[libvirt] Memory free in libvirt JNA

[Benjamin Wang (gendwang)]

Hi,
   I wrote a code to verify the memory leak problem as following.
C code in so:
void checkJNAMemLeak1(int **head, int *length)
{
    long i = 0;

    *head = (int *)malloc(sizeof(int) * 100000000);
    for(i=0; i<100000000; i++)
    {
        (*head)[i] = 1;
    }

    *length = 100000000;
}

Java code:
	public static void testJNAMemLeak1()
	{
		PointerByReference head = new PointerByReference();
		IntByReference length = new IntByReference();
		
		while(true)
		{
			libben.checkJNAMemLeak1(head, length);
			System.out.println(length.getValue());
			sleep(1);
			
		}
	}

When we check memory by top command, the virt and res will increase very quickly. When we check with jconsole, there is no memory in Java heap. Even I execute GC manually by jconsole. Nothing happen.

If I change java code as following:
	public static void testJNAMemLeak1()
	{
		PointerByReference head = new PointerByReference();
		IntByReference length = new IntByReference();
		
		while(true)
		{
			libben.checkJNAMemLeak1(head, length);
			System.out.println(length.getValue());
			sleep(1);
			
			libc.free(head.getValue());
		}
	}

	public static void testJNAMemLeak1()
	{
		PointerByReference head = new PointerByReference();
		IntByReference length = new IntByReference();
		
		while(true)
		{
			libben.checkJNAMemLeak1(head, length);
			System.out.println(length.getValue());
			sleep(1);
			
			libc.free(head.getValue());
		}
	}

Then everything works well. The virt and res will not increase.
I think we must provide the free functions for all the memory allocated by libvirt.

B.R.
Benjamin Wang


-----Original Message-----
From: Benjamin Wang (gendwang) 
Sent: 2012年9月7日 15:22
To: libvir-list at redhat.com
Cc: 'veillard at redhat.com'; Yang Zhou (yangzho)
Subject: RE: Memory free in libvirt JNA

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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