7.29. ioctl VIDIOC_G_EXT_CTRLS, VIDIOC_S_EXT_CTRLS, VIDIOC_TRY_EXT_CTRLS¶

7.29.1. Name¶

VIDIOC_G_EXT_CTRLS - VIDIOC_S_EXT_CTRLS - VIDIOC_TRY_EXT_CTRLS - Get or set the value of several controls, try control values

7.29.2. Synopsis¶

int ioctl(int fd, VIDIOC_G_EXT_CTRLS, struct v4l2_ext_controls *argp)¶

int ioctl(int fd, VIDIOC_S_EXT_CTRLS, struct v4l2_ext_controls *argp)¶

int ioctl(int fd, VIDIOC_TRY_EXT_CTRLS, struct v4l2_ext_controls *argp)¶

7.29.3. Arguments¶

fd: File descriptor returned by open().
argp: Pointer to struct v4l2_ext_controls.

7.29.4. Description¶

These ioctls allow the caller to get or set multiple controls atomically. Control IDs are grouped into control classes (see Control classes) and all controls in the control array must belong to the same control class.

Applications must always fill in the count, which, controls and reserved fields of struct v4l2_ext_controls, and initialize the struct v4l2_ext_control array pointed to by the controls fields.

To get the current value of a set of controls applications initialize the id, size and reserved2 fields of each struct v4l2_ext_control and call the VIDIOC_G_EXT_CTRLS ioctl. String controls controls must also set the string field. Controls of compound types (V4L2_CTRL_FLAG_HAS_PAYLOAD is set) must set the ptr field.

If the size is too small to receive the control result (only relevant for pointer-type controls like strings), then the driver will set size to a valid value and return an ENOSPC error code. You should re-allocate the memory to this new size and try again. For the string type it is possible that the same issue occurs again if the string has grown in the meantime. It is recommended to call ioctls VIDIOC_QUERYCTRL, VIDIOC_QUERY_EXT_CTRL and VIDIOC_QUERYMENU first and use maximum+1 as the new size value. It is guaranteed that that is sufficient memory.

N-dimensional arrays are set and retrieved row-by-row. You cannot set a partial array, all elements have to be set or retrieved. The total size is calculated as elems * elem_size. These values can be obtained by calling VIDIOC_QUERY_EXT_CTRL.

To change the value of a set of controls applications initialize the id, size, reserved2 and value/value64/string/ptr fields of each struct v4l2_ext_control and call the VIDIOC_S_EXT_CTRLS ioctl. The controls will only be set if all control values are valid.

To check if a set of controls have correct values applications initialize the id, size, reserved2 and value/value64/string/ptr fields of each struct v4l2_ext_control and call the VIDIOC_TRY_EXT_CTRLS ioctl. It is up to the driver whether wrong values are automatically adjusted to a valid value or if an error is returned.

When the id or which is invalid drivers return an EINVAL error code. When the value is out of bounds drivers can choose to take the closest valid value or return an ERANGE error code, whatever seems more appropriate. In the first case the new value is set in struct v4l2_ext_control. If the new control value is inappropriate (e.g. the given menu index is not supported by the menu control), then this will also result in an EINVAL error code error.

The driver will only set/get these controls if all control values are correct. This prevents the situation where only some of the controls were set/get. Only low-level errors (e. g. a failed i2c command) can still cause this situation.

v4l2_ext_control¶

struct v4l2_ext_control¶
__u32	`id`		Identifies the control, set by the application.
__u32	`size`		The total size in bytes of the payload of this control. This is normally 0, but for pointer controls this should be set to the size of the memory containing the payload, or that will receive the payload. If VIDIOC_G_EXT_CTRLS finds that this value is less than is required to store the payload result, then it is set to a value large enough to store the payload result and `ENOSPC` is returned. Note For string controls, this `size` field should not be confused with the length of the string. This field refers to the size of the memory that contains the string. The actual length of the string may well be much smaller.
__u32	`reserved2`[1]		Reserved for future extensions. Drivers and applications must set the array to zero.
union	(anonymous)
	__s32	`value`	New value or current value. Valid if this control is not of type `V4L2_CTRL_TYPE_INTEGER64` and `V4L2_CTRL_FLAG_HAS_PAYLOAD` is not set.
	__s64	`value64`	New value or current value. Valid if this control is of type `V4L2_CTRL_TYPE_INTEGER64` and `V4L2_CTRL_FLAG_HAS_PAYLOAD` is not set.
	char *	`string`	A pointer to a string. Valid if this control is of type `V4L2_CTRL_TYPE_STRING`.
	__u8 *	`p_u8`	A pointer to a matrix control of unsigned 8-bit values. Valid if this control is of type `V4L2_CTRL_TYPE_U8`.
	__u16 *	`p_u16`	A pointer to a matrix control of unsigned 16-bit values. Valid if this control is of type `V4L2_CTRL_TYPE_U16`.
	__u32 *	`p_u32`	A pointer to a matrix control of unsigned 32-bit values. Valid if this control is of type `V4L2_CTRL_TYPE_U32`.
	void *	`ptr`	A pointer to a compound type which can be an N-dimensional array and/or a compound type (the control’s type is >= `V4L2_CTRL_COMPOUND_TYPES`). Valid if `V4L2_CTRL_FLAG_HAS_PAYLOAD` is set for this control.

v4l2_ext_controls¶

struct v4l2_ext_controls¶
union	(anonymous)
	__u32	`ctrl_class`	The control class to which all controls belong, see Control classes. Drivers that use a kernel framework for handling controls will also accept a value of 0 here, meaning that the controls can belong to any control class. Whether drivers support this can be tested by setting `ctrl_class` to 0 and calling VIDIOC_TRY_EXT_CTRLS with a `count` of 0. If that succeeds, then the driver supports this feature.
	__u32	`which`	Which value of the control to get/set/try. `V4L2_CTRL_WHICH_CUR_VAL` will return the current value of the control and `V4L2_CTRL_WHICH_DEF_VAL` will return the default value of the control. Note You can only get the default value of the control, you cannot set or try it. For backwards compatibility you can also use a control class here (see Control classes). In that case all controls have to belong to that control class. This usage is deprecated, instead just use `V4L2_CTRL_WHICH_CUR_VAL`. There are some very old drivers that do not yet support `V4L2_CTRL_WHICH_CUR_VAL` and that require a control class here. You can test for such drivers by setting ctrl_class to `V4L2_CTRL_WHICH_CUR_VAL` and calling VIDIOC_TRY_EXT_CTRLS with a count of 0. If that fails, then the driver does not support `V4L2_CTRL_WHICH_CUR_VAL`.
__u32	`count`	The number of controls in the controls array. May also be zero.
__u32	`error_idx`	Set by the driver in case of an error. If the error is associated with a particular control, then `error_idx` is set to the index of that control. If the error is not related to a specific control, or the validation step failed (see below), then `error_idx` is set to `count`. The value is undefined if the ioctl returned 0 (success). Before controls are read from/written to hardware a validation step takes place: this checks if all controls in the list are valid controls, if no attempt is made to write to a read-only control or read from a write-only control, and any other up-front checks that can be done without accessing the hardware. The exact validations done during this step are driver dependent since some checks might require hardware access for some devices, thus making it impossible to do those checks up-front. However, drivers should make a best-effort to do as many up-front checks as possible. This check is done to avoid leaving the hardware in an inconsistent state due to easy-to-avoid problems. But it leads to another problem: the application needs to know whether an error came from the validation step (meaning that the hardware was not touched) or from an error during the actual reading from/writing to hardware. The, in hindsight quite poor, solution for that is to set `error_idx` to `count` if the validation failed. This has the unfortunate side-effect that it is not possible to see which control failed the validation. If the validation was successful and the error happened while accessing the hardware, then `error_idx` is less than `count` and only the controls up to `error_idx-1` were read or written correctly, and the state of the remaining controls is undefined. Since VIDIOC_TRY_EXT_CTRLS does not access hardware there is also no need to handle the validation step in this special way, so `error_idx` will just be set to the control that failed the validation step instead of to `count`. This means that if VIDIOC_S_EXT_CTRLS fails with `error_idx` set to `count`, then you can call VIDIOC_TRY_EXT_CTRLS to try to discover the actual control that failed the validation step. Unfortunately, there is no `TRY` equivalent for VIDIOC_G_EXT_CTRLS.
__u32	`reserved`[2]	Reserved for future extensions. Drivers and applications must set the array to zero.
struct `v4l2_ext_control` *	`controls`	Pointer to an array of `count` v4l2_ext_control structures. Ignored if `count` equals zero.

Control classes¶
`V4L2_CTRL_CLASS_USER`	0x980000	The class containing user controls. These controls are described in User Controls. All controls that can be set using the VIDIOC_S_CTRL and VIDIOC_G_CTRL ioctl belong to this class.
`V4L2_CTRL_CLASS_MPEG`	0x990000	The class containing MPEG compression controls. These controls are described in Codec Control Reference.
`V4L2_CTRL_CLASS_CAMERA`	0x9a0000	The class containing camera controls. These controls are described in Camera Control Reference.
`V4L2_CTRL_CLASS_FM_TX`	0x9b0000	The class containing FM Transmitter (FM TX) controls. These controls are described in FM Transmitter Control Reference.
`V4L2_CTRL_CLASS_FLASH`	0x9c0000	The class containing flash device controls. These controls are described in Flash Control Reference.
`V4L2_CTRL_CLASS_JPEG`	0x9d0000	The class containing JPEG compression controls. These controls are described in JPEG Control Reference.
`V4L2_CTRL_CLASS_IMAGE_SOURCE`	0x9e0000	The class containing image source controls. These controls are described in Image Source Control Reference.
`V4L2_CTRL_CLASS_IMAGE_PROC`	0x9f0000	The class containing image processing controls. These controls are described in Image Process Control Reference.
`V4L2_CTRL_CLASS_FM_RX`	0xa10000	The class containing FM Receiver (FM RX) controls. These controls are described in FM Receiver Control Reference.
`V4L2_CTRL_CLASS_RF_TUNER`	0xa20000	The class containing RF tuner controls. These controls are described in RF Tuner Control Reference.

7.29.5. 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.

EINVAL: The struct v4l2_ext_control id is invalid, the struct v4l2_ext_controls which is invalid, or the struct v4l2_ext_control value was inappropriate (e.g. the given menu index is not supported by the driver). This error code is also returned by the VIDIOC_S_EXT_CTRLS and VIDIOC_TRY_EXT_CTRLS ioctls if two or more control values are in conflict.
ERANGE: The struct v4l2_ext_control value is out of bounds.
EBUSY: The control is temporarily not changeable, possibly because another applications took over control of the device function this control belongs to.
ENOSPC: The space reserved for the control’s payload is insufficient. The field size is set to a value that is enough to store the payload and this error code is returned.
EACCES: Attempt to try or set a read-only control or to get a write-only control.