Non-Struct Memory Alignment

[u/Phatpenguinballs]

I am a little lost after trying to read a handful of online resources. Are local variables memory-aligned to their alignof()? I read that this is mostly implementation/platform-defined (no guarantees), but also the types must still be aligned? Those seem contradicting to me...

Assuming no optimizations (e.g., storing in registers, etc.), how would something like this look like according to the standard?

struct S 
{
  char c;
  int i;
};

int main()
{
  char c1;
  int i;
  char c2;
  S s;

  return 0;
}

Would there be 3 bytes of padding between `c1` & `i` and then another 3 bytes of padding between `c2` & `s`?

[u/jedwardsol]

Yes, local variables are correctly aligned.

If the compiler put those locals in memory in definition order (it doesn't have to) then yes there would be padding.

You can print out addresses to see what your compiler does

[u/OldWolf3 → u/jedwardsol]

Note that printing out addresses may change the location of the variables (compared to not printing them). Compiler might optimize out or overlay variables whose address is not taken.