The following table summarizes the possible process states in UNIX operating system. In other words, a UNIX process at any given point of time has one of these nine states.
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User Running
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Executing in user mode.
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Kernel Running
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Executing in kernel mode.
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Ready to Run, in memory
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Ready to run as soon as the kernel schedules it.
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A sleep in memory
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Unable to execute until an event occurs; process is in main memory (a blocked state).
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Read to Run Swapped
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Process is ready to run, but the swapper must swap the process into main memory before the kernel can schedule it to execute.
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Sleeping, Swapped
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The process is awaiting an event and has been swapped to secondary storage (a blocked state).
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Preempted
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Process is returning from kernel to user mode, but the kernel preempts it and does a process switch to schedule another process.
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Created
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Process is newly created and not yet ready to run.
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Zombie
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Process no longer exists, but it leaves a record for its parent process to collect.
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The Zombie state denotes that a process was terminated.
Why does UNIX need to use a state to represent a process that no longer exists?
(Interpretation:
How is the Zombie state used?
Is the Zombie state really useful?
Please give an example to show how the Zombie state may be significant or useful.)