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

[Mark H Weaver <mhw@netris.org>]

Hi David,

David Pirotte <david@altosw.be> writes:

> 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")

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.

If you look at the code in foreign.c, specifically
'scm_string_to_pointer' and 'scm_from_pointer', you'll see that the C
pointer is wrapped within a heap-allocated foreign pointer object, and a
finalizer is being set on that heap object.  'make-c-struct' copies the
C pointers from those foreign pointer objects, but not not keep a
reference to the objects themselves.

In a later message, you wrote:

> 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
> Copyright (C) 1995-2017 Free Software Foundation, Inc.
>
> Guile comes with ABSOLUTELY NO WARRANTY; for details type `,show w'.
> This program is free software, and you are welcome to redistribute it
> under certain conditions; type `,show c' for details.
>
> 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 you specifically saving a reference to the Scheme string
objects, but that's not what's needed.  What's needed is to hold
references to the foreign pointer objects returned by 'string->pointer'
for as long as the C strings are needed.

      Regards,
        Mark