_Alignas for struct members using clang & C11

Question

I'm having some trouble with -Wpadded using C11 and structs.

I've already read Structure member alignment with _Alignas, and I looked in the clang docs and saw that it IS supported now.

Also, I'm using a very new version of clang that I built from trunk recently.

$ clang --version
clang version 3.3 (trunk 175473)
Target: x86_64-unknown-linux-gnu
Thread model: posix

The problem I'm running into is this:

#include <stdlib.h>
#include <stdalign.h>

struct foo{
   void* a;
   int b;
};

int main() {
   struct foo instance;

   instance.a = NULL;
   instance.b = 2;

   return 0;
}

Which throws me this warning:

$ clang -Weverything -std=c11 t.c 
t.c:4:8: warning: padding size of 'struct foo' with 4 bytes to alignment boundary [-Wpadded]
struct foo{
       ^
1 warning generated.

Now isn't this what _Alignas is for? I tried putting it before the int member declaration, like so:

struct foo{
   void* a;
   _Alignas(void*) int b;
};

But the same warning remains. I also tried putting the _Alignas in various places, to no avail. What am I missing here?

I know I could just ignore this particular warning and I understand why padding is important, so I'm not interested in workarounds or explanations about what padding is. I want to know how to change my C in a portable, standards conformant way so that the warning is no longer emitted.

Answer 1

-Weverything prints all diagnostic messages required by C as well as some diagnostics not required by C. The diagnostic that is printed here is not required by C: its purpose is informative and your program is already strictly conforming. C says an implementation is free to produce additional diagnostic messages as long as it does not fail to translate the program.