Ivi_SetAttrReadCallbackViReal64
Usage
ViStatus Ivi_SetAttrReadCallbackViReal64(ViSession vi, ViAttr attributeID, ReadAttrViReal64_CallbackPtr readCallback);
Purpose
This function sets the read callback function for a ViReal64 attribute. The IVI engine calls the read callback function when you request the current value of the attribute and the cache value is invalid.
If you do not want the IVI engine to invoke a read callback, specify VI_NULL for the Read Callback parameter.
You can set the read callback function when you create the attribute with Ivi_AddAttributeViReal64.
Parameters
| Name | Type | Description | ||
|---|---|---|---|---|
| vi | ViSession |
The ViSession handle that you obtain from Ivi_SpecificDriverNew. The handle identifies a particular IVI session. |
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| attributeID | ViAttr |
Pass the ID of an attribute for this parameter. The include file for a specific instrument driver defines constant names for all of the user-accessible attributes that apply to the driver. This includes attributes that the IVI engine defines, attributes that the instrument class defines, and attributes that are specific to the particular instrument. Each defined constant name begins with PREFIX_ATTR_, where PREFIX is the specific instrument prefix. For each IVI engine attribute, the specific driver include file uses the same constant name that appears in ivi.h, except that the specific instrument prefix replaces the IVI prefix. For example, ivi.h defines IVI_ATTR_CACHE, and the Fluke 45 include file, fl45.h, defines the following constant name: #define FL45_ATTR_CACHE IVI_ATTR_CACHE For each instrument class attribute, the specific driver include file uses the same constant name that appears in the instrument class include file, except that the specific instrument prefix replaces the class prefix. For example, the DMM class include file, ividmm.h, defines IVIDMM_ATTR_RANGE, and fl45.h defines the following constant name: #define FL45_ATTR_RANGE IVIDMM_ATTR_RANGE For each specific instrument attribute, the specific driver include file defines a constant name and assigns a value that is an offset from IVI_SPECIFIC_PUBLIC_ATTR_BASE. For example, fl45.h defines the following constant name: #define FL45_ATTR_HOLD_THRESHOLD \ (IVI_SPECIFIC_PUBLIC_ATTR_BASE + 3L) For each attribute that is private to an instrument driver, the instrument driver source file defines a constant name and assigns a value that is an offset from IVI_SPECIFIC_PRIVATE_ATTR_BASE. For example, hp34401a.c defines the following constant name: #define HP34401A_ATTR_TRIGGER_TYPE \ (IVI_SPECIFIC_PRIVATE_ATTR_BASE + 1L) |
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| readCallback | ReadAttrViReal64_CallbackPtr |
Specify the read callback function you want the IVI engine to invoke when you request the current value of the attribute. You must define the read callback function in the source code for the specific instrument driver. The function must have the following prototype: ViStatus _VI_FUNC Callback(ViSession vi,ViSession io, Upon entry to the callback, *value contains the cache value. Upon exit from the callback, *value must contain the actual current value.
If you do not want to use a read callback function, pass VI_NULL. |
Return Value
Contains the status code that the function call returns. IVI engine functions can return error and warning values from several sets of status codes. Some status codes are unique to the IVI engine. Other status codes are the same codes that VISA Library functions return. Still others are error or warning values that functions in specific instrument drivers return. Each set of status codes has its own numeric range.
Regardless of the source of the status code, 0 always indicates success, a positive value indicates a warning, and a negative value indicates an error.