... process¹

A process is the object created by an operating system for (managing) the execution of a program.

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... checker²

To be honest, if you wanted to do this you wouldn't use the fork  command. Instead you would probably use the exec command - but the exec  command uses fork to create the process, so fork is still the focus of attention.

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... perhaps³

Actually, the use of fork above is not in the traditional form. Traditionally $C$ and $C^{++}$ programmers would put the assignment on line 10 into the selection condition on line 11 as follows.

if ((pid = fork()) > 0)

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... (zero)!⁴

The example clearly contains a third possible scenario: the fork  command could also return a negative value. This is done when some error occurred during process creation; for example, there is not enough memory to create another process.

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