Question

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

P |------ ---- > c1 |------------>c2

                             |------- ------>c3 ---------->c4

                           

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

A sample run of the program shows

Your program should output something similar to what is shown above. You could optionally use wait/waitpid/sleep/exit in your program.

Comment your code to show where you created p, c1, c2, c3, c4, etc.

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: c2
                     */
                    printf("c2 : [0x%x], c2's parent : [0x%x]\n",
                           getpid(), getppid());
                }
                else
                {
                    /*
                     * Context c1
                     */
                    {
                        pid_t pid = fork();

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

                            {
                                pid_t pid = fork();

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

                        }
                    }
                }
            }
        }
    }


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


    return 0;
}

Following is a sample run :

./multiple_forks 
P : [0x1f10]
c1 : [0x1f11], c1's parent : [0x1f10]
c2 : [0x1f12], c2's parent : [0x1f11]
c3 : [0x1f13], c3's parent : [0x1f11]
c4 : [0x1f14], c4's parent : [0x1f13]

As seen,

• The parent-process ID for c1 (0x1f10) is equal to the process ID of P.
• The parent-process IDs for c2 and c3 are equal (0x1f11) to the process ID of c1.
• The parent-process ID for c4 (0x1f13) is equal to the process ID of c3.