lib: add listnode_add_head()

Provide a new convenience function that adds an element to the beginning
of a list.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

lib/linklist.c

@@ -70,6 +70,26 @@ void listnode_add(struct list *list, void *val)
 	list->count++;
 }
 
+void listnode_add_head(struct list *list, void *val)
+{
+	struct listnode *node;
+
+	assert(val != NULL);
+
+	node = listnode_new();
+
+	node->next = list->head;
+	node->data = val;
+
+	if (list->head == NULL)
+		list->head = node;
+	else
+		list->head->prev = node;
+	list->head = node;
+
+	list->count++;
+}
+
 void listnode_add_sort(struct list *list, void *val)
 {
 	struct listnode *n;

lib/linklist.h

@@ -82,6 +82,19 @@ extern struct list *list_new(void);
  */
 extern void listnode_add(struct list *list, void *data);
 
+/*
+ * Add a new element to the beginning of a list.
+ *
+ * Runtime is O(1).
+ *
+ * list
+ *    list to operate on
+ *
+ * data
+ *    element to add
+ */
+extern void listnode_add_head(struct list *list, void *data);
+
 /*
  * Insert a new element into a list with insertion sort.
  *