Q:

Where does fork() start from

So as far as i know fork creates a duplicate of the process it's called from but it also copy's it's program counter so it continues from the line after it's called but why is this code printing hello world twice when it's before the fork
#include <stdio.h>
#include <sys/wait.h>
int main()
{
    printf("Hello World");

    fork();

    wait(NULL);
    return 0;
}

A:

printf doesn't actually print -- it actually just puts data into a buffer to be printed later.  It will actually be printed when the buffer gets flushed, which can happen in a variety of ways.
In your case, the buffer flush doesn't happen until after the fork, so both the parent and the child have a copy of the string to be printed in the buffer when they fork, and both end up printing it.

