Concurrent Processes using fork() - c

Question

I am given the following code:
main()
{
int i, rc;
for (i = 0; i<=1; i++)
{
if( (rc=fork()) == 0)
{
printf("Child %d executing\n",i);
} /*end if*/
} /*end for*/
}
printf("All children created\n");
I am also given the solution of the possible permutations of which outputs may occur.
Child 0 executing |
Child 1 executing | Child 1
All children created |
Child 1 executing | Child 2
All children created |
Child 1 executing | Grand child
All children created |
All children created | Parent
I know that these outputs are created by each process but I am just having trouble tracing them to understand HOW these outputs occur. I know fork() creates a process and if (fork() == 0) means it is a child process, but if anyone can help me understand where the answers beyond Child 0 executing | come about thanks. I believe the | is just a description of which process is currently being ran. How come child 1 can create a "grand child" but child 0 can not?

Answer 1

First, the code and behavior will be easier to understand if the loop is unrolled. The code then becomes:
int rc;
if ((rc = fork()) == 0)
printf("Child 0 executing\n");
if ((rc = fork()) == 0)
printf("Child 1 executing\n");
printf("All children created\n");
Then, to help understand what's going, the best is to draw the process hierarchy as a tree. Here is an ASCII version of it:
main
/|
/ |
/ |\
child0 | \
| | \
| | child1
/| | |
/ | | |
/ | | end
/ end |
/ |
child1 end
|
|
end
In the graph, child0 is the printf statement displaying "Child 0 executing", child1 is the statement "Child 1 executing" and "end" is the printf statement displaying "All children created".
As you can see from the graph, you'll get 1x child0, 2x child1 and 4x "All children created".
UPDATE #bkennedy
Here is another view that shows the process view only, with P0 being the main (original) process and "end" indicating each process' completion:
P0
/|
/ |
/ |\
P1 | \
| | \
| | P2
/| | |
/ | | |
/ | | end
/ end |
/ |
P3 end
|
|
end
There are really 4 processes: P0 (main), P1, P2 and P3.
P1 is the first child of P0; it displays "Child 0 executing".
P2 is the second child of P0; it displays "Child 1 executing". P2 never creates any children, it just finishes with the printf statement.
P3 is the first (and only) child of P1.
Each process displays "All children created" when they finish.
Remember:
P0 (main) goes through 2 fork calls, hence the 2 children.
P1 goes through 1 fork call, hence the single child (P3).
P2 never goes through a fork call.
That's it, there is no other process creation. I'm not sure how to explain this better to you.

Answer 2

After the first fork and the if statement that it's in, the "child 0 executing" process and its silent parent process main go on to the next if statement. There they both produce a "child 1 executing" process while remaining silent. All of those four processes, ie main, the "child 0 executing" process, main's "child 1 executing" process" and the "child 0 executing" process's "child 1 executing process", go on after the second fork and the if statement that it's in to print "all children created".
One of "Child 0 executing" is output, two of "Child 1 executing" are output, and four of "All children created" are output.
How come child 1 can create a "grand child" but child 0 can not?
No "child 1" creates a child. Main and "child 0" each create a "child 1". The output apparently calls "child 0"'s "child 1" the "Grand child" of main. So no "child 1" creates a child, and both main and "child 0" produce a "child 1". So your question is wrong about both "child 1" (of which there are two) and "child 0".
HOW these outputs occur
The quoted "output" seems to call main Parent, the "child 0" process Child 0, main's "child 1" process Child 2 and the "child 0" process's "child 1" process Grand child. So you need to distinguish between what the "output" is naming the processes and what those processes are outputing. It also has three lines outputing "Child 1 executing", which is wrong. But you should be telling us what that "output" is supposed to be.