Question

I have executed a block of code. And it is as shown below:

#include<stdio.h>

main() {
int i=0;
fork();
printf("The value of i is:%d\n",++i);
fork();
printf("The value of j is:%d\n",++i);
fork();
wait();
}

And I got following output:

The value of i is:1
The value of j is:2
The value of i is:1
The value of j is:2
The value of j is:2
pckoders@ubuntu:~$ The value of j is:2

Can anyone explain to me what role fork() and wait() functions play here?

Answer 1

The program generates a tree of processes. At every fork, this tree branches in two. If you grab a piece of paper, it’s not very hard to draw this tree; the only thing that’s hard is getting the i values right due to your use of prefix ++. If you let each of the process sleep for a few seconds at the end, you can also observe the tree using the pstree program.

Each one of the processes then runs the wait system call, which waits for any one of its child processes (child nodes in the process tree) to finish.