CODEWITHSUNDEEP
cmemory-management
hexiecs
hexiecs

Reputation: 322

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

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.

Upvotes: 11

Views: 1955

Answers (2)

skyking
skyking

Reputation: 14390

It's because sizeof tells where the next element in an array would be placed. That is if you have the declaration

struct A a[2];

You would need both a[0].c and a[1].c to be aligned at 4096 bytes.

Strictly speaking the compiler could manage to make this happen with a size of 4096, but it doesn't probably because struct A will inherit the alignment requirement and put two ints before the .c field which too has to be aligned which inserts 4080ish bytes of padding between .b and .c and then 4080ish bytes of padding after .d.

The way the compiler could have done this (without rearranging struct members) is to extend the concept of alignment. Instead of just having requirements that the address must fall on an address of the form N*4096 it could extend this with an offset requiring it to fall on an adress of the form N*4096-2*sizeof(int). Giving struct A such a requirement would result in that the .c element would naturally become 4096 bytes aligned without requiring padding between the .b and .c (too).

Upvotes: 4

rodrigo
rodrigo

Reputation: 98328

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.

Upvotes: 12

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