Structure padding and packing?

Question

Consider: struct mystruct_A { char a; int b; char c; } x; struct mystruct_B { int b; char a; } y;The sizes of the structures are 12 and 8 respectively.Are these structures padded or packed?When does padding or packing take place?

Answer 1

For mystruct_A, its elements are padded so that each one of them are aligned with the 4 byte boundary.

struct mystruct_A {

char a; // 1 byte + 3 bytes of padding

int b; // 4 bytes

char c // 1 byte + 3 bytes of padding

} x;

Similarly, for mystruct_B, the elements are padded so that each one of them are aligned with the 4 byte boundary.

struct mystruct_B {

int b; // 4 bytes

char a // 1 byte + 3 bytes of padding

} y;

Thus their sizes are 12 bytes and 8 bytes respectively.

Here, the compiler is NOT doing any optimization by packing the members. With such optimization, the packed structure would become

struct mystruct_A_packed {

char a; // 1 byte

char c; // 1 byte + 2 bytes of padding

int b; // 4 bytes

} x_packed;

The size of x_packed would be 8 bytes.