[Python-Dev] GC head alignment issue

[Paul Svensson]

On Thu, 11 Oct 2001, Tim Peters wrote:

>[Tim]
>> My head hurts, so I'm not sure this is strictly legit C
>> (drop-in replacement for the current PyGC_Head declaration):
>>
>> /* GC information is stored BEFORE the object structure */
>> typedef union _gc_head {
>> 	struct {
>> 		union _gc_head *gc_next; /* not NULL if object is tracked */
>> 		union _gc_head *gc_prev;
>> 		int gc_refs;
>> 	};
>> 	double dummy;  /* force worst-case alignment */
>> } PyGC_Head;
>
>[Guido]
>> Alas, gcc gives me warnings about this:
>>
>> ../Include/objimpl.h:274: warning: unnamed struct/union that
>> defines no instances
>>
>> and references to the fields fail with errors:
>>
>> ../Objects/cellobject.c: In function `PyCell_New':
>> ../Objects/cellobject.c:14: union has no member named `gc_next'
>> [and many more]
>
>Then:
>
>    If that's not legit, we could make it legit by naming the struct
>    member and interpolating the name into all existing references.
>
>This is straightforward, if a bit tedious ... OK, I did that.  Requires
>changes to the track and untrack macros, plus a ton of edits inside
>gcmodule.c.  But that's it, and they're all straightforward changes (i.e.,
>low chance of breakage).  Should I check it in?

The usual thing to do in this situation would be:

typedef union _gc_head {
	struct {
		union _gc_head *internal_gc_next;
		union _gc_head *internal_gc_prev;
		int internal_gc_refs;
	} gc_internals;
	double dummy;  /* force worst-case alignment */
} PyGC_Head;

#define gc_next	gc_internals.internal_gc_head
#define gc_next	gc_internals.internal_gc_head
#define gc_refs	gc_internals.internal_gc_refs


No need to change any other code.

	/Paul