Question

I’m working on an exercise on the textbook “Operating System Concepts 7th Edition”, and I’m a bit confused about how does fork() work. From my understanding, fork() creates a child process which runs concurrently with its parent. But then, how do we know exactly which process runs first? I meant the order of execution.

Problem
Write a C program using fork() system call that generates the Fibonacci sequence in the child process. The number of sequence will be provided in the command line.

This is my solution:

#include <sys/types.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

void display_fibonacci_sequence( int n ) {
    int i = 0;
    int a = 1;
    int b = 1;
    int value;
    printf( "%d, %d, ", a, b );
    for( ;i < n - 2; ++i ) {
        value = a + b;
        printf( "%d, ", value );
        a = b;
        b = value;
    }
    printf( "\n" );
}

int main( int argc, char** argv ) {
    int n;
    pid_t pid;
    pid = fork();
    if( argc != 2 ) {
        fprintf( stderr, "Invalid arguments" );
        exit( -1 );
    }
    n = atoi( argv[1] );

    if( pid < 0 ) {
        fprintf( stderr, "Fork failed" );
        exit( -1 );
    }
    else if( pid == 0 ) {
        display_fibonacci_sequence( n );
    }
    else { // parent process
        // what do we need to do here? 
    }
}

To be honest, I don’t see any difference between using fork and not using fork. Besides, if I want the parent process to handle the input from user, and let the child process handle the display, how could I do that?

Answer 1

We don’t know which runs first, the parent or the child. This is why the parent generally has to wait for the child process to complete if there is some dependency on order of execution between them.

In your specific problem, there isn’t any particular reason to use fork(). Your professor probably gave you this just for a trivial example.

If you want the parent to handle input and the child to calculate, all you have to do is move the call to fork() below the point at which you handle the command-line args. Using the same basic logic as above, have the child call display_fibonacci_sequence, and have the parent simply wait