diff --git a/ipc/util.h b/ipc/util.h
index 2a03d8c..fa7e7a0 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -79,6 +79,8 @@
 #define IPC_MSG_IDS	1
 #define IPC_SHM_IDS	2
 
+#define ipcid_to_idx(id) ((id) % SEQ_MULTIPLIER)
+
 /* must be called with ids->mutex acquired.*/
 int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);
 int ipc_get_maxid(struct ipc_ids *);