diff --git a/src/structs.h b/src/structs.h
index 6482e30..9d7bc92 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -963,7 +963,8 @@
  */
 typedef struct
 {
-    char	v_type;	/* see below: VAR_NUMBER, VAR_STRING, etc. */
+    char	v_type;	    /* see below: VAR_NUMBER, VAR_STRING, etc. */
+    char	v_lock;	    /* see below: VAR_LOCKED, VAR_FIXED */
     union
     {
 	varnumber_T	v_number;	/* number value */
@@ -981,6 +982,10 @@
 #define VAR_LIST    4	/* "v_list" is used */
 #define VAR_DICT    5	/* "v_dict" is used */
 
+/* Values for "v_lock". */
+#define VAR_LOCKED  1	/* locked with lock(), can use unlock() */
+#define VAR_FIXED   2	/* locked forever */
+
 /*
  * Structure to hold an item of a list: an internal variable without a name.
  */
@@ -1013,6 +1018,7 @@
     listitem_T	*lv_first;	/* first item, NULL if none */
     listitem_T	*lv_last;	/* last item, NULL if none */
     listwatch_T	*lv_watch;	/* first watcher, NULL if none */
+    char	lv_lock;	/* zero, VAR_LOCKED, VAR_FIXED */
 };
 
 /*
@@ -1032,6 +1038,7 @@
 #define DI_FLAGS_RO	1 /* "di_flags" value: read-only variable */
 #define DI_FLAGS_RO_SBX 2 /* "di_flags" value: read-only in the sandbox */
 #define DI_FLAGS_FIX	4 /* "di_flags" value: fixed variable, not allocated */
+#define DI_FLAGS_LOCK	8 /* "di_flags" value: locked variable */
 
 /*
  * Structure to hold info about a Dictionary.
@@ -1040,6 +1047,7 @@
 {
     int		dv_refcount;	/* reference count */
     hashtab_T	dv_hashtab;	/* hashtab that refers to the items */
+    char	dv_lock;	/* zero, VAR_LOCKED, VAR_FIXED */
 };