Question

considering the following instruction:

for(int i = 0; i < 3; i++)
    fork();

I try to figure out the number of created process and the steps of graph creation.

so the code above is equivalent than:

fork();
fork();
fork();

The official answer to this problem is illustrated with this graph:

I am not able to visualize how this graph is created.

This is how I would have drawn the graph.

1. so, the first fork will create a child copy (p2) of the parent process (p1). We have 2 process.

2. the second fork, will duplicate p1 and p2 parents, by creating child process (p3 and p4).

3. The third fork, will duplicate p1, p2, p3 and p4, by creating child process (p5, p6, p7, p8)

How can I obtain the same graph as my teacher?

Answer 1

If you want a graph similar to the official answer, try to stop thinking about how things run concurrently and instead concentrate on the generations of processes (parents, children, grandchildren and so on).

At the start, there is one process p0, with three forks to go. When doing those three forks, it creates p1 with two forks to go, p2 with one fork to go and p3 with no forks left. Then p0 exits (only p1, p2 and p3 remain).

We can toss away p3 since it has no forks left, leaving just p1 and p2).

Process p1 then executes its second fork producing p4 with one fork left, then executes the third fork making p5 with no forks left. p1 is now done and exits (p2, p4 and p5 remain).

Similar to p3, p5 can be tossed because it has no forks left. This leaves p2 and p4.

Similarly, p2 had one fork left so it creates p6 with no forks left. Then both p2 and p6 exit due to having no forks left, leaving p4.

Process p4 had one fork left so it creates p7 with no forks, and they both then exit.

By drawing the chart with the depth based on parentage rather than when processes are started (although the starting time^(a) controls where the process exists horizontally at a specific depth, eg, see p1, p2 and p3), your diagram should match the one given.

So think of it this way:

  Sequence within generation -------->
G
e           ______p00______
n          /       |       \
e       p01       p02       p03
r      /   \       |
a   p04     p05   p06
t    |
i   p07
o
n
|
V

---

^(a) Keep in mind that starting time as defined here is when the process comes into existence – the order in which processes do actual useful work also depends on the vagaries of scheduling.