Assign memory to structure using structure

Question

Suppose I have the following code:

typedef struct _SingleList {
    struct _SingleList *link;
    int data;
} SingleList;


SingleList *prepend(SingleList *list, int data) {
    SingleList a;
    SingleList *newNode = &a; // Note, here I assign memory using structure not malloc.
    newNode->data = data;
    newNode->link = list;
    return newNode;
}

As, you can see in above prepend function, I assign memory using address of a instead of malloc and it works perfectly, when I call prepend function. Here's a sample:

int main(void) {
    SingleList *list = NULL;
    list = prepend(list, 10);
    printf("%d", list->data);

    list = prepend(list, 20);
    printf("\n%d", list->link->data);

    list = prepend(list, 30);
    printf("\n%d", list->link->link->data);

}

So, how it works and if it works for others too, then why we use malloc() for linked list instead of simply assigning a structure.

Answer 1

In the function prepend, the return value newNode is a pointer that points to a local variable a. When the function ends, it's undefined behavior to access it. It just happens to work in your machine in this simple code, you can't rely on it.