Remove dots (".") and supernumerary slashes ("/"s) from paths

path_normalise.c

int
path_normalise(size_t n, const char *const fname, const norm[n])
{
	char *lim;		/* Limit of normalised filename. */
	char *end;		/* Current end of normalised filename. */

	assert(*fname != '\0');

	lim = norm + n - 1U;
	end = norm;

	/*
	 * Some implementations of stpncpy fail to terminate strings.
	 * Also permits to use assignments instead of stpncpy in some cases.
	 */
	(void) memset(norm, '\0', n);

	for (const char *cur = fname; *cur != '\0'; cur += strspn(cur, "/")) {
		size_t len;	/* Length of current filename segment. */

		len = strcspn(cur, "/");
		if (len > 0) {
			if (len > 1 || *cur != '.') {
				/* 
				 * `<=` to account for the path seperator.
				 * Assumes that NAME_MAX < PATH_MAX.
				 */
				if ((size_t) (lim - end) <= len)
					return -1;
				if (end > norm || *fname == '/')
					*(end++) = '/';
				end = stpncpy(end, cur, len);
			}

			cur += len;
		}
	}

	if (end == norm)
		/* 
		 * If this point is reached, fname either
		 * - starts with '.' and points to the working directory or
		 * - starts with '/' and points to the root directory.
		 */
		*end = *fname;

	return 0;
}