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

From: David Pirotte
Subject: Re: make-c-struct and pointer->string
Date: Tue, 2 Apr 2019 15:03:42 -0300

Hello Mark,

> >>  '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?  

> In my previous email I described some practical difficulties with
> implementing this suggestion, but I should rather emphasize a more
> important point:

> 'make-c-struct' is ultimately meant for passing data to C, and in
> general you should assume that Boehm GC will _not_ see references from C
> data structures to your GC-allocated objects.

> In fact, Boehm GC does _not_ scan objects allocated using C 'malloc',
> nor C++ 'new', nor 'make-c-struct'.  So, for purposes of garbage
> collection, you should normally assume that passing pointers to C land
> is equivalent to passing them to a block hole.

> In general, when passing GC-allocated objects to C code, unless the C
> code in question was written with Boehm GC in mind, you must always
> think about how long the object must be kept alive, and arrange to keep
> a reference to it for at least that long, and preferably no longer than
> necessary or else you'll have space leaks.

> I'm open to suggestions of how to make this better, although I'm also
> fairly confident that interfacing between GC-managed and non-GC-managed
> code is a thorny problem with no silver bullets.

Answering the second of your 2 emails, but I did read the first as well ...

I agree with you, I think I somehow was only looking at one (of the many) way 
may use make-c-struct/parse-c-struct, and indeed I now see that my request would
even lead to mem problem, since most of the time, C may change these pointers 
or any
other struct field(s) content ...

Thanks again for your time and very good explanations.


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