a.) process creation - Process creation in UNIX
The seven-state logical process model we considered in a previous lecture can accommodate the UNIX process model with some modifications, actually becoming a ten state model.
First, as we previously observed, UNIX executes most kernel services within a process's context, by implementing a mechanism which separates between the two possible modes of execution of a process. Hence our previously unique ``Running'' state must actually be split in a ``User Running'' state and a ``Kernel Running'' state. Moreover a process preemption mechanism is usually implemented in the UNIX scheduler to enforce priority. This allows a process returning from a system call (hence after having run in kernel mode) to be immediately blocked and put in the ready processes queue instead of returning to user mode running, leaving the CPU to another process. So it's worth considering a ``Preempted'' state as a special case of ``Blocked''. Moreover, among exited processes there's a distinction between those which have a parent process that waits for their completion (possibly to clean after them), and those which upon termination have an active parent that might decide to wait for them sometime in the future (and then be immediately notified of its children's termination) . These last processes are called ``Zombie'', while the others are ``Exited''. The difference is that the system needs to maintain an entry in the process table for a zombie, since its parent might reference it in the future, while the entry for an exited (and waited for) process can be discarded without further fiddling. So the much talked about ``Zombie'' processes of UNIX are nothing but entries in a system table, the system having already disposed of all the rest of their image. This process model is depicted in fig. 5.
b.)process termination - Processes terminate in one of two ways:
- Normal Termination occurs by a return from main or when requested by an explicit call to exit or _exit.
- Abnormal Termination occurs as the default action of a signal or when requested by abort
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