FRMIVAL_TYPE_STEPWISE by the driver. Of the union only the stepwise member is valid.
  • Continuous: This is a special case of the step-wise type above. The function returns success if the given index value is zero and EINVAL for any other index value. The v4l2_frmivalenum.type field is set to V4L2_FRMIVAL_TYPE_CONTINUOUS by the driver. Of the union only the stepwise member is valid and the step value is set to 1.

When the application calls the function with index zero, it must check the type field to determine the type of frame interval enumeration the device supports. Only for the V4L2_FRMIVAL_TYPE_DISCRETE type does it make sense to increase the index value to receive more frame intervals.

Note

The order in which the frame intervals are returned has no special meaning. In particular does it not say anything about potential default frame intervals.

Applications can assume that the enumeration data does not change without any interaction from the application itself. This means that the enumeration data is consistent if the application does not perform any other ioctl calls while it runs the frame interval enumeration.

Note

Frame intervals and frame rates: The V4L2 API uses frame intervals instead of frame rates. Given the frame interval the frame rate can be computed as follows:

frame_rate = 1 / frame_interval

7.16.5. Structs

In the structs below, IN denotes a value that has to be filled in by the application, OUT denotes values that the driver fills in. The application should zero out all members except for the IN fields.

v4l2_frmival_stepwise
struct v4l2_frmival_stepwise
struct v4l2_fract min Minimum frame interval [s].
struct v4l2_fract max Maximum frame interval [s].
struct v4l2_fract step Frame interval step size [s].
v4l2_frmivalenum
struct v4l2_frmivalenum
__u32 index IN: Index of the given frame interval in the enumeration.
__u32 pixel_format IN: Pixel format for which the frame intervals are enumerated.
__u32 width IN: Frame width for which the frame intervals are enumerated.
__u32 height IN: Frame height for which the frame intervals are enumerated.
__u32 type OUT: Frame interval type the device supports.
union { (anonymous) OUT: Frame interval with the given index.
struct v4l2_fract discrete Frame interval [s].
struct v4l2_frmival_stepwise stepwise  
}    
__u32 reserved[2] Reserved space for future use. Must be zeroed by drivers and applications.

7.16.6. Enums

v4l2_frmivaltypes
enum v4l2_frmivaltypes
V4L2_FRMIVAL_TYPE_DISCRETE 1 Discrete frame interval.
V4L2_FRMIVAL_TYPE_CONTINUOUS 2 Continuous frame interval.
V4L2_FRMIVAL_TYPE_STEPWISE 3 Step-wise defined frame interval.

7.16.7. 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.