C语言中的Fork（）程序

Question

so I am attempting to solve a problem in my book regarding the fork system call in C.

Here is the following code:

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

int main() {  

int i = 1;

   if (fork ()) //parent process will return PID of child.
        i++;
   else if (fork()) //child process (becomes parent)
            i--;
       else //grandchild process returns 0
            i++;

 printf("%d\n", i);
}

After going through the code, I got 2 0 2 as the solution. I am confused though since I don't know which order is correct? Can the child processes get printed out before the parent does? If so, another viable solution could be 0 2 2 or 2 2 0. How do I know if all the conditional statements will be executed? I know in the first conditional, the if statement will be executed and will be the parent process (which will return the process ID of the child, and the value of i will increment to 2). How would I know if the else if and else code will be executed? Sorry, I am just confused and trying to wrap my head around this as this is a new topic for me.

Any help would be greatly appreciated. Thank you in advance.

Answer 1

The child process can be printed before the parent finishes. Remember, in the parent process, the call to fork() returns a nonzero number, while in the child it will return 0.

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

int main() {  

int i = 1;

   if (fork ())
        i++;
   else if (fork())
            i--;
       else
            i++;

 printf("%d\n", i);
}

For the above code, the parent will execute the first if() statement, the child will execute the else if statement and the grandchild will execute the else statement. The child/grandchild/parent can be printed in any order. So, 2 0 2, 2 2 0 and 0 2 2 are all viable solutions.