tag_header
struct tag_header hdr;
#define tag_size(type) ((sizeof(struct tag_header) + sizeof(struct type)) >> 2)
sizeof(struct tag_header)) >> 2;
sizeof(struct tag_header));
struct tag_header hdr1;
struct tag_header hdr2;
struct tag_header hdr3;
size = (char *)tag - atags_copy + sizeof(struct tag_header);