Can the order of execution of fork() be determined?

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You are asking many questions, I’ll try to answer them in a convenient order.

First question

To be honest, I don’t see any difference between using fork and not
using fork.

That’s because the example is not a very good one. In your example the parent doesn’t do anything so the fork is useless.

Second

else {
    // what do we need to do here? 
}

You need to wait(2) for the child to terminate. Make sure you read that page carefully.

Third

I want the parent process to handle the input from user, and let the
child process handle the display

Read the input before the fork and “handle” the display inside if (pid == 0)

Fourth

But then, how do we know exactly which process runs first?

Very few programs should concern themselves with this. You can’t know the order of execution, it’s entirely dependent on the environment. TLPI says this:

After a fork(), it is indeterminate which process—the parent or the
child—next has access to the CPU. On a multiprocessor system, they may both simultaneously get access to a CPU.

Applications that implicitly or explicitly rely on a particular
sequence of execution in order to achieve correct results are open to
failure due to race conditions

That said, an operating system can allow you to control this order. For instance, Linux has /proc/sys/kernel/sched_child_runs_first.

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