Tracing a program with multiple forks in if/else block

Question

I'm trying to trace this program. I see that it outputs 4 times when I run it, but I don't understand why.

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
int main() 
{
    if (fork() == 0)
        fork();
    else 
    {
        fork();
        fork();
        printf("%d\n", getpid());
    }
}

To my knowledge, the if/else forks my program, then if we're in the child, it forks again. If we're in the parent, it runs the code in the else block. I get confused when I try to trace after this point, though.

• Once the else statement forks again, aren't we in another child?

• When do the children stop being spawned?

Answer 1

After getting it explained for me, I understand it now.

The first fork will spawn a child (let's call it c1):

if (fork() == 0)

The return value of fork is 0 when you're in the child. So, c1 will execute the if statement block:

fork();

The child created here, c2 (as well as c1) will both die, as they aren't going to execute the else block.

---

Meanwhile, the parent process will execute the else block.

fork();

This creates another child of the original parent process (c3). c3 will execute the next fork in the else block.

fork();

Now, we'll have c4, too.

---

Meanwhile, the original parent process will still have a fork that hasn't run. This creates the final child process, c5.

At the end of the run, there will be 4 prints: The original parent process, c3, c4, and c5.