alignas | Programming Place Plus　Ｃ言語編　標準ライブラリのリファレンス

#include <stdalign.h>
#include <stddef.h>
#include <stdio.h>

struct Data_tag {
    short a;
    char b;
    int c;
};

struct Data2_tag {
    short a;
    alignas(8) char b;
    int c;
};

int main(void)
{
    printf("%zu %zu %zu\n",
        offsetof(struct Data_tag, a),
        offsetof(struct Data_tag, b),
        offsetof(struct Data_tag, c)
    );
    printf("%zu %zu %zu\n",
        offsetof(struct Data2_tag, a),
        offsetof(struct Data2_tag, b),
        offsetof(struct Data2_tag, c)
    );
}