C: Memory Allocation on Stack for Nested Structures

Question

Suppose I define a structure type as follows:

typedef struct Child {
    int a;
    char b[10];
} Child;

Because the compiler knows the space requirements for all of Child's members, I don't need to use malloc.

Child child; /* Allocates space on the stack for child and its members. */

Next, I define a structure type Parent that includes Child.

typedef struct Parent {
    int c;
    char d[10];
    Child child;
} Parent;

When I declare a variable of type Parent, is enough stack memory allocated for members of both the parent and the child structures?

Parent parent; /* Allocates space for all members of parent, including the nested child? */
parent.child.a = 5; /* Is this assignment guaranteed to work? */

(This is an elaboration of the question Nested Structures memory allocation.)

Answer 1

Yes, declaring a Parent variable allocates space for all members, including the nested Child. Accessing nested members like parent.child.a is valid and doesn't require separate allocation.

#include <stdio.h>

typedef struct Child {
    int a;
    char b[10];
} Child;

typedef struct Parent {
    int c;
    char d[10];
    Child child;
} Parent;

int main() {
    Parent parent;
    parent.child.a = 5;  // Valid: Space for child.a is already allocated.
    printf("%d\n", parent.child.a); // Output: 5
    return 0;
}