Consider the example

#include <stdio.h>
#include <unistd.h>

int main()
{
    int id;
    printf("Hello, World!\n");

    id = fork();
    if (id > 0) {
        /*parent process*/
        printf("This is parent section [Process id: %d].\n", getpid());
    }
    else if (id == 0) {
        /*child process*/
        printf("fork created [Process id: %d].\n", getpid());
        printf("fork parent process id: %d.\n", getppid());
    }
    else {
        /*fork creation faile*/
        printf("fork creation failed!!!\n");
    }

    return 0;
}

Output:

Hello, World!
This is parent section [Process id: 1252].
fork created [Process id: 1253].
fork parent process id: 1252.

Explanation:

This is parent section [Process id: 1252].
1252 is the parent process id which is the process id of the main function too; this is the parent section of new created fork.

fork created [Process id: 1253].
This message will print under the fork section and assigns a new id to the newly created child process.

fork parent process id: 1252.
This message will also print under the fork section, it shows parent process id of the newly created child process which is equivalent to parent process’s id (main process id). Hence we can say newly created child process is the child process of main which is known as parent process.

Another Example using fork()

In this example, we will print natural numbers from 1 to 10 using for loop and we will create a fork() - you will see numbers will be printed twice, because of for(), it will create duplicate copy of the process.

#include <stdio.h>
#include <unistd.h>

int main()
{
    int id, i;

    printf("Start of main...\n");

    id = fork();
    if (id > 0) {
        /*parent process*/
        printf("Parent section...\n");
    }
    else if (id == 0) {
        /*child process*/
        printf("\nfork created...\n");
    }
    else {
        /*fork creation faile*/
        printf("\nfork creation failed!!!\n");
    }

    printf("Printing the numbers from 1 to 10\n");
    for (i = 1; i <= 10; i++)
        printf("%d ", i);
    printf("\n");
    printf("End of the main function...\n");

    return 0;
}

Output:

Start of main...
Parent section... 
Printing the numbers from 1 to 10 
1 2 3 4 5 6 7 8 9 10
End of the main function... 

fork created... 
Printing the numbers from 1 to 10 
1 2 3 4 5 6 7 8 9 10
End of the main function...