Structure Confusion

The handful of structures thus far discussed is confusing not because of their sheer number (there are only four) or their complexity (they are all fairly simple), but because they are all very interrelated. In the world of kobjects, it is hard to discuss one structure without involving the others. With the basics of each structure covered, however, you can develop a firm understanding of their relationships.

The important guy here is the kobject, represented by struct kobject. The kobject is used to introduce basic object propertiessuch as reference counting, parent-child relationship, and object nameto kernel data structures. The kobject can provide these features in a standard unified way. kobjects, in and of themselves, are not overly useful. Instead, kobjects are typically embedded in other data structures.

kobjects are associated with a specific ktype, which is represented by struct kobj_type and pointed at by the ktype variable inside of the kobject. ktypes define some default properties of related kobjects: deconstruction behavior, sysfs behavior, and default attributes.

kobjects are then grouped into sets, called ksets. ksets are represented by struct kset. ksets provide two functions. First, their embedded kobjects act as a base class for a group of kobjects. Second, ksets aggregate together related kobjects. In sysfs, kobjects are the individual directories in the filesystem. Related directoriessay, perhaps all subdirectories of a given directorymight be in the same kset.

Subsystems represent large pieces of the kernel and are collections of ksets. They are represented by struct subsystem. The root level directories in sysfs are all subsystems.

Figure 17.1 depicts the relationship between these data structures.