container_of: Add container_of_or_null()

[ccan] / ccan / container_of / container_of.h

diff --git a/ccan/container_of/container_of.h b/ccan/container_of/container_of.h
index 0449935056f51a84af2a84f5b2dd7fcbf48c7678..47a34d853b4c73f0280e360fea413480e4df8229 100644 (file)
--- a/ccan/container_of/container_of.h
+++ b/ccan/container_of/container_of.h
@@ -36,6 +36,42 @@
           - container_off(containing_type, member))                    \
          + check_types_match(*(member_ptr), ((containing_type *)0)->member))
 
+
+/**
+ * container_of_or_null - get pointer to enclosing structure, or NULL
+ * @member_ptr: pointer to the structure member
+ * @containing_type: the type this member is within
+ * @member: the name of this member within the structure.
+ *
+ * Given a pointer to a member of a structure, this macro does pointer
+ * subtraction to return the pointer to the enclosing type, unless it
+ * is given NULL, in which case it also returns NULL.
+ *
+ * Example:
+ *     struct foo {
+ *             int fielda, fieldb;
+ *             // ...
+ *     };
+ *     struct info {
+ *             int some_other_field;
+ *             struct foo my_foo;
+ *     };
+ *
+ *     static struct info *foo_to_info_allowing_null(struct foo *foo)
+ *     {
+ *             return container_of_or_null(foo, struct info, my_foo);
+ *     }
+ */
+static inline char *container_of_or_null_(void *member_ptr, size_t offset)
+{
+       return member_ptr ? (char *)member_ptr - offset : NULL;
+}
+#define container_of_or_null(member_ptr, containing_type, member)      \
+       ((containing_type *)                                            \
+        container_of_or_null_(member_ptr,                              \
+                              container_off(containing_type, member))  \
+        + check_types_match(*(member_ptr), ((containing_type *)0)->member))
+
 /**
  * container_off - get offset to enclosing structure
  * @containing_type: the type this member is within