Re: are statically allocated structs always aligned to a machine word on x86/x86_64?

[Jonathan Wakely <jwakely.gcc@gmail.com>]

On 21 August 2015 at 19:39, john smith wrote:
> I didn't find any information about alignment requirements for
> statically allocated objects in GCC and x86-64 manual (or I have
> missed because the manual is huge). I noted that sometimes variables
> such as int are not aligned on word boundary in x86 and x86_64 but I
> have never seen a struct that wouldn't be allocated at address that
> isn't a multiple or 4/8.

Three of these structs are not word-aligned:

#include <stdio.h>
struct A { char c; };
struct A a[4];

int main()
{
  for (int i=0; i<4; ++i)
    printf("%p\n", a+i);
}


> I am asking this question because I would
> like to know whether it's safe to assume that struct will be always
> assigned at a word boundary and therefore it's possible to correctly
> calculate a struct size without running a program.

sizeof?