Question

I have written a Hello, world program with JNI. Java calls c program with a string, c program prints that string. Here is the java program

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class JNISample {
    public native void leakMem(String str);

    static {
        System.loadLibrary("sample");
    }


    public static void main(String args[]) throws IOException {
        JNISample sample = new JNISample();
        sample.leakMem("Hello world!");
    }
}

Corresponding c program

#include <stdlib.h>
#include <jni.h>
#include "JNISample.h"

JNIEXPORT void JNICALL Java_JNISample_leakMem(JNIEnv *env, jobject jobj, jstring givenStr) {
    const char *javaStr = env->GetStringUTFChars (givenStr, 0);

    printf("Received String: %s\n", javaStr);

    env->ReleaseStringUTFChars (givenStr, javaStr);
}

When I executed this program with valgrind, with following command

valgrind --trace-children=yes --show-reachable=yes --leak-check=full java -Djava.library.path=. JNISample 10 2> log

I have observed definitely lost bytes in the log, here is the valgrind’s memory summery

LEAK SUMMARY:
==5385==    definitely lost: 5,246 bytes in 36 blocks
==5385==    indirectly lost: 5,072 bytes in 23 blocks
==5385==      possibly lost: 154,317 bytes in 131 blocks
==5385==    still reachable: 6,164,933 bytes in 831 blocks
==5385==         suppressed: 0 bytes in 0 blocks

Why there are definitely lost bytes with such a small program without any memory allocations? Is there any problem with JNI usage or JNI leaks memory? I have used JDK6 on Ubuntu 11.10 64 bit machine.

Answer 1

I cannot answer your question specifically. However, my experience with JNI is that it does a lot of things internally that can confuse valgrind. If the point at which the memory is being lost is inside JNI there wouldn’t be a lot you could do about it, anyway. However, it’s still a pain in the proverbial, since the number of different suppression needed to give you a clean run is likely to be enormous (hundreds).

Good luck.