FD_EMPTY
fd->flags |= FD_EMPTY;
fd->flags &= ~FD_EMPTY;
if (fd->flags & FD_EMPTY)
fd->flags |= FD_EMPTY;
fd->flags = FD_EMPTY;
fd->flags |= FD_EMPTY;
fd->flags |= FD_EMPTY;
!(fd->flags & FD_EMPTY)) ||
if (fd->flags & FD_EMPTY)
fd->flags |= FD_EMPTY;