JNI and pointer deletion
 

Hello All
Some time ago I did some modifications at a JNI function and I am
facing some problems, so I need some tips hot to solve it.
My java code calls a C++ code, that calls the java code again many
times.
In the first call to C++ from java, a C++ object is created (named
"problem"), and passed to Java (casted as a jlong), in order to, in
the next call to C++, retrieve the C++ object again.
The "create" function is like that:
[code]
JNIEXPORT jlong JNICALL create
(JNIEnv *env, jobject obj_this,
jint n, jint m,
jint nele_jac, jint nele_hess,
jint index_style, jboolean call_finalize_solution, jboolean
call_intermediate_callback)
{
/* create the IpoptProblem */
Jipopt* problem=new Jipopt(env, obj_this, n, m, nele_jac, nele_hess,
index_style,
call_finalize_solution, call_intermediate_callback);
if(problem == NULL){
return 0;
}

// return our class
return (jlong)problem;
}
[\code]

The communication between them is perfect, except for one fact, the
destroy function that delete the pointer "problem"; inside C++ code
lead to JVM crash. the delete function is like that:
[code]
JNIEXPORT void JNICALL destroy
(JNIEnv *env,
jobject obj_this,
jlong pipopt){
// cast back our class
Jipopt *problem = (Jipopt *)pipopt;

if(problem!=NULL){
delete problem;
}
}
[\code]
But there is also an interesting thing: if I call the destroy
function this way, the crash does not happen:
[code]
JNIEXPORT void JNICALL destroy
(JNIEnv *env,
jobject obj_this,
jlong pipopt){
// cast back our class
Jipopt *problem = (Jipopt *)pipopt;

if(problem!=NULL){
problem = NULL;
delete problem;
}
}
[\code]
I mean, if I point the allocated pointer to NULL before its deletion,
the crash does not occur, if I just delete it (like in the first
code), the crash happens. I think that the possible source of this
problem is the fact that the java code still holds it reference to
jlong reference. Is it right? Pointing "problem" to NULL would not
lead to memory leak if I call this function many times? Is this the
right way to do this deletion? Is this deletion necessary or the JVM
"takes care" of this deletion issue?

Bye

Re: JNI and pointer deletion
 

On Wed, 23 Apr 2008 08:31:45 -0700 (PDT), Edsoncv wrote:
> The communication between them is perfect, except for one fact, the
> destroy function that delete the pointer "problem"; inside C++ code
> lead to JVM crash.
[...]
> I mean, if I point the allocated pointer to NULL before its
> deletion, the crash does not occur, if I just delete it (like in the
> first code), the crash happens.

Setting the pointer to NULL before calling delete means that you
aren't actually deleting the object at all. "delete NULL" has no
effect at all, so your object is still allocated. Do this enough times
and you will run out of memory.

If delete (when the pointer is not NULL) causes a crash, then there
are three likely reasons:

- the object has already been deleted. Deleting it a second time
isn't allowed.

- the pointer has changed (i.e. you have changed it) since it was
allocated, so you are attempting to delete something "random" that
was never allocated with new in the first place.

- you've done something bad with a *different* pointer (such as a
reference to a java object in one of your many calls to java from
the c++ code) that has corrupted the metadata used by new and
delete, and delete crashes as a result.

> I think that the possible source of this problem is the fact that
> the java code still holds it reference to jlong reference.

That's true. You should clear the jlong pointer stored on the java
side when you delete the object, or there is a risk that you attempt
to use it again after deleting the object (or delete it a second
time).

/gordon

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Re: JNI and pointer deletion
 

On Wed, 23 Apr 2008 08:31:45 -0700 (PDT), Edsoncv wrote:
> Jipopt* problem=new Jipopt(env, obj_this, n, m, nele_jac, nele_hess,
> index_style, call_finalize_solution, call_intermediate_callback);

One thing that I missed in my previous response...

You are passing a copy of "env" to the Jipopt constructor, and
presumably use it to invoke JNI functions later. That's a really bad
idea, and potentially violates an important JNI rule: the env pointer
is only valid in the context it was created for. Violating that rule
will almost guarantee a crash.

See sections 10.14, 8.1.1 and 8.1.4 here:
http://java.sun.com/docs/books/jni/html/jniTOC.html

/gordon

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