attributes in the definition of max_align_t of libstdc++

Question

max_align_t is defined as follows in libstdc++:

typedef struct {
  long long __max_align_ll __attribute__((__aligned__(__alignof__(long long))));
  long double __max_align_ld __attribute__((__aligned__(__alignof__(long double))));
} max_align_t;

Aren't those attributes redundant? I got the same result without those attributes:

typedef struct {
  long long __max_align_ll;
  long double __max_align_ld;
} max_align_t;

The question is 'Is there any reason those attributes specified?.'

Answer 1

The attributes force the type to be correctly aligned if it is included as a member of another struct and compiled with -fpack-struct or a packing #pragma

e.g.

typedef struct {
  long long __max_align_ll __attribute__((__aligned__(__alignof__(long long))));
  long double __max_align_ld __attribute__((__aligned__(__alignof__(long double))));
} max_align_t;

typedef struct {
  long long __max_align_ll;
  long double __max_align_ld;
} max_align2_t;

struct A {
  char c;
  max_align_t ma;
};

struct A2 {
  char c;
  max_align2_t ma;
};

static_assert( sizeof(A) == sizeof(A2), "" );

With -fpack-struct the assertion fails, showing that the attributes prevent the max_align_t type being incorrectly aligned.