When declaring variable is not completed, why compiler know its size?

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After pointer pa declaring, right now,to use it in the function malloc(),

why compiler can know its size? And compiler how to parse under code:

int *pa = malloc(10 * sizeof *pa); // allocate an array of 10 int

and

struct Node {
    int value;
    struct Node *prev;
    struct Node *next;  
}

prev and next is struct Node *, but now struct Node is undefined.

Why compiler is able to allocate enough size for *prev, *next?

c
3 Answers

Because they are pointers. Compiler knows how much memory to allocate for pointers.

user_42657140

In case of the structure declaration shown here, the member variables you are interested in are pointer-to-some-type.

For any given environment, a pointer (to type) holds a defined size. While allocating memory for a variable of the type, compiler need not know the actual size for the type, it just needs to know the size required for a pointer to that type and it knows that.

user_21739170

Compiler does not allocate enough space for a Node structure, inside a Node structure. Though it does know how much a pointer needs, and allocates enough space for two pointers and in integer.

user_2752210
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