When extending a padded struct, why can't extra fields be placed in the tail padding?

Question

Let\'s consider the structs :

struct S1 {
    int a;
    char b;
};

struct S2 {
    struct S1 s;       /* struct needed to make this compile as C without ty         


        

Answer 1

Let's consider some code:

struct S1 {
    int a;
    char b;
};

struct S2 {
    S1 s;
    char c;
};

Let's consider what would happen if sizeof(S1) == 8 and sizeof(S2) == 8.

struct S2 s2;
struct S1 *s1 = &(s2.s);
memset(s1, 0, sizeof(*s1));

You've now overwritten S2::c.

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For array alignment reasons, S2 also cannot have a size of 9, 10, or 11. So the next valid size is 12.