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

[Neil Jerram]

On Thu, 28 Mar 2019 at 15:15, Amirouche Boubekki
<address@hidden> wrote:
>
> Eventually, I can reproduced the issue with guile-next from guix:
>
> scheme@(guile-user) [1]> (make-c-struct (list '* '*) (list (string->pointer 
> "hello ") (string->pointer "there!")))
> $5 = #<pointer 0x1d1ca70>
> scheme@(guile-user) [1]> (parse-c-struct $5 (list '* '*))
> $6 = (#<pointer 0x166a950> #<pointer 0x166a9f0>)
> scheme@(guile-user) [1]> (map pointer->string $6)
> $7 = ("hello " "there!")
> scheme@(guile-user) [1]> (make-c-struct (list '* '*) (list (string->pointer 
> "hello") (string->pointer "there!")))
> $8 = #<pointer 0x1ce2a40>
> scheme@(guile-user) [1]> (parse-c-struct $8 (list '* '*))
> $9 = (#<pointer 0x166a810> #<pointer 0x166a910>)
> scheme@(guile-user) [1]> (map pointer->string $9)
> $10 = ("hello" "there!")
> scheme@(guile-user) [1]> (make-c-struct (list '* '*) (list (string->pointer 
> "hello") (string->pointer "there!")))
> $11 = #<pointer 0x1ce2a10>
> scheme@(guile-user) [1]> (parse-c-struct $11 (list '* '*))
> $12 = (#<pointer 0x166a850> #<pointer 0x166a690>)
> scheme@(guile-user) [1]> (map pointer->string $12)
> $13 = ("?<\"?\x0e\x7f" "@?f\x01")
> scheme@(guile-user) [1]>
> scheme@(guile-user)>
>
>
> Le mar. 26 mars 2019 à 14:19, David Pirotte <address@hidden> a écrit :
>>
>> Hello,
>>
>>         guile 2.2.4.1-cdb19
>>
>> ,use (system foreign)
>>
>> ;; this fails
>>
>> 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")
>>
>> ;; but this works
>>
>> scheme@(guile-user)> (map pointer->string (parse-c-struct (make-c-struct 
>> (list '* '*) (list (string->pointer "hello ") (string->pointer "there!"))) 
>> (list '* '*)))
>> $19 = ("hello " "there!")

I guess it may be GC.  I've had problems with code like

   (procedure->pointer void (lambda (...) ...) args...)

because procedure->pointer does not protect the (lambda ...) from being GC'd.

For example: 
http://git.savannah.nongnu.org/cgit/ossaulib.git/commit/?id=1d31a87b53e9375b5fdd5120b7d409e6dde6aa47

I wonder if there's a similar problem here with the string->pointer data.

Best wishes,
     Neil