Structure alignment not as expected-CodePudding

I have a structure

struct abc{
int32_t status;
union A;
};

sizeof(int32_t) is 4. sizeof(union A) is 24.

I was expecting that status should get aligned and take total 8 bytes (considering 64 bit compiler) but the size of structure abc is coming as 28 and not 32. Can someone explain why ? Note it is not pragma packed.

union A{
int8_t a;
struct b;
struct c;
}

struct b
{
int8_t a;
int32_t b2;
int32_t b3;
int32_t b4;
}

struct c
{
struct b;
int32_t c1;
int32_t c2;
}

CodePudding user response：

The largest field in any substructure is 4 bytes in size, so the structures will have 4 byte alignment.

Going from the bottom up, the largest field in struct b is a int32_t, so struct b has 4 byte alignment.

struct c contains int32_t which is 4 bytes and struct b which has 4 byte alignment, so struct c has 4 byte alignment.

union A has a uint8_t which has 1 byte alignment, struct b which has 4 byte alignment, and struct c which has 4 byte alignment, so union A has 4 byte alignment.

Finally, struct abc has a int32_t which is 4 bytes and union A which has 4 byte alignment, so struct abc has 4 byte alignment.