[libvirt] Memory free in libvirt JNA

[Benjamin Wang (gendwang)]

Hi Claudio,
   Thanks for you informing about new JNA version. I try to use JNA provided API. It works well. The updated code is as following:

     public SchedParameter[] getSchedulerParameters() throws LibvirtException {
         IntByReference nParams = new IntByReference();
         SchedParameter[] returnValue = new SchedParameter[0];
-        String scheduler = libvirt.virDomainGetSchedulerType(VDP, nParams);
+        Pointer pScheduler = libvirt.virDomainGetSchedulerType(VDP, 
+ nParams);
         processError();
-        if (scheduler != null) {
+        if (pScheduler != null) {
+            String scheduler = pScheduler.getString(0);
+            Native.free(Pointer.nativeValue(pScheduler));
             virSchedParameter[] nativeParams = new virSchedParameter[nParams.getValue()];
             returnValue = new SchedParameter[nParams.getValue()];
             libvirt.virDomainGetSchedulerParameters(VDP, nativeParams, nParams);

If there is no issue. I recommend to use this solution to enhance all JNA code.

BTW:
Not all the returned String should be freed by JNA. For example,  In Domain Class, the result returned by the method getName/ getUUIDString
can't be freed. Because the reference were not allocated temporarily. We must analyze case by case.

B.R.
Benjamin Wang


-----Original Message-----
From: Claudio Bley [mailto:cbley at av-test.de] 
Sent: 2012年10月11日 23:36
To: Benjamin Wang (gendwang)
Cc: veillard at redhat.com; libvir-list at redhat.com; Yang Zhou (yangzho)
Subject: Re: [libvirt] Memory free in libvirt JNA

At Thu, 11 Oct 2012 08:37:23 +0000,
Benjamin Wang (gendwang) wrote:
> 
> Hi Claudio,
>    Sorry for my late response.
>    I have gone through Claudio's solution. It's good. But I think this is not a common solution. There are two points:
> 1. This solution must use Pointerbyreference to encapsulate the
>    Pointer. This is not clean.

Yes, as I said, this adds another level of indirection --- which is pretty useless in Java.

> 2. Libvirt provides virFree method. But a common library could not
>    provide memory management functions.

Sorry, I don't get your point here.

> My proposal is as following:
> 1. Add a new Class Libc.java
> public interface Libc  extends Library{
> 	Libc INSTANCE = (Libc) Native.loadLibrary("c", Libc.class);
> 	
> 	public void free(Pointer p);
> }

Not every platform has a shared library called "c". On Windows this would be msvcrt.dll for the Microsoft runtime. So, you would need to branch on the platform to load whatever library seems appropriate.

Also, I just discovered that since version 3.3.0 JNA provides a public free method itself.

Since I get crashes when using callback functions with JNA 3.2.7 in certain circumstances it is better just to require a newer version of JNA, IMHO.

I'll post a few patches with improvements and memory fixes tomorrow.

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