Why does hello world print twice only and why is a's largest value 2

Question

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

int main() {
    int a=0;
    int rc=fork();
    a++;
    if(rc=0)
    {
        rc=fork();

        a++;
    }
    else
    {
       a++;
    }
    printf("Hello World");
    printf("%d",a);

    return 0;
}

Answer 1

Why does hello world print twice only and why is a's largest value 2

To answer your question (after correcting your (rc = 0) vs. (rc == 0) issue), it may help to diagram what is happening in your code. For example:

    parent
      |
      a = 1
      if (rc == 0)
      +--------------- child 1
      |                   |
      else                +--------------- child 2
        a++               a++              a++
      a = 2               a = 2            a = 2
      Hello World2        Hello World2     Hello World2

In the parent process, you fork and then increment a with a++ before your test of if (rc == 0) to give direction to the child process. The original parent sees a++ again due to the else and "Hello World2" is the result.

In the first-child, you fork again, but the child-1 (as parent) and child-2 both increment a++; before dropping out of the conditional with "Hello World2" the result for both.

It appears you were attempting something similar to:

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

int main (void) {

    int a = 1,
        rc = fork();

    if (rc == 0) {
        a++;
        rc = fork();

        if (rc == 0)
            a++;
    }

    printf ("Hello World %d\n", a);
}

Example Use/Output

$ ./bin/forkcount
Hello World 1
Hello World 2
Hello World 3

Sometimes a pencil and paper is as helpful as a keyboard. Let me know if you have further questions.