Re: C calling Scheme and garbage collection

[David Pirotte <david@altosw.be>]

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Hello,

>...
> In my experience, it's definitely not ok to capture a pointer to a
> scheme object and store it for later use without protecting the scheme
> object from gc by holding a reference.

I recently had a similar conversation, here is one of the emails of the thread, the
one that lists a series of examples, and out of those, only the last example works:

	https://lists.gnu.org/archive/html/guile-devel/2019-03/msg00013.html

In that last example, I am holding a reference to both the data and their pointer,
but it seemed to me, after further local tests, that it is sufficient to hold on a
reference to the pointers, not the pointers and the data:

scheme@(guile-user)> ,use (system foreign)
scheme@(guile-user)> (define ptr-1 (string->pointer "Hello"))
scheme@(guile-user)> (define ptr-2 (string->pointer "there!"))
scheme@(guile-user)> (make-c-struct (list '* '*) (list ptr-1 ptr-2))
$2 = #<pointer 0x5623b599a350>
scheme@(guile-user)> (parse-c-struct $2 (list '* '*))
$3 = (#<pointer 0x5623b51b1110> #<pointer 0x5623b591e6f0>)
scheme@(guile-user)> (map pointer->string $3)
$4 = ("Hello" "there!")
scheme@(guile-user)> (gc)
scheme@(guile-user)> (map pointer->string $3)
$5 = ("Hello" "there!")

Is this correct, or am I just lucky here? What is certain is that holding a
reference to the data (only) does not work, as shown in the examples
in the 'pointed' email ...

David

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