Question

Using the program below, explain what the output will be at LINE A. 1 #include 2#include 3#include 4 5 int value - 128; 6 7 int main 8 9 pid.t pid; 10 11 pid=fork 12 13 if (pid 0) { /* child process */ 14 15 16 value +8; return 0; 17 else if (pid > 0) parent process / 18 19 20 21 wait (NULL); printf ("PARENT: return 0; value =%d\n" ,value); /* LINEA */

Answer 1

Answer:

This is the complete correct program:

#include <stdio.h>

#include<sys/types.h>

#include<unistd.h>

int value = 128;

int main()

{

  pid_t pid;

  pid=fork();

  if (pid==0) /* child process */

  {

  value +=8;

  return 0; }

  else if (pid > 0) {/* parent process */

 wait (NULL);

 printf ("PARENT: value =%d\n" ,value); /* LINEA */

 return 0;

}

}

The output of the LINE A is:

PARENT: value = 128

Explanation:

The fork() function used in the program creates a new process and this process is the child process. The child process is same as the original process having its own address space or memory.

In the child process the value of pid is 0. So the if condition checks if pid==0. Then the child process adds 8 to the value of its variable according to the following statement  

value +=8;

Now the original process has value = 128. In else if part the parents process has the value of pid greater than zero and this portion of the program is of the parent process :

else if (pid > 0)

{ wait (NULL);

printf ("PARENT: value =%d\n" ,value);

return 0; }

So the value 128 is printed at the end in the output.

wait(NULL) is used to wait for the child process to terminate so the parent process waits untill child process completes.

So the conclusion is that even if the value of the variable pid is changed in the child process but it will not affect the value in the variable of the parent process.