+int path_make_relative(const char *from_dir, const char *to_path, char **_r) {
+ char *r, *p;
+ unsigned n_parents;
+
+ assert(from_dir);
+ assert(to_path);
+ assert(_r);
+
+ /* Strips the common part, and adds ".." elements as necessary. */
+
+ if (!path_is_absolute(from_dir))
+ return -EINVAL;
+
+ if (!path_is_absolute(to_path))
+ return -EINVAL;
+
+ /* Skip the common part. */
+ for (;;) {
+ size_t a;
+ size_t b;
+
+ from_dir += strspn(from_dir, "/");
+ to_path += strspn(to_path, "/");
+
+ if (!*from_dir) {
+ if (!*to_path)
+ /* from_dir equals to_path. */
+ r = strdup(".");
+ else
+ /* from_dir is a parent directory of to_path. */
+ r = strdup(to_path);
+
+ if (!r)
+ return -ENOMEM;
+
+ path_kill_slashes(r);
+
+ *_r = r;
+ return 0;
+ }
+
+ if (!*to_path)
+ break;
+
+ a = strcspn(from_dir, "/");
+ b = strcspn(to_path, "/");
+
+ if (a != b)
+ break;
+
+ if (memcmp(from_dir, to_path, a) != 0)
+ break;
+
+ from_dir += a;
+ to_path += b;
+ }
+
+ /* If we're here, then "from_dir" has one or more elements that need to
+ * be replaced with "..". */
+
+ /* Count the number of necessary ".." elements. */
+ for (n_parents = 0;;) {
+ from_dir += strspn(from_dir, "/");
+
+ if (!*from_dir)
+ break;
+
+ from_dir += strcspn(from_dir, "/");
+ n_parents++;
+ }
+
+ r = malloc(n_parents * 3 + strlen(to_path) + 1);
+ if (!r)
+ return -ENOMEM;
+
+ for (p = r; n_parents > 0; n_parents--, p += 3)
+ memcpy(p, "../", 3);
+
+ strcpy(p, to_path);
+ path_kill_slashes(r);
+
+ *_r = r;
+ return 0;
+}
+