Question

I have tried implementing an os program. Here is the code:

#include<sys/types.h>
#include<stdio.h>
#include<unistd.h>

int main()
{
    pid_t pid, pid1;

    pid = fork();

    if(pid<0)
    {
            fprintf(stderr,"Fork Failed");
            return 1;
    }

    else if(pid == 0) /* child process */
    {
            pid1 = getpid();
            printf("child: pid = %d\n",pid);
            printf("child: pid1 = %d\n",pid1);

    }

    else   /* parent process */
    {
            pid1 = getpid();
            printf("parent: pid = %d\n",pid);
            printf("parent: pid1 = %d\n",pid1);

    }

    return 0;
}

and its o/p:

parent: pid = 1836
parent: pid1 = 1835
child: pid = 0
child: pid1 = 1836

can somebody explain me how is it working , i.e. the sequence of the execution for the if/else-if/else statements written in the code. I would think once the else if condition becomes true then else part is not executed, however here it has executed the parent process part i.e. else part and then the child part ….. how come?

Answer 1

You should read up on fork(). Once you hit a fork() statement a second process is started, it has a copy of everything the parent process has but it can run a separate execution, and the return it sees from the fork is different than what the parent sees.

 int main()
 {
   pid_t pid, pid1;
                   <--- up to here you have one process running
   pid = fork();   <--- when this returns you have two processes:
                          parent has pid = child's pid               child has pid = 0


   if(pid<0)       <--- child and parent both check this, it's not true so they move on
   {
     ....
   }
   else if(pid == 0)<--- this is true for the child, not the parent
   {
     ....           <--- child will now execute this code
   }
   else             <-- nothing else was true for the parent so it sees this
     ....           <-- and executes this code

So yes, you are correct, once you hit the if, or the else if or the else you’re not going to get into another branch of the code, in a single process’ execution. You’re seeing the else if and the else because you have two processes running.

note how the pid1‘s are different, because getpid() is returning which process is running that code, and you can see you have two different processes, one picks the else if the other picks the else.