target: provide container_of()

Provide a cleaner way to handle single inheritance of targets
in C, using the same model Linux does:  structs containing other
structs, un-nested via calls to a "container_of()" macro that
are packaged in typesafe inline functions.

Targets already use this containment idiom, but make it much
more complicated because they un-nest using embedded "void *"
pointers ... in chains of up to five per target, which is all
pure needless complication.  (Example: arm92x core, arm9tdmi,
arm7_9, armv4_5 ... on top of the base "target" class.)

Applying this scheme consistently simplifies things, and gets
rid of many error-prone untyped pointers.  It won't change any
part of the type model though -- it just simplifies things.
(And facilitates more cleanup later on.)

Rule of thumb:  where there's an X->arch_info void* pointer,
access to that pointer can and should be removed.  It may be
convenient to set up pointers to some of the embedded structs;
and shrink their current "*_common" names (annoyingly long).

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

src/target/target.h

@@ -26,6 +26,8 @@
 #ifndef TARGET_H
 #define TARGET_H
 
+#include <stddef.h>
+
 #include "breakpoints.h"
 #include "algorithm.h"
 #include "command.h"
@@ -34,6 +36,19 @@ struct reg_s;
 struct trace_s;
 struct command_context_s;
 
+
+/**
+ * Cast a member of a structure out to the containing structure.
+ * @param ptr The pointer to the member.
+ * @param type The type of the container struct this is embedded in.
+ * @param member The name of the member within the struct.
+ *
+ * This is a mechanism which is used throughout the Linux kernel.
+ */
+#define container_of(ptr, type, member) ({			\
+	const typeof( ((type *)0)->member ) *__mptr = (ptr);	\
+	(type *)( (char *)__mptr - offsetof(type,member) );})
+
 /*
  * TARGET_UNKNOWN = 0: we don't know anything about the target yet
  * TARGET_RUNNING = 1: the target is executing user code