C program - Structure variable data packing and alignment

Question

What will be the output of the program on a 32-bit machine (using GCC)? Explain.

#include<stdio.h>

int main() {

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

     struct node *p, *q;
     p = (struct node *) malloc(sizeof(struct node));
     q = (struct node *) malloc(sizeof(struct node));
     printf("%d, %d\n", sizeof(p), sizeof(q));
     return 0;
}

The output shows

4, 4.

Is the above program related to structure member alignment padding and data packing?

Answer 1

No, you are just printing the size of the pointers. It's not related to the internal member layout of structures.

Answer 2

On a 32-bit system the stored addresses are always 32 bits big. If you're printing the size of a pointer you're basically just printing the size of the address it points to (32 Bit -> 4 Byte).

If you want to know the size of the struct do something like this:

struct node p;
struct node q = {4, &p};

printf("%zu, %zu\n", sizeof(p), sizeof(q));