Controlling Forks in C

Question

I have a c file that looks like this:

#include <stdio.h> 
#include <sys/types.h> 
#include <unistd.h> 
int main () 
{ 
    pid_t child_pid; 
    printf ("The PID is %d\n", (int) getpid ()); 
    child_pid = fork (); 
    if (child_pid != 0)
    { 
     printf ("this is the parent process, with PID %d\n", 
     (int)getpid()); 
     printf ("the child's PID is %d\n", (int) child_pid); 
    } 
    else 
     printf ("this is the child process, with PID %d\n", 
     (int)getpid()); 
return 0; 
}

I need to modify it to produce a a hierarchy that looks like

• 0
  • >1
  • >2
  • >>3
  • >>4
  • >>>5

Basically a tree structure where each second child makes two new children. As far as I understand it, when I fork a process, each process will run concurrently. Adding a fork in the if statement seems to work and creates processes 0 to 2 correctly, since only the parent will create a new fork. But I have no idea how to make process 2 fork and not 1. Any ideas?

Answer 1

Well, process 1 will be created by the first fork. Process 2 will be created by the fork inside the if-statement. So to let process 2 fork too, you fork again inside the if-statement if the second fork did not return 0.

An illustration:

if(fork) {
    // Inside process 0
    if(fork) {
        // still in process 0
    } else {
        // in process 2
        if(fork) {
          // still in process 2
        } else {
          // in prcess 3
        }
        // and so on
    }
} else {
    // Inside process 1
}

Answer 2

Children get a copy of the parent's state at the time of the fork.

So if the parent has a counter or other property then the children will see the value at the time of they were forked (but not if the parent subsequently alters it).

Answer 3

I don't know why you want to do it this way, but normally only the parent process performs the fork. This may be easier to design too. When you perform fork() inside a for-loop you'll have direct control over the processes created.

Please be aware that fork() is a relatively expensive operation, especially if you want to create many processes. More lightweight alternatives vfork and threads are available but I can not judge if they also fit your needs.