New variant of container_of: container_of_var, for list.h

container_of/container_of.h

@@ -32,4 +32,34 @@
 	  - check_types_match(*(member_ptr), ((containing_type *)0)->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
+ * @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.
+ *
+ * Example:
+ *	struct info
+ *	{
+ *		int some_other_field;
+ *		struct foo my_foo;
+ *	};
+ *
+ *	struct info *foo_to_info(struct foo *foop)
+ *	{
+ *		struct info *i = container_of_var(foo, i, my_foo);
+ *		return i;
+ *	}
+ */
+#ifdef HAVE_TYPEOF
+#define container_of_var(member_ptr, var, member) \
+	container_of(member_ptr, typeof(*var), member)
+#else
+#define container_of_var(member_ptr, var, member) \
+	((void *)((char *)(member_ptr) - offsetof(containing_type, member)))
+#endif
+
 #endif /* CCAN_CONTAINER_OF_H */

container_of/test/compile_fail-bad-type.c

@@ -13,6 +13,7 @@ int main(int argc, char *argv[])
 	char *p;
 
 #ifdef FAIL
+	/* p is a char *, but this gives a struct foo * */
 	p = container_of(intp, struct foo, a);
 #else
 	p = (char *)intp;

container_of/test/compile_fail-types.c

@@ -12,6 +12,7 @@ int main(int argc, char *argv[])
 	int *intp = &foo.a;
 
 #ifdef FAIL
+	/* b is a char, but intp is an int * */
 	foop = container_of(intp, struct foo, b);
 #else
 	foop = NULL;

container_of/test/compile_fail-var-types.c

+#include "container_of/container_of.h"
+#include <stdlib.h>
+
+struct foo {
+	int a;
+	char b;
+};
+
+int main(int argc, char *argv[])
+{
+	struct foo foo = { .a = 1, .b = 2 }, *foop;
+	int *intp = &foo.a;
+
+#ifdef FAIL
+	/* b is a char, but intp is an int * */
+	foop = container_of_var(intp, foop, b);
+#else
+	foop = NULL;
+#endif
+	return intp == NULL;
+}

container_of/test/run.c

@@ -12,8 +12,10 @@ int main(int argc, char *argv[])
 	int *intp = &foo.a;
 	char *charp = &foo.b;
 
-	plan_tests(2);
+	plan_tests(4);
 	ok1(container_of(intp, struct foo, a) == &foo);
 	ok1(container_of(charp, struct foo, b) == &foo);
+	ok1(container_of_var(intp, &foo, a) == &foo);
+	ok1(container_of_var(charp, &foo, b) == &foo);
 	return exit_status();
 }