Re: [Haskell-cafe] FFI Ptr Question

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 wrote:

Quoth Michael Jones ,

...

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.

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