Forking inside conditional statements

Question

How would I calculate how many "hello world" are printed after the execution of this program. Based on how fork works, we know that fork returns non-zero value as a parent process, and a child process returns 0.

I have worked with fork() before, and calculating fork count when in loops, but have not done anything with conditional statements, how would I begin to break this problem down?

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

int main() {
    (fork() && fork() || fork() && fork() || fork() && fork() && fork());
    printf("hello world");
    return 0;
}

Answer 1

To better understand what's going on you should try to draw a tree with parent and children processes.

Consider that after a fork():

• the parent process return a number > 0
• child process return a number == 0

that means:

• && operator allows multiple children for the same parent because as soon as the pid=0 (child process) the result is 0 and the condition is no longer considered
• || operator allows single child for the parent processes because as soon as the pid!=0 (parent process) the result is !=0 and the condition is no longer considered

Visualizing the trees related with much simpler cases than yours:

fork() && fork() && fork()

                                  P
                                / | \
                              C1 C2 C3
       

fork() || fork() || fork()

                                      P
                                     /
                                    C1
                                   /
                                  C2
                                 /
                               C3

As a suggestion, if you want to understand more about what's going on and be able to build the tree, replace your printf() with:

printf("hello world - pid %d, ppid %d\n", getpid(), getppid());

so you can understand more easily how the process tree has been created.