why not use macro to get the address of sdshdr?

I find that almost in every function, you write "struct sdshdr _sh = (void_)(s - sizeof *sh);" to get the address of sdshdr, I wonder why don't you make it macro?
Like this :
#define SDS_ADDR(s) (void *)((s) - sizeof(struct sdshdr))
struct sdshdr *sh = SDS_ADDR(s);