Use __attribute__((aligned())) in struct, why the result of sizeof is this?

Question

This is my test code:

#include <cstdio>
struct A {
    int  a;
    int  b;
    int  c __attribute__((aligned(4096)));
    int  d;
}t;
int main()
{
    printf("%d\n",sizeof(t));

    return  0;
}

The result is 8192, but I can't figure out the reason.

Answer 1

There are a few facts about alignment in structs that are worth mentioning:

1. The size of a type is always a multiple of its alignment.
2. The alignment of a struct is always a multiple of the alignment of all its members.

So, since one of these members has an alignment of 4096, the alignment of the struct itself is at least 4096. It will most likely be exactly that.

But since it needs a padding of 4080 bytes before c, the size of the struct is at least 4104, but it must be a multple of 4096, its alignment. So it grows to 8192.