I don't understand this diagram of fork()

Question

How we can get this process with this condition??schema of process?

    int main (int argc, char **argv) {
    int i;
    int pid;
       for (i= 0; i < 3; i++) {
            pid = fork();

            if (pid < 0) break;// with this condition i dont understand??

       }
           while (wait(NULL) != -1);

Answer 1

fork() splits a process in two, and returns 0 (if this process is the child), or the PID of the child (if this process is the parent), or -1 if the fork failed. So, this line:

if (pid < 0) break;

Says "exit the loop if we failed to create a child process".

The diagram is a little confusing because of the way the processes (circles) correspond to the fork() calls in the loop. The three child processes of the main process are created when i is 0, 1, and 2 respectively (see the diagram at the bottom of this post).

Since the loop continues in both the parent and the child process from the point fork was called, this is how the forks happen:

• i == 0: fork is called in the original parent. There are now two processes (the top one, and the left one).
• i == 1: fork is called in the two existing processes. New children are the leftmost child on the second layer from the bottom, and the middle child on the third layer from the bottom. There are now four processes
• i == 2: fork is called in all existing processes. New children are all remaining nodes (the bottom node, the two rightmost nodes in the second layer from the borrom, and the rightmost node in the third layer from the bottom)
• i == 3: All 8 processes exit the loop

Here is the diagram again, with numbers indicating what the value of i was in the loop when the process was created:

                 -1  <--- this is the  parent that starts the loop
              /   |  \
             0    1   2
           /  \   |
          1    2  2
          |
          2

Answer 2

To understand your diagram you must rely on the behavior of fork: it splits the process in two, creating another process identical to the first (except for the PID) in a new memory location.

If you call it in a loop that's what happen:

When i=0 the first process will be split, creating another process that will start running from exactly this point on (so will skip the first loop). Focusing on the first process, it will continue the loop, generating another process when i=1. The second process, thus, will start from i=1, so will skip the first two loops. The first process will be split last time for i=2. The last copy created, however, will start running from i=2, so it will exit the loop and will not generate anything.

The first copy created will start the loop from i=1, generating two process, while the second copy will start from i=2, generating only one copy.

You can continue this reasoning and understand the rest of the diagram.

As others pointed out, if (pid < 0) is just a check to see if there are errors and does not modify the logic of the code.