Re: make-c-struct and pointer->string

[David Pirotte <david@altosw.be>]

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Hi Mark,

Thanks for your help.

> > ...
> > scheme@(guile-user)> (make-c-struct (list '* '*) (list (string->pointer "hello
> > ") (string->pointer "there!"))) $16 = #<pointer 0x55a3d54d54d0>
> > scheme@(guile-user)> (parse-c-struct $16 (list '* '*))
> > $17 = (#<pointer 0x55a3d5d12170> #<pointer 0x55a3d5d0a640>)
> > scheme@(guile-user)> (map pointer->string $17)
> > $18 = ("?g?գU" "`!?գU")  

> The Guile manual states:

>  -- Scheme Procedure: string->pointer string [encoding]
>      Return a foreign pointer to a nul-terminated copy of STRING in the
>      given ENCODING, defaulting to the current locale encoding.  The C
>      string is freed when the returned foreign pointer becomes
>      unreachable.

> Note the last sentence.  When the returned foreign pointer (object)
> becomes unreachable, the C string is freed.  The problem here is that
> you're not keeping a reference to those foreign pointer objects.

Yes, and I was assuming that make-c-struct was holding a reference to 'its
children', including pointers.

> If you look at the code in foreign.c, specifically ...

>  'make-c-struct' copies the C pointers from those foreign pointer objects, but not
> not keep a reference to the objects themselves.

To me, this sounds very counter intuitive, actually, it sounds like a bug,
make-c-struct should be holding a reference to the pointers it receives: i seems to
me that only when the c-struct itself becomes unreachable, that these pointers could
be freed?

If I am wrong, and it seems I am (wrong), we really should think about writing
something about this in the manual (and explaining why would be nice).

Cheers,
David

ps:	in the last part of my email, I did precisely what you suggest in your
	answer, and showed that it works - if we hold a reference to these pointers
	'by ourselves' - and asked if that was what is what was expected from us and
	Neil Jerram to (also) find that 'very odd'

> > ...
> > Ok, let's hold-on to the pointers (which I thought would not be necessary
> > after holding-on c-struct and/or parsed):

> > scheme@(guile-user)> (define p1 (string->pointer str-1))
> > scheme@(guile-user)> (define p2 (string->pointer str-2))
> > scheme@(guile-user)> (make-c-struct (list '* '*) (list p1 p2))
> > $19 = #<pointer 0x5584467a5d70>
> > scheme@(guile-user)> (parse-c-struct $19 (list '* '*))
> > $20 = (#<pointer 0x5584468b0190> #<pointer 0x5584468b4430>)
> > scheme@(guile-user)> p1
> > $21 = #<pointer 0x5584468b0190>
> > scheme@(guile-user)> p2
> > $22 = #<pointer 0x5584468b4430>
> > scheme@(guile-user)> (map pointer->string $20)
> > $23 = ("Hello" "there!")

> > That worked, but I wonder if this is what is expected from us (users)?

> Yes, that would seem very odd to me, as I thought that a 'pointer' really
> was just an address.

> (Hopefully some more light will be thrown when the maintainers are back
> around again!)

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