Accessing Device Attributes

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.

Modifying an Attribute 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:

• If the new value differs from the default value and no object currently exists in the CuAt object class, any method you write must add an object into the CuAt object class to identify the new value.
• If the new value differs from the default value and an object already exists in the CuAt object class, any method you write must update the CuAt object with the new value.
• If the new value is the same as the default value and an object exists in the CuAt object class, any method you write must delete the CuAt object for the attribute.
• If the new value is the same as the default value and no object exists in the CuAt object class, any method you write does not need to do anything.

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.

Related Information

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.