Factorial calculation using fork() in C for Linux

Write a Unix C program using the fork() system call that generates the factorial and gives a sequence of series like 1, 2, 6, 24, 120… in the child process.
The number of the sequence is provided in the command line.
Examples:

Input :gfg@ubuntu:~/$ gcc -o fork fork.c
       gfg@ubuntu:~/$ ./fork 6

Output :1 
        1  2 
        1  2  6 
        1  2  6  24 
        1  2  6  24  120 
        1  2  6  24  120  720 
        After deletion sum
        1  3  9  33  153  873 Done

Input :gfg@ubuntu:~/$ gcc -o fork fork.c
       gfg@ubuntu:~/$ ./fork -2

Output :negative number entered -2

To create child process fork()is used. fork() returns :

• < 0 fail to create child (new) process
• = 0 for child process
• > 0 i.e process ID of the child process to the parent process. When >0 parent process will execute.

Inside Child process: If input is 6, the first six numbers of the factorial sequence will be given as output by the child process. Because the parent and child processes have their own copies of the data, it will be necessary for the child to output the sequence.
Inside Parent process: The parent invokes the wait () call to wait for the child process to complete before exiting the program. Perform necessary error checking to ensure that a non-negative number is not passed on the command line.

// C program to illustrate factorial calculation
// using fork() in C for Linux
#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <sys/types.h>
#include <string.h>
#include <stdlib.h>

int main(int argc , char *argv[] )
{
    pid_t pid;

    if (argc != 2)
    {
        printf("arg missing or exceeding\n");
        exit(0);
    }

    // atoi converts string to integer
    if ( atoi ( argv[1] ) <0 )
    {
        printf("negative number entered %d", atoi(argv[1]));
        exit(0);
    }

    pid=fork();

    if ( pid<0 )
    {
        printf("failed to create child\n");
        exit(0);
    }

    else if ( pid==0 )
    {
        //Child Process
        int ans = 0, i, j, k = 2, n;

        // atoi converts string to integer
        n = atoi(argv[1]);
        int arr[n],sum[n];

        arr[0] = 1;

        // generating factorial series
        for (i=1 ; i<n; i++)
        {
            arr[i] = arr[i-1]*k;
            k++;
        }

        // printing and sum of each sub-series
        for (j=0; j<n; j++)
        {
            sum[j] = 0;
            for (i=0; i<=j; i++)
            {
                printf(" %d ",arr[i]);
                sum[j]+=arr[i];
            }
            printf("\n");
        }

        for (i=0; i<n; i++)
        {
            if ((sum[i]%2) == 0)
                sum[i] = -1;
        }

        printf("After deletion sum\n");
        for (i=0; i<n; i++)
        {
            if (sum[i] > 0)
                printf(" %d ", sum[i]);
        }
        exit(0);
    }

    // parent process
    else
    {
        wait(NULL);

        // waiting for child process to end
        printf("Done\n");
    }
}

Compiling Code code saved with name fork.c

gfg@ubuntu:~/$ gcc -o fork fork.c

Input:

gfg@ubuntu:~/$ ./fork 5

Output:

 1 
 1  2 
 1  2  6 
 1  2  6  24 
 1  2  6  24  120 
After deletion sum
 1  3  9  33  153 Done

Firstly, command line argument is accepted. Then the number of arguments are passed and checked if the argument is positive.
After that, generate the factorial series.

References:

• http://www.csl.mtu.edu/cs4411.ck/www/NOTES/process/fork/create.html
• http://www.csl.mtu.edu/cs4411.ck/www/NOTES/process/fork/wait.html