declaration in local vs global

#include<stdio.h>
#include<stdlib.h>

struct node
{
    int data;
    struct node* link;
};

void insert_last(struct node **head, int value)
{
    struct node* new = malloc(sizeof(struct node));
    new->data = value;

    if( !*head )
    {
        new->link = NULL;
        *head = new;
    }
    else
    {
        struct node* last = *head;
        while(last->link)
        {
            last = last->link;
        }
        new->link = NULL;
        last->link = new;
    }
}

    struct node *head;

int main()
{
    insert_last( & head,  5);
    insert_last( & head, 10);
    insert_last( & head, 15);
    printf("%d  ", head->data);
    printf("%d  ", head->link->data);
    printf("%d  ", head->link->link->data);

}

If I Declare struct node *head inside of the main the programm is not working.
what is the reason ?
> if i declare globally its working, otherwise not working.
> I repeating question because stackoverflow asking add more details(> If I Declare struct node *head in side of the main the programm is not working.
what is the reason ?
> if i declare globally its working, otherwise not working.)

>Solution :

There is a initialization bug: In insert_last(), the variable head is tested without having been initialized explicitely. If head is declared as global, it is located in a global section that is most probably initialized to 0 by the loader (when the program is started); if head is declared in the function main() then it is located in the stack of the function and is not set to 0.