Processes created by a program have a parent/child relationship. When a process creates multiple children,
these children have sibling relationships. Several fields must be introduced in a process descriptor to represent
these relationships; Processes 0 and 1 are created by the kernel; as we will see later in the chapter, process 1
(init) is the ancestor of all other processes.
ULK --- Chap3 Processes: Relationships Among Processes
原文：http://www.cnblogs.com/miaoyong/p/4948959.html