[racket] ffi/unsafe and free

Thanks both for the help. I feel a bit silly now, as I did read the
ffi memory management page... ahh well. Excuses, excuses, eh?
Thankfully, in this case, the pointer isn't kept by the c library, as
it is just wanting somewhere to store a string.

Thanks again,

Neil


On 26 January 2012 14:12, Michael W <mwilber at uccs.edu> wrote:
> Hey there, Neil!
>
> When racket allocates something, it lives in memory that's
> managed by the garbage collector unless you say otherwise. By
> contrast, when the C library allocates something, racket's
> garbage collector has no clue about it -- that's what (free) is
> for.
>
> By default, (malloc) allocates a pointer that's managed by the
> garbage collector. So, you're (free)ing it yourself, but the
> garbage collector comes along later, notices that you haven't
> kept that pointer around, and frees it again.
>
> See the "mode" argument in the malloc documentation:
> http://docs.racket-lang.org/foreign/foreign_pointer-funcs.html?q=malloc#(def._((quote._~23~25foreign)._malloc))
>
> There are two ways around this. First, you can:
> (malloc _pointer 'raw)
> and then be extra careful to (free) it yourself later. Or, you
> can just (define x (malloc _pointer)) and let the garbage
> collector deal with it, being extra careful that the xmms
> libraries don't keep that pointer around after racket's garbage
> collector frees it.
>
> This post by Thomas Chust helped me understand:
> https://lists.racket-lang.org/users/archive/2011-June/045967.html
> (see about halfway down)
>
> 3 hours ago, Neil Caldwell wrote:
>> Hi
>>
>> I've just started playing with racket, and decided to start with a
>> small (that's what I'm telling myself, anyway) project in the form of
>> an xmms2 client. To do this, I'm having to interface with
>> libxmmsclient. Long story short, I'm having a problem with free, and
>> was hoping to get some advice.
>>
>> If I put the following code into ptr.rkt:
>> ####################################
>>
>> #lang racket
>>
>> (require ffi/unsafe)
>>
>> (free (malloc _pointer))
>>
>> ####################################
>>
>> and then run it I get a double free error.
>>
>> ####################################
>>
>> $ racket ptr.rkt
>> *** glibc detected *** /home/neil/apps/racket-5.2/bin/racket: double
>> free or corruption (out): 0xb45d4488 ***
>> ======= Backtrace: =========
>> /lib/libc.so.6(+0x6b6d1)[0xb76ed6d1]
>>
>> ####################################
>>
>> I haven't had much luck googling, unfortunatly (I could be search the
>> wrong things)
>>
>> Is this a problem with racket (v 5.2), my setup (compiled it myself
>> with all the default switches), glibc version incompatibility, or am I
>> just doing something plain wrong?
>>
>> Thanks for the help,
>>
>> Neil
>
> --
> Hope that helps,
>    _mike
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