include: add list_last_entry to get the tail of a list

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Chase Douglas <chase.douglas@canonical.com>

include/list.h

@@ -262,6 +262,21 @@ list_is_empty(struct list *head)
 #define list_first_entry(ptr, type, member) \
     list_entry((ptr)->next, type, member)
 
+/**
+ * Retrieve the last list entry for the given listpointer.
+ *
+ * Example:
+ * struct foo *first;
+ * first = list_last_entry(&foo->mylist, struct foo, mylist);
+ *
+ * @param ptr The list head
+ * @param type Data type of the list element to retrieve
+ * @param member Member name of the struct list field in the list element.
+ * @return A pointer to the last list element.
+ */
+#define list_last_entry(ptr, type, member) \
+    list_entry((ptr)->prev, type, member)
+
 #define __container_of(ptr, sample, member)				\
     (void *)((char *)(ptr)						\
 	     - ((char *)&(sample)->member - (char *)(sample)))

test/list.c

@@ -103,14 +103,20 @@ test_list_append(void)
 
     c = list_first_entry(&parent.children, struct child, node);
     assert(memcmp(c, &child[0], sizeof(struct child)) == 0);
+    c = list_last_entry(&parent.children, struct child, node);
+    assert(memcmp(c, &child[0], sizeof(struct child)) == 0);
 
     list_append(&child[1].node, &parent.children);
     c = list_first_entry(&parent.children, struct child, node);
     assert(memcmp(c, &child[0], sizeof(struct child)) == 0);
+    c = list_last_entry(&parent.children, struct child, node);
+    assert(memcmp(c, &child[1], sizeof(struct child)) == 0);
 
     list_append(&child[2].node, &parent.children);
     c = list_first_entry(&parent.children, struct child, node);
     assert(memcmp(c, &child[0], sizeof(struct child)) == 0);
+    c = list_last_entry(&parent.children, struct child, node);
+    assert(memcmp(c, &child[2], sizeof(struct child)) == 0);
 
     i = 0;
     list_for_each_entry(c, &parent.children, node) {