[Haskell-cafe] FFI Ptr Question

[Michael Jones]

Donn,

My intent was to hold the strings in a TVar, with a thread keeping it up to date, in this case, filtering, adding, etc.

I need to look at how TVar works. I assume that a read from a TVar produces a value, and if the read from TVar is done in the callback, the pointer has to be freed when the callback exits, or by the C++ code.

I also recall seeing some code in wxWidgets where after the callback to read is made, it calls another callback to set. It may be intended as a way to free memory. If every read is followed by a set, the set could could free the pointer.

Either way, the design has dynamic data getting updated by a thread at a rate of change much larger than human interaction (scrolling).

Mike

On Apr 6, 2015, at 10:47 AM, Donn Cave <donn at avvanta.com> wrote:

> Quoth Michael Jones <mike at proclivis.com>,
> 
>> The problem is newCWString creates a string that must be freed, and
>> wxWidgets does not free it. This results in a memory leak.
>> 
>> In a real application, I'll probably store a String in a TVar so that
>> some thread can keep it up to date. I could use other TVars to hold
>> the CWString and free it each time a new value is created. But, it
>> would be better if there was some way to guarantee it is freed, even
>> with exceptions, etc. The best would be if when the function returns,
>> it automatically freed the string.
>> 
>> Is there a way express the callback function so that it frees the string
>> after the return?
> 
> Are you sure that's what you want?  I may be misunderstanding "after
> the return" - to me, that sounds like right after your getValue callback.
> That would be approximately the same as freeing it within the callback,
> right after allocation and initialization.  That's not ideal at all.
> You'll need to store the string in your application, it seems to me,
> and pass that value via the callback rather than allocating a new copy
> each time.
> 
> 	Donn
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