A child can return an integer to its parent on completion and all children must have their completion checked by their parent. Children whose parents exit before them become children of the system init process. Processes can form other associations for the purpose of communication. These associations can be formed after the processes have been created and can change dynamically. In UNIX the ordering of processes imposed by their creation order is called the process heirarchy and the dynamic process associations are called process groups.
Some systems have both or neither.
Signals Signals are OS abstractions of interrupts. They are indications to a process that some asynchronous event has occurred. Processes can generally define handler procedures that are invoked when a signalis delivered. Only one process is inthis state on a given processor. The process is ready to run These states may be defined implicitly.