cleanup_pathname(struct archive_write_disk *a)
{
	char *dest, *src;
	char separator = '\0';

	dest = src = a->name;
	if (*src == '\0') {
		archive_set_error(&a->archive, ARCHIVE_ERRNO_MISC,
		    "Invalid empty pathname");
		return (ARCHIVE_FAILED);
	}

#if defined(__CYGWIN__)
	cleanup_pathname_win(a);
#endif
	/* Skip leading '/'. */
	if (*src == '/')
		separator = *src++;

	/* Scan the pathname one element at a time. */
	for (;;) {
		/* src points to first char after '/' */
		if (src[0] == '\0') {
			break;
		} else if (src[0] == '/') {
			/* Found '//', ignore second one. */
			src++;
			continue;
		} else if (src[0] == '.') {
			if (src[1] == '\0') {
				/* Ignore trailing '.' */
				break;
			} else if (src[1] == '/') {
				/* Skip './'. */
				src += 2;
				continue;
			} else if (src[1] == '.') {
				if (src[2] == '/' || src[2] == '\0') {
					/* Conditionally warn about '..' */
					if (a->flags & ARCHIVE_EXTRACT_SECURE_NODOTDOT) {
						archive_set_error(&a->archive,
						    ARCHIVE_ERRNO_MISC,
						    "Path contains '..'");
						return (ARCHIVE_FAILED);
					}
				}
				/*
				 * Note: Under no circumstances do we
				 * remove '..' elements.  In
				 * particular, restoring
				 * '/foo/../bar/' should create the
				 * 'foo' dir as a side-effect.
				 */
			}
		}

		/* Copy current element, including leading '/'. */
		if (separator)
			*dest++ = '/';
		while (*src != '\0' && *src != '/') {
			*dest++ = *src++;
		}

		if (*src == '\0')
			break;

		/* Skip '/' separator. */
		separator = *src++;
	}
	/*
	 * We've just copied zero or more path elements, not including the
	 * final '/'.
	 */
	if (dest == a->name) {
		/*
		 * Nothing got copied.  The path must have been something
		 * like '.' or '/' or './' or '/././././/./'.
		 */
		if (separator)
			*dest++ = '/';
		else
			*dest++ = '.';
	}
	/* Terminate the result. */
	*dest = '\0';
	return (ARCHIVE_OK);
}
The software uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the software does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.