[Gambas-devel] New(er) enumeration API

[Tobias Boege]

On Wed, 03 Oct 2012, Beno?t Minisini wrote:
> Le 03/10/2012 20:56, Tobias Boege a ?crit :
> > On Wed, 03 Oct 2012, Tobias Boege wrote:
> >> Hi,
> >>
> >> today while debugging the new List code for gb.data I encountered problems
> >> with the new enumeration API. There are _few_ places to learn from (I took
> >> CResult.c) but apparently something doesn't quite work out with it:
> >>
> >> I want to run through all references I manage for a chunk in a List (a
> >> 'chunk' contains some Variants). These are the 'Current' pointer and all the
> >> enumerations currently active. The ultimate goal is to update members of
> >> these reference structures just because the elements they point to in the
> >> chunk changed their location. The convenience macro looks like this:
> >>
> >> #define for_all_persistent_vals(list, vp, es, ebuf)                     \
> >> 	for (ebuf = GB.BeginEnum(list), vp = &list->current, es = NULL;	\
> >> 	vp ? 1 : (GB.EndEnum(ebuf), 0);					\
> >> 		!GB.NextEnum() ? (es = (struct enum_state*) GB.GetEnum(),\
> >> 			vp = &es->next) : (vp = NULL))
> >>
> >> (admittely not a very appealing piece of code)
> >>
> >> where 'list' is the Gambas List object, 'vp' is a pointer to the structure
> >> I want to update, 'es' is my struct enum_state and 'ebuf' is whatever I get
> >> back from GB.BeginEnum() and have to pass to GB.EndEnum().
> >>
> >> The thing now is that, even if no enumeration runs in the Gambas code, I get
> >> some 'es' which contains information that could actually have been valid (in
> >> fact, it refers to the last enumerated object in the last enumeration).
> >> For some reason, the important part (the pointer to the chunk which sits at
> >> offset 0 from the 'es' pointer) is set to NULL and thus, my library
> >> segfaults on NULL pointer dereference.
> >>
> >
> > Actually, I set it myself to NULL which indicates the end of enumeration to
> > the List_next routine. But why does that 'es' appear again without being in
> > enumeration?
> >
> > I have a really bad feeling now since I only get the 'es' filled if I put in
> > some basically unrelated Gambas code before the List.Take() from which the
> > above problem originates...
> >
> > I just want to know: Is it possible under normal circumstances to get an old
> > non-running enumerator from that GB.*Enum() interface?
> >
> >> Benoit, can you please enlighten me if I am doing something wrong with these
> >> functions?
> >
> 
> I can't know. See me the full code because you can't do what you want 
> between GB.BeginEnum() ... GB.EndEnum() (i.e. you can't do something 
> that may call another enumeration-related function).
> 
> Moreover, I'm not sure that your spaghetti macro is right. Can you 
> rewrite it as clear as possible (i.e. without using the ',' operator)?

Here it goes:

ebuf = GB.BeginEnum(list);
vp   = &list->current; /* Begin with Current */
es   = NULL; /* Just signal that vp is not an enumerator */
while (vp) {
	if (!GB.NextEnum())
		break; /* In the macro it does this by setting vp = NULL */
	es = GB.GetEnum();
	vp = &es->next;
	/* Here goes the loop body */
}
GB.EndEnum(list);

I'm modifying only the 'vp' structures in there.

Regards,
Tobi