Question

Write a C program to create a series of processes, as shown below in the diagram.

P - ------------------ > c1 ------------>c3

   |                                          |

   |------------------> c2 |----- >c4

In other words, the parent process p creates processes c1 and c2, c1 creates c3 and c4.

Answer 1

We use the fork() call, which returns a value of type pid_t. The distinguishing factor is :

• If the return value returned is equal to 0, it means we are in the child-process's context.
• If the return value returned is not equal to 0, it means we are in the parent-process's context.

Following is the code in multiple_forks.c :

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

int main()
{
    /*
     * P
     */
    printf("P : [0x%x]\n", getpid());

    {
        pid_t pid = fork();

        if(pid == 0)
        {
            /*
             * Context: c1
             */
            printf("c1 : [0x%x], c1's parent : [0x%x]\n",
                   getpid(), getppid());

            {
                pid_t pid = fork();

                if(pid == 0)
                {
                    /*
                     * Context: c3
                     */
                    printf("c3 : [0x%x], c3's parent : [0x%x]\n",
                           getpid(), getppid());
                }
                else
                {
                    /*
                     * Context c1
                     */
                    {
                        pid_t pid = fork();

                        if(pid == 0)
                        {
                            /*
                             * Context: c4
                             */
                            printf("c4 : [0x%x], c4's parent : [0x%x]\n",
                                   getpid(), getppid());

                        }
                    }
                }
            }
        }
        else
        {
            /*
             * Context: P
             */
            {
                pid_t pid = fork();

                if(pid == 0)
                {
                    /*
                     * Context: c2
                     */
                    printf("c2 : [0x%x], c2's parent : [0x%x]\n",
                           getpid(), getppid());
                }
            }
        }
    }


    /*
     * Loop indefinitely, so that no process dies.
     */
    while(1)
    {
        sleep(1000);
    }


    return 0;
}

Following is the compilation step :

gcc multiple_forks.c -o multiple_forks

Following is a sample run :

./multiple_forks 
P : [0x1e6a]
c1 : [0x1e6b], c1's parent : [0x1e6a]
c2 : [0x1e6c], c2's parent : [0x1e6a]
c3 : [0x1e6d], c3's parent : [0x1e6b]
c4 : [0x1e6e], c4's parent : [0x1e6b]

As seen,

• The parent-process IDs for c1 and c2 are equal (0x1e6a) to the process ID of P.
• The parent-process IDs for c3 and c4 are equal (0x1e6b) to the process ID of c1.

We see the same pattern across multiple runs, as follows :

./multiple_forks 
P : [0x1e82]
c2 : [0x1e84], c2's parent : [0x1e82]
c1 : [0x1e83], c1's parent : [0x1e82]
c3 : [0x1e85], c3's parent : [0x1e83]
c4 : [0x1e86], c4's parent : [0x1e83]


./multiple_forks 
P : [0x1e87]
c1 : [0x1e88], c1's parent : [0x1e87]
c2 : [0x1e89], c2's parent : [0x1e87]
c3 : [0x1e8a], c3's parent : [0x1e88]
c4 : [0x1e8b], c4's parent : [0x1e88]


./multiple_forks 
P : [0x1e8c]
c1 : [0x1e8d], c1's parent : [0x1e8c]
c2 : [0x1e8e], c2's parent : [0x1e8c]
c3 : [0x1e8f], c3's parent : [0x1e8d]
c4 : [0x1e90], c4's parent : [0x1e8d]