JNI, memory leak and JNI memory deallocation

[sedj]

Hi,

I've got a .so C library on which is called via JNI. This takes a few parameters, and creates a char* which points to about 300K ish of allocated memory (its basically a file). I then create a jbyteArray from this char*, copying in the contents. I then free the char*. This jbyteArray is then returned to the calling Java method.

However, there appears to be a substantial memory leak somewhere, and I don't believe it is within the native C code.

Does anyone have any idea how JNI frees a pointer to a byte array (the jbyteArray) after the JNI call is finished, and the calling Java method returns ?

The Java code is basically doing :

public void doSomething(OutputStream os) {
  byte[] bytes = myNativeMethod();
  OutputStream os = ...
  os.write(bytes);
  os.flush();
  os.close();
  bytes = null;
}

and the native C is basically :

JNIEXPORT jbyteArray JNICALL myClass_nativeMethod((JNIEnv* env, jobject obj, ...) {

  char *myPtr;
  jbyteArray jb;
  int size;

  myPtr = make_me_a_big_300K_pointer(&size);

  jb = (*env)->NewByteArray(env, size);
  (*env)->SetByteArrayRegion(env, jb, 0, size, (jbyte*)myPtr);
  free(myPtr);
  return jb;
}

Any insight would be cool !

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[Diancecht]

AFAIK, you can only make direct references to memory from within the native code. Once you return from there, is the garbage collector the one who will take care of it.

I'm not sure if you need to call a release operation from your native code in your byte array.

HTH

Dian

 

[stefanwagner]

public void doSomething(OutputStream os) 
{
  byte[] bytes = myNativeMethod();
// (...)
  bytes = null;
}

Since you leave the scope, where 'bytes' is defined, the nulling is perfect nonsense.


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[sedj]

Cheers for that Stefan !!!

 

[stefanwagner]

Sorry I didn't recognized it was you.
Looking at the c-code, I only get confused by those pointers and see that I cannot help here.

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[Diancecht]

Diving on my mind (I used to work with JNI), I remember that, appart from getting and releasing memory from the C code (malloc, free and so on), you could also use JNI calls to do that.

Probably when you do a newByteArray that's what you're doing. Then, the free(Ptr) frees the memory for C code, but not for JNI linker, so maybe what you need is a ReleaseByteArrayElements before returning.

HTH.

Dian

 

[sedj]

Oops. Just found the memory leak in my C code.
Thanks for the replies guys !

 

[haslo]

What was it? I was wondering...

haslo@haslo.ch - www.haslo.ch​

 

[sedj]

In effect it was this :

AnImagePtr myPtr;
AnotherPtr anPtr;
FILE* in;

for (i = 0; i < something; i++) {
  in = fopen(someFiles[i], "r");
  myPtr = someFunction(in);
  anotherFunction(myPtr, anPtr);
  free(anPtr);
  // forgot to free myPtr
}

It was a bit harder to spot than that though, as there was a lot more code. I should stick with Java !!!

 

[stefanwagner]

Gratulation.
That remains me, why I prefer java over c++.

seeking a job as java-programmer in Berlin:

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[chrissie1]

I think your problem is the variable naming. something, somefunction are a nono.

Christiaan Baes
Belgium

If you want to get an answer read this FAQ faq796-2540
There's no such thing as a winnable war - Sting

 

[haslo]

That was pseudocode ;-) - and sedj's problem was the dangling myPtr, as he pointed out in his post.

haslo@haslo.ch - www.haslo.ch​

 

[chrissie1]

yes,yes just kidding. but in his first post he frees myptr and in the second he doesn't free myptr but anptr. I think poor old sedj is getting a bit confussed. 1nd I don't like posters then forget half of the question and then don't even give the answer.

Christiaan Baes
Belgium

If you want to get an answer read this FAQ faq796-2540
There's no such thing as a winnable war - Sting

 

[sedj]

OK chrissie, calm down, calm down - its nearly the weekend.

 

[Diancecht]

<ironic>
Yes, I also don't like people like sedj that spend all day asking questions
</ironic>

 

[sedj]

I know, I'm such a mooch - its just "take take take" with me !!!



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