[racket] make-sized-byte-string and GC

This looks right as far as I can tell, except that you need to return
`res` after the `memcpy`.

At Sun, 26 Jan 2014 00:08:09 +0400, Roman Klochkov wrote:
>  So, to make string, that is freed by reciever, I should make something like
> 
> (define _string/transfer 
>        (make-ctype _bytes
>                         (λ (v) 
>                           (define b (if (string? v) (string->bytes/utf-8 v) v))
>                           (define length (bytes-length b))
>                           (define res (malloc 'raw length))
>                           (memcpy res b length))                         (λ (v) 
> (register-finalizer v free))))
> 
> Or finalizer will not work (because v is _pointer, but the type returns 
> _bytes), and instead I have to make
> (λ (v)
>    (define res (cast v _pointer _bytes))
>    (register-finalizer v free))
> 
> ?
> 
> 
> Суббота, 25 января 2014, 14:37 -05:00 от Ryan Culpepper <ryanc at ccs.neu.edu>:
> >On 01/25/2014 01:28 PM, Roman Klochkov wrote:
> >> Is making bytestring from pointer adds the pointer to GC?
> >> 
> >>
> >>  > (define x (malloc 'raw 10))
> >>  > x
> >> #<cpointer>
> >>  > (define b (make-sized-byte-string x 10))
> >>  > (cpointer-gcable? b)
> >> #t
> >>  > (cpointer-gcable? x)
> >> #f
> >>  > (cast x _pointer _int32)
> >> 173726656
> >>  > (cast b _pointer _int32)
> >> 173726656
> >>
> >> So b and x points to the same block of 10 bytes, but value of b is
> >> GCable and value of x is not.
> >> I assume, that when b will be changed, then the bytestring will be
> >> collected and accessing x will give segfault. Am I right?
> >
> >I think it's a bug that (cpointer-gcable? b) returns true, since the FFI 
> >generally treats bytestrings as pointers to the memory that stores their 
> >contents, and in this case that memory is not managed by the GC.
> >
> >The bytestring object itself (which consists of a header, a pointer, and 
> >a length, IIRC) is collectible, but then so is the cpointer object 
> >(which consists of a header, a pointer, and some other stuff, like a tag 
> >list).
> >
> >So no, you should not expect a segfault. On the other hand, if you free 
> >x and use b afterwards, then you should expect a segfault or some other 
> >form of memory corruption.
> >
> >Ryan
> >
> 
> 
> -- 
> Roman Klochkov
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