xstrndup
buf = xstrndup(label, slash - label);
extern char *xstrndup(const char *s, size_t len);
fragment = xstrndup(&str[start], pos - start);
fragment = xstrndup(&str[pos], end - pos);
tmp = xstrndup(str, len);
alias->builtin_modname = xstrndup(modname, modnamelen);