On Fri, 29 Mar 2019, 09:46 David Pirotte, <david@altosw.be> wrote:

> Hi again,
>
> > ...
> > Following your explanation and example, I tried this and thought it
> would work
> > then, but it also failed:
>
> > GNU Guile 2.2.4.1-cdb19
>
> > Enter `,help' for help.
> > scheme@(guile-user)> ,use (system foreign)
> > scheme@(guile-user)> (define str-1 "Hello")
> > scheme@(guile-user)> (define str-2 "there!")
> > scheme@(guile-user)>  (make-c-struct (list '* '*) (list
> (string->pointer str-1)
> > (string->pointer str-2))) $2 = #<pointer 0x55ae02e57830>
> > scheme@(guile-user)> (parse-c-struct $2 (list '* '*))
> > $3 = (#<pointer 0x55ae02f9e3c0> #<pointer 0x55ae02f8b050>)
> > scheme@(guile-user)> (map pointer->string $3)
> > $4 = ("" "`\v?\x02?U")
>
> Here are another couple of examples:
>
> scheme@(guile-user)> ,use (system foreign)
> scheme@(guile-user)> (define str-1 "Hello")
> scheme@(guile-user)> (define str-2 "there!")
> scheme@(guile-user)>  (make-c-struct (list '* '*) (list (string->pointer
> str-1) (string->pointer str-2)))
> $2 = #<pointer 0x55844635a0b0>
> scheme@(guile-user)> (parse-c-struct $2 (list '* '*))
> $3 = (#<pointer 0x558445edf610> #<pointer 0x558445edf630>)
> scheme@(guile-user)> (map pointer->string $3)
> $4 = ("Hello" "there!")
>
> But then:
>
> scheme@(guile-user)> str-1
> $5 = "Hello"
> scheme@(guile-user)> str-2
> $6 = "there!"
> scheme@(guile-user)> (map pointer->string $3)
> $7 = ("?\\?q\x7f" " ??E?U")
> scheme@(guile-user)> (map pointer->string $3)
> $8 = ("?\\?q\x7f" " ??E?U")
>
> I guess that what the GC recollected are the pointers, or even $3, the list
> Let's hold on it then:
>
> scheme@(guile-user)> (define c-struct (make-c-struct (list '* '*) (list
> (string->pointer str-1) (string->pointer str-2))))
> scheme@(guile-user)> (parse-c-struct c-struct (list '* '*))
> $9 = (#<pointer 0x558445edf400> #<pointer0x558445ef8df0>)
> scheme@(guile-user)> (map pointer->string $9)
> $10 = ("\x02" "0??E?U")
>
> Not good either. Let's hold on the parse-c-struct to then:
>
> scheme@(guile-user)> (define parsed (parse-c-struct c-struct (list '*
> '*)))
> scheme@(guile-user)> parsed
> $16 = (#<pointer 0x558445edf400> #<pointer 0x558445ef8df0>)
> scheme@(guile-user)> (map pointer->string $16)
> $17 = ("0??E?U" "?\x01?F?U")
> scheme@(guile-user)> (map pointer->string parsed)
> $18 = ("\x03" "?\x01?F?U")
>
> Didn't do it either
> 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!)

Best wishes,
     Neil


> Cheers,
> David
>