Operations on processes: creation and termination · CAP366

A parent creates children with fork(). The child is a copy of the parent; fork returns 0 in the child, the child's PID in the parent, and -1 on error. The child may then call exec() to load another program, and the parent may wait() for it. A process ends with exit().

Counting processes: n unconditional sequential forks give 2^n processes in total (2^n - 1 children). Example: fork(); fork(); fork(); gives 8 processes. With a condition, trace each branch: if (fork()==0) fork(); fork(); gives 4 + 2 = 6 processes.

Term	Meaning
Zombie	Has terminated, but still occupies a process-table entry until the parent calls wait()
Orphan	Parent exited while it still runs; adopted by init

Exam pitfall: a zombie is finished and uses no CPU; it cannot be killed by a signal because it is already dead. An orphan is still running, and the two terms are often swapped.

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Question 1 · Tier 1, Recall

Which Unix system call creates a new process by duplicating the calling process?

fork()
exec()
wait()
exit()

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A. fork() creates a child that is a copy of the parent. exec() is usually called afterwards to load a different program.

Why not B: exec replaces the current program image and creates no new process.

Why not C: wait only makes the parent block until a child terminates.

Why not D: exit terminates the calling process.

Question 2 · Tier 2, Understand

Which of the following is a feature of an orphan process?

It is waiting for I/O
It has no PCB
Its parent has terminated while it is still running
It has finished but its parent has not yet read its exit status

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C. A zombie has finished but is still in the process table awaiting wait() by the parent; an orphan has lost its parent (adopted by init).

Key points

Try two questions