C fork dealing with global variable

Question

I'm not understanding the output of this program:

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>

int i = 0;

int main()
{
    while(i<3)
    {
        fork();

        printf("%d\n",i);
        ++i;
    }
}

The output is:

0
1
2
2
1
2
0
1
2
2
2
1
2
2

Can please someone tell me how I should tackle this issue in order to fully understand why I'm getting this output?

Answer 1

Fork will make a copy of the process. An independent copy of the process. So, if a global variable contains 3 at the time you fork, each copy of the process gets their very own 3. And if they modify, their modifications are completely independent.

Answer 2

Change your code to this and the output should make a lot more sense:

#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>

int i = 0;

int main()
{
    while (i < 3)
    {
        fork();
        printf("pid = %d, i = %d\n", getpid(), i);
        ++i;
    }
    return 0;
}