xdirname
size_t xdirname(char *, const char *, size_t);
(void)xdirname(dp, fname, sizeof(dp));
(void)xdirname(bp->b_cwd, newname, sizeof(bp->b_cwd));
(void)xdirname(bp->b_cwd, fname, sizeof(bp->b_cwd));
(void)xdirname(dp, fn, sizeof(dp));