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 4490184c2529b97ced2b9e3611e0f3671bae19f7..47a34d853b4c73f0280e360fea413480e4df8229 100644 (file)
--- a/ccan/container_of/container_of.h
+++ b/ccan/container_of/container_of.h
@@ -1,3 +1,4 @@
+/* CC0 (Public domain) - see LICENSE file for details */
 #ifndef CCAN_CONTAINER_OF_H
 #define CCAN_CONTAINER_OF_H
 #include <stddef.h>
@@ -32,19 +33,53 @@
 #define container_of(member_ptr, containing_type, member)              \
         ((containing_type *)                                           \
          ((char *)(member_ptr)                                         \
-          - container_off((member_ptr), containing_type, member)))
+          - container_off(containing_type, member))                    \
+         + check_types_match(*(member_ptr), ((containing_type *)0)->member))
+
 
 /**
- * container_off - get offset to enclosing structure
+ * 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
+ * @member: the name of this member within the structure.
+ *
  * Given a pointer to a member of a structure, this macro does
  * typechecking and figures out the offset to the enclosing type.
  *
- * Note that @member_ptr is not evaluated.
- *
  * Example:
  *     struct foo {
  *             int fielda, fieldb;
@@ -57,18 +92,17 @@
  *
  *     static struct info *foo_to_info(struct foo *foo)
  *     {
- *             size_t off = container_off(foo, struct info, my_foo);
+ *             size_t off = container_off(struct info, my_foo);
  *             return (void *)((char *)foo - off);
  *     }
  */
-#define container_off(member_ptr, containing_type, member)             \
-       (offsetof(containing_type, member)                              \
-        + check_types_match(*(member_ptr), ((containing_type *)0)->member))
+#define container_off(containing_type, member) \
+       offsetof(containing_type, member)
 
 /**
  * container_of_var - get pointer to enclosing structure using a variable
  * @member_ptr: pointer to the structure member
- * @var: a pointer to a structure of same type as this member is within
+ * @container_var: a pointer of same type as this member's container
  * @member: the name of this member within the structure.
  *
  * Given a pointer to a member of a structure, this macro does pointer
@@ -82,12 +116,30 @@
  *     }
  */
 #if HAVE_TYPEOF
-#define container_of_var(member_ptr, var, member) \
-       container_of(member_ptr, typeof(*var), member)
+#define container_of_var(member_ptr, container_var, member) \
+       container_of(member_ptr, typeof(*container_var), member)
+#else
+#define container_of_var(member_ptr, container_var, member)    \
+       ((void *)((char *)(member_ptr)  -                       \
+                 container_off_var(container_var, member)))
+#endif
+
+/**
+ * container_off_var - get offset of a field in enclosing structure
+ * @container_var: a pointer to a container structure
+ * @member: the name of a member within the structure.
+ *
+ * Given (any) pointer to a structure and a its member name, this
+ * macro does pointer subtraction to return offset of member in a
+ * structure memory layout.
+ *
+ */
+#if HAVE_TYPEOF
+#define container_off_var(var, member)         \
+       container_off(typeof(*var), member)
 #else
-#define container_of_var(member_ptr, var, member)              \
-       ((void *)((char *)(member_ptr)                          \
-                 - ((char *)&(var)->member - (char *)(var))))
+#define container_off_var(var, member)                 \
+       ((const char *)&(var)->member - (const char *)(var))
 #endif
 
 #endif /* CCAN_CONTAINER_OF_H */