The predefined device attributes for each type of predefined device are stored in the Predefined Attribute (PdAt) object class. The objects in the PdAt object class identify the default values as well as other possible values for each attribute. The Customized Attribute (CuAt) object class contains only attributes for customized device instances that have been changed from their default values.
When a customized device instance is created by a Define method, its attributes assume the default values. As a result, no objects are added to the CuAt object class for the device. If an attribute for the device is changed from the default value by the Change method, an object to describe the attribute's current value is added to the CuAt object class for the attribute. If the attribute is subsequently changed back to the default value, the Change method deletes the CuAt object for the attribute.
Any device methods that need the current attribute values for a device must access both the PdAt and CuAt object classes. If an attribute appears in the CuAt object class, then the associated object identifies the current value. Otherwise, the default value from the PdAt attribute object identifies the current value.
When modifying an attribute value, methods you write must obtain the objects for that attribute from both the PdAt and CuAt object classes.
Any method you write must be able to handle the following four scenarios:
Your methods can use the getattr and putattr subroutines to get and modify attributes. The getattr subroutine checks both the PdAt and CuAt object classes before returning an attribute to you. It always returns the information in the form of a CuAt object even if returning the default value from the PdAt object class.
Use the putattr subroutine to modify these attributes.
getattr subroutine, putattr subroutine.
Customized Attribute (CuAt) Object Class in AIX Technical Reference: Kernel and Subsystems Volume 1.
Predefined Attribute (PdAt) Object Class in AIX Version 4.3 Technical Reference: Kernel and Subsystems Volume 1.