Question

Complete the following C code by filling all “???”sto write a C program to create a child process. Before the child process is created, an integer variable A is defined and assigned value 10. Then in the child process, A is reduced by 5. After the child process completes, the parent process increases A by 5. Then what will A’s value be just before the parent process completes? Why? Write the code and run it to verify your answer.

#include <stdio.h>
// include the head files that allow us to use fork()
#include <???                                >
#include <???                                >

// define integer variable A and assign value 10 to it
???


int main()
{
        // define a variable pid that holds the return value of fork() 
        ???


        // call fork() to create a child process
        ???


        // check the returned value of fork() for call failure, being child process or parent process
if (???                 )
        { 
                // call failure, output to stderr ¡°Fork Failed¡±
                ???


                return 1;
        }
        else if (???                    )
        { 
                // this process is the child process, reduce A¡¯s value by 5
                ???

                
                // output to screen ¡°In child process, A=(A)¡±, where (A) is A¡¯s value
                ???

        }
        else
        {
// this process is the parent process, first wait for the child process to complete
                ???


// then increate A¡¯s value by 5
                ???


                // output to screen ¡°In parent process, A=(A)¡±, where (A) is A¡¯s value               
                ???

        }
        
        return 0;
}

Answer 1

Here is the code and output, explanation for the output is after the output and the code

Output: we can see that output is always 15 in the parent process

CODE:

 #include<stdio.h> #include<sys/types.h> #include<unistd.h> int main() { //A=10 int A=10; //create a child process if(fork()==0) { //if control is in child process, decrease A by 5 A=A-5; //print the value printf("Child process A=%d\n",A); } else { //if control is in child process, increase A by 5 A=A+5; //print the value printf("Parent process A=%d\n",A); } return 0; }

Explanation:

fork() returns 0 in child process and returns the PID(process id) of child process inside parent process.

Assuming that we have an OS that uses virtual memory(any modern OS), the child process gets the exact copy of the virtual memory that parent has.

In simple terms, child process will have exact replica of all variables of parent process in a separate location.

If we print the address of A in both child and parent, we will have same address, but in reality, physically A of child and parent will be in completely different locations.

So, even though we use same variable A in child and parent, they are actually in different memory locations

So, in parent process, A=A+5; 15 will be printed.

in child process, A=A-5;  5 will be printed.

Extras:

Having multiple processes is very different than having multiple threads.

When we have multiple threads in a process, all the threads share same memory space, the output will become complecated in that case.