diff --git a/src/wayland-private.h b/src/wayland-private.h
index ea702584..2113d837 100644
--- a/src/wayland-private.h
+++ b/src/wayland-private.h
@@ -46,6 +46,7 @@ void wl_map_init(struct wl_map *map);
 void wl_map_release(struct wl_map *map);
 uint32_t wl_map_insert_new(struct wl_map *map, uint32_t side, void *data);
 int wl_map_insert_at(struct wl_map *map, uint32_t i, void *data);
+int wl_map_reserve_new(struct wl_map *map, uint32_t i);
 void wl_map_remove(struct wl_map *map, uint32_t i);
 void *wl_map_lookup(struct wl_map *map, uint32_t i);
 void wl_map_for_each(struct wl_map *map, wl_iterator_func_t func, void *data);
diff --git a/src/wayland-util.c b/src/wayland-util.c
index 107b6dbb..eacf9026 100644
--- a/src/wayland-util.c
+++ b/src/wayland-util.c
@@ -214,6 +214,39 @@ wl_map_insert_at(struct wl_map *map, uint32_t i, void *data)
 	return 0;
 }
 
+WL_EXPORT int
+wl_map_reserve_new(struct wl_map *map, uint32_t i)
+{
+	union map_entry *start;
+	uint32_t count;
+	struct wl_array *entries;
+
+	if (i < WL_SERVER_ID_START) {
+		entries = &map->client_entries;
+	} else {
+		entries = &map->server_entries;
+		i -= WL_SERVER_ID_START;
+	}
+
+	count = entries->size / sizeof *start;
+
+	if (count < i)
+		return -1;
+
+	if (count == i) {
+		wl_array_add(entries, sizeof *start);
+		start = entries->data;
+		start[i].data = NULL;
+	} else {
+		start = entries->data;
+		if (start[i].data != NULL) {
+			return -1;
+		}
+	}
+
+	return 0;
+}
+
 WL_EXPORT void
 wl_map_remove(struct wl_map *map, uint32_t i)
 {