Why do I have to use malloc when I have a pointer to a struct?

Question

 struct element {
     int val;
     int z;
 };
 typedef struct element ELEM;

Look at this example:

 int main()
 {

    ELEM z;
    z =         


        

Answer 1

You only need to use malloc if you intend to dynamically allocate memory. Otherwise, your pointer must point to something. If you intend to use your pointer when it doesn't point to valid memory, it is undefined behavior.

struct Elem
{
    int val;
};

int main() {
    struct Elem elem;
    struct Elem* ptr; // can't use pointer, points to nothing
    ptr = &elem; // good, points to elem
    ptr->val = 5;
    printf("%d", ptr->val);
    ptr = malloc(sizeof(struct Elem)); // good, points to object on the heap
    ptr->val = 10;
    printf("%d", ptr->val);
    free(ptr);
    return 0;
}