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.

If request_fd is set to a not-yet-queued request file descriptor and which is set to V4L2_CTRL_WHICH_REQUEST_VAL, then the controls are not applied immediately when calling VIDIOC_S_EXT_CTRLS, but instead are applied by the driver for the buffer associated with the same request. If the device does not support requests, then EACCES will be returned. If requests are supported but an invalid request file descriptor is given, then EINVAL will be returned.

An attempt to call VIDIOC_S_EXT_CTRLS for a request that has already been queued will result in an EBUSY error.

If request_fd is specified and which is set to V4L2_CTRL_WHICH_REQUEST_VAL during a call to VIDIOC_G_EXT_CTRLS, then it will return the values of the controls at the time of request completion. If the request is not yet completed, then this will result in an EACCES 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.
  v4l2_area * p_area A pointer to a struct v4l2_area. Valid if this control is of type V4L2_CTRL_TYPE_AREA.
  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, V4L2_CTRL_WHICH_DEF_VAL will return the default value of the control and V4L2_CTRL_WHICH_REQUEST_VAL indicates that these controls have to be retrieved from a request or tried/set for a request. In the latter case the request_fd field contains the file descriptor of the request that should be used. If the device does not support requests, then EACCES will be returned.

Note

When using V4L2_CTRL_WHICH_DEF_VAL be aware that 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.

__s32 request_fd File descriptor of the request to be used by this operation. Only valid if which is set to V4L2_CTRL_WHICH_REQUEST_VAL. If the device does not support requests, then EACCES will be returned. If requests are supported but an invalid request file descriptor is given, then EINVAL will be returned.
__u32 reserved[1]

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, or 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), or the which field was set to V4L2_CTRL_WHICH_REQUEST_VAL but the given request_fd was invalid or V4L2_CTRL_WHICH_REQUEST_VAL is not supported by the kernel. 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, or (if the which field was set to V4L2_CTRL_WHICH_REQUEST_VAL) the request was queued but not yet completed.
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, or to get a control from a request that has not yet been completed.

Or the which field was set to V4L2_CTRL_WHICH_REQUEST_VAL but the device does not support requests.