Question

Sorry for a stupid question, but if I need to ensure the alignment of a structure / class / union, should I add attribute((aligned(align))) to typedef declaration?

class myAlignedStruct{} __attribute__ ((aligned(16)));
typedef myAlignedStruct myAlignedStruct2; // Will myAlignedStruct2 be aligned by 16 bytes or not?

Answer 1

should I add attribute((aligned(align))) to typedef declaration?

No… typedefs are just pseudonyms or aliases for the actual type specified, they don’t exist as a separate type to have different alignment, packing etc..

#include <iostream>

struct Default_Alignment
{
    char c;
};

struct Align16
{
    char c;
} __attribute__ ((aligned(16)));

typedef Align16 Also_Align16;

int main()
{
    std::cout << __alignof__(Default_Alignment) << '\n';
    std::cout << __alignof__(Align16) << '\n';
    std::cout << __alignof__(Also_Align16) << '\n';
}

Output:

1
16
16