[PATCH 1/2] utils: Document wl_container_of

[Aaron Faanes]

Modified this documentation to be much less verbose than before. I also added
an example, used better doxygen style, and mentioned that wl_container_of is
useful outside of just wl_list.

---
 src/wayland-util.h | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/src/wayland-util.h b/src/wayland-util.h
index de2e464..f85c521 100644
--- a/src/wayland-util.h
+++ b/src/wayland-util.h
@@ -111,6 +111,33 @@ int wl_list_length(const struct wl_list *list);
 int wl_list_empty(const struct wl_list *list);
 void wl_list_insert_list(struct wl_list *list, struct wl_list *other);
 
+/**
+ * Retrieves a pointer to the container of a given item.
+ *
+ * This macro allows conversion from a pointer to a contained item to its
+ * container. This is useful if you have a contained item like a wl_list,
+ * wl_listener, or wl_signal, and would like to retrieve the struct that
+ * contains it.
+ *
+ * To demonstrate, given a wl_list contained by a wl_resource, the wl_resource
+ * can be retrieved as follows:
+ *
+ * \code
+ * struct wl_list *some_link = ...;
+ * struct wl_resource *resource = NULL;
+ *
+ * resource = wl_container_of(some_link, resource, link);
+ * \endcode
+ *
+ * \param ptr A valid pointer to the contained item.
+ *
+ * \param sample A pointer to the type of content that the list item stores.
+ * Sample does not need be a valid pointer; a null pointer will suffice.
+ *
+ * \param member The named location of ptr within the sample type.
+ *
+ * \return The container for the specified pointer.
+ */
 #ifdef __GNUC__
 #define wl_container_of(ptr, sample, member)				\
 	(__typeof__(sample))((char *)(ptr)	-			\
-- 
1.8.3.1