V4L2命令之 VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU

http://linuxtv.org/downloads/v4l-dvb-apis/vidioc-queryctrl.html

Name

VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU — Enumerate controls and menu control items

Synopsis

int ioctl(int fd,
int request,
struct v4l2_queryctrl *argp);

 

int ioctl(int fd,
int request,
struct v4l2_querymenu *argp);

 

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU

argp

Description

To query the attributes of a control applications set the id  field of a struct v4l2_queryctrl and call the VIDIOC_QUERYCTRL ioctl with a pointer to this structure. The driver fills the rest of the structure or returns an EINVAL error code when theid  is invalid.

It is possible to enumerate controls by calling VIDIOC_QUERYCTRL with successive id  values starting from V4L2_CID_BASE up to and exclusive V4L2_CID_BASE_LASTP1. Drivers may return EINVAL if a control in this range is not supported. Further applications can enumerate private controls, which are not defined in this specification, by starting at V4L2_CID_PRIVATE_BASE and incrementing id  until the driver returns EINVAL.

In both cases, when the driver sets the V4L2_CTRL_FLAG_DISABLED flag in the flags  field this control is permanently disabled and should be ignored by the application.^[25]

When the application ORs id  with V4L2_CTRL_FLAG_NEXT_CTRL the driver returns the next supported control, or EINVAL if there is none. Drivers which do not support this flag yet always return EINVAL.

Additional information is required for menu controls: the names of  the menu items. To query them applications set the id  and index  fields of struct  v4l2_querymenu and call the VIDIOC_QUERYMENU  ioctl with a pointer to this structure. The driver fills the rest of the structure or returns anEINVAL error code when theid  or index is invalid. Menu items are enumeratedby callingVIDIOC_QUERYMENU with successive index  values from struct v4l2_queryctrl minimum  to maximum,  inclusive. Note that it is possible forVIDIOC_QUERYMENU to return anEINVAL error code for someindices between minimum  and maximum. In that case that particular menu item is not supported by this driver. Also note that the minimum  value is not necessarily 0.

See also the examples in the section called “User Controls”.

Table A.90. struct v4l2_queryctrl

__u32idIdentifies the control, set by the application. SeeTable 1.1, “Control IDs” for predefined IDs. When the ID is ORed with V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and returns the first control with a higher ID. Drivers which do not support this flag yet always return an EINVAL error code.__u32typeType of control, see Table A.92, “enum v4l2_ctrl_type”.__u8name[32]Name of the control, a NUL-terminated ASCII string. This information is intended for the user.__s32minimumMinimum value, inclusive. This field gives a lower bound forV4L2_CTRL_TYPE_INTEGER controls and the lowest valid index forV4L2_CTRL_TYPE_MENU controls.ForV4L2_CTRL_TYPE_STRING controls the minimum value gives the minimum length of the string. This lengthdoes not include the terminating zero. It may not be valid for any other type of control, including V4L2_CTRL_TYPE_INTEGER64 controls. Note that this is asigned value.__s32maximumMaximum value, inclusive. This field gives an upper bound forV4L2_CTRL_TYPE_INTEGER controls and the highest valid index forV4L2_CTRL_TYPE_MENUcontrols. For V4L2_CTRL_TYPE_BITMASK controls it is the set of usable bits.ForV4L2_CTRL_TYPE_STRING controls the maximum value gives the maximum length of the string. This lengthdoes not include the terminating zero. It may not be valid for any other type of control, including V4L2_CTRL_TYPE_INTEGER64 controls. Note that this is asigned value.__s32step

This field gives a step size for V4L2_CTRL_TYPE_INTEGER controls. For V4L2_CTRL_TYPE_STRING controls this field refers to the string length that has to be a multiple of this step size.It may not be valid for any other type of control, includingV4L2_CTRL_TYPE_INTEGER64controls.

Generally drivers should not scale hardware control values. It may be necessary for example when the name orid  implya particular unit and the hardware actually accepts only multiples of said unit. If so, drivers must take care values are properly rounded when scaling, such that errors will not accumulate on repeatedread-write cycles.

This field gives the smallest change of an integer control actually affecting hardware. Often the informationis needed when the user can change controls by keyboard or GUIbuttons, rather than a slider. When for example a hardware register accepts values 0-511 and the driver reports 0-65535, step should be128.

Note that although signed, the step value is supposed to be always positive.

__s32default_valueThe default value of a V4L2_CTRL_TYPE_INTEGER,_BOOLEAN or_MENU control.Not valid for other types of controls. Drivers reset controls only when the driver is loaded, not later, in particular not when the func-open; is called.__u32flagsControl flags, see Table A.93, “Control Flags”.__u32reserved[2]Reserved for future extensions. Drivers must set the array to zero.

Table A.91. struct v4l2_querymenu

__u32
idIdentifies the control, set by the application from the respective struct v4l2_queryctrlid.__u32
indexIndex of the menu item, starting at zero, set by the application.union



__u8name[32]Name of the menu item, a NUL-terminated ASCIIstring. This information is intended for the user. This field is validforV4L2_CTRL_FLAG_MENU type controls.
__s64valueValue of the integer menu item. This field is valid forV4L2_CTRL_FLAG_INTEGER_MENU type controls.__u32
reservedReserved for future extensions. Drivers must setthe array to zero.

Table A.92. enum v4l2_ctrl_type

TypeminimumstepmaximumDescriptionV4L2_CTRL_TYPE_INTEGERanyanyanyAn integer-valued control ranging from minimum to maximum inclusive. The step value indicates the increment between values which are actually different on the hardware.V4L2_CTRL_TYPE_BOOLEAN011A boolean-valued control. Zero corresponds to"disabled", and one means "enabled".V4L2_CTRL_TYPE_MENU≥ 01N-1The control has a menu of N choices. The names ofthe menu items can be enumerated with theVIDIOC_QUERYMENU ioctl.V4L2_CTRL_TYPE_INTEGER_MENU≥ 01N-1The control has a menu of N choices. The values of the menu items can be enumerated with theVIDIOC_QUERYMENU ioctl. This is similar toV4L2_CTRL_TYPE_MENU except that instead of strings, the menu items are signed 64-bit integers.V4L2_CTRL_TYPE_BITMASK0n/aanyA bitmask field. The maximum value is the set of bits that canbe used, all other bits are to be 0. The maximum value is interpreted as a __u32,allowing the use of bit 31 in the bitmask.V4L2_CTRL_TYPE_BUTTON000A control which performs an action when set.Drivers must ignore the value passed withVIDIOC_S_CTRL and return anEINVAL error code on aVIDIOC_G_CTRL attempt.V4L2_CTRL_TYPE_INTEGER64n/an/an/aA 64-bit integer valued control. Minimum, maximumand step size cannot be queried.V4L2_CTRL_TYPE_STRING≥ 0≥ 1≥ 0The minimum and maximum string lengths. The step sizemeans that the string must be (minimum + N * step) characters long forN ≥ 0. These lengths do not include the terminating zero, so in order topass a string of length 8 to VIDIOC_S_EXT_CTRLS you need to set thesize field of struct v4l2_ext_control to 9. For VIDIOC_G_EXT_CTRLS you canset the size field to maximum + 1.Which character encoding is used will depend on the string control itself andshould be part of the control documentation.V4L2_CTRL_TYPE_CTRL_CLASSn/an/an/aThis is not a control. WhenVIDIOC_QUERYCTRL is called with a control IDequal to a control class code (seeTable A.63, “Control classes”) + 1, theioctl returns the name of the control class and this control type.Older drivers which do not support this feature return anEINVAL error code.

Table A.93. Control Flags

V4L2_CTRL_FLAG_DISABLED0x0001This control is permanently永久地 disabled and should be ignored by the application. Any attempt to change the control will result in anEINVAL error code.V4L2_CTRL_FLAG_GRABBED0x0002This control is temporarily unchangeable, for example because another application took over control of there spective resource. Such controls may be displayed specially in auser interface. Attempts to change the control may result in an EBUSY error code.V4L2_CTRL_FLAG_READ_ONLY0x0004This control is permanently readable only. Any attempt to change the control will result in anEINVAL error code.V4L2_CTRL_FLAG_UPDATE0x0008A hint that changing this control may affect thevalue of other controls within the same control class. Applicationsshould update their user interface accordingly.V4L2_CTRL_FLAG_INACTIVE0x0010This control is not applicable to the currentconfiguration and should be displayed accordingly in a user interface.For example the flag may be set on a MPEG audio level 2 bitratecontrol when MPEG audio encoding level 1 was selected with anothercontrol.V4L2_CTRL_FLAG_SLIDER0x0020A hint that this control is best represented as aslider-like element in a user interface.V4L2_CTRL_FLAG_WRITE_ONLY0x0040This control is permanently writable only. Anyattempt to read the control will result in anEACCES error code error code. Thisflag is typically present for relative controls or action controls wherewriting a value will cause the device to carry out a given action(e. g. motor control) but no meaningful value can be returned.V4L2_CTRL_FLAG_VOLATILE0x0080This control is volatile, which means that the value of the controlchanges continuously. A typical example would be the current gain value if the deviceis in auto-gain mode. In such a case the hardware calculates the gain value based onthe lighting conditions which can change over time. Note that setting a new value fora volatile control will have no effect. The new value will just be ignored.

Return Value

On success 0 is returned, on error-1 and theerrno variable is set appropriately. The generic error codes are described at theGeneric Error Codes chapter.

EINVAL

The struct v4l2_queryctrlidis invalid. The struct v4l2_querymenuid  is invalid or index is out of range (less thanminimum  or greater thanmaximum )or this particular menu item is not supported by the driver.

EACCES

An attempt was made to read a write-only control.