[libvirt] Memory free in libvirt JNA

[Benjamin Wang (gendwang)]

Hi,
    Overview Part of JNA API describes as following:
1. Description1:
If the native method returns char* and actually allocates memory, a return type of Pointer should be used to avoid leaking the memory. It is then up to you to take the necessary steps to free the allocated memory.

2. Description2:
Declare the method as returning a Structure of the appropriate type, then invoke Structure.toArray(int) to convert to an array of initialized structures of the appropriate size. Note that your Structure class must have a no-args constructor, and you are responsible for freeing the returned memory if applicable in whatever way is appropriate for the called function.

And the example code shows as following:
// Original C code
struct Display* get_displays(int* pcount);
void free_displays(struct Display* displays);

// Equivalent JNA mapping
Display get_displays(IntByReference pcount);
void free_displays(Display[] displays);
...
IntByReference pcount = new IntByReference();
Display d = lib.get_displays(pcount);
Display[] displays = (Display[])d.toArray(pcount.getValue());
...
lib.free_displays(displays);


That's to say. All the memory allocated by native code must be freed explicitly in JNA part. We must add some free memory methods to support the memory-freeing.
Any comments?

B.R.
Benjamin Wang





-----Original Message-----
From: Daniel Veillard [mailto:veillard at redhat.com] 
Sent: 2012年8月20日 14:25
To: Benjamin Wang (gendwang)
Cc: stoty at tvnet.hu; Daniel.Schwager at dtnet.de
Subject: Re: Memory free in libvirt JNA

On Mon, Aug 20, 2012 at 05:15:45AM +0000, Benjamin Wang (gendwang) wrote:
> Hi Veillard,
>   Thanks for your reply. I checked the current Libvirt-JNA 
> implementation. I find that a method named "free" defined in Domain 
> class which is used to free the domain object. If this is mandatory, that's to say, we should a lot of methods into the current Libvirt-jna implementation to free the memory which is allocated by libvirt API. Please correct me!

  As far as I understat free() is aliased as finalize() on that object so the java runtime will call free() automatically on garbage collection. I'm not a java expert, check some Java litterature for more details about how this is done and the cases where

free() might be better called directly.

Daniel

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