Question

Here is the native C++ method.

JNIEXPORT jboolean JNICALL Java_xpnp_XpNamedPipe_readBytes
(JNIEnv* pEnv, jclass cls, jlong pipeHandle, jbyteArray readBufferJava, jint bytestoread, jint timeoutMsecs){

    jbyte* readBuffer = NULL;
    try {
        readBuffer = pEnv->GetByteArrayElements(readBufferJava, NULL);
        if (readBuffer == NULL) {
            throw std::bad_alloc();
        }
        int retval = XPNP_readBytes ((XPNP_PipeHandle)pipeHandle, (char*)readBuffer, bytestoread, timeoutMsecs);

        std::cout<<"this is what I read: " << readBuffer << "\n";
        std::flush(std::cout);

        return (retval <= 0) ? 0 : retval;
    }catch (std::exception& except) {
        // setErrorInfo(except.what());
    }
    return 0;
}

This method prints the correct text of readBuffer that it reads from the call XPNP_readBytes, but passes an array of all zeros to Java! Any idea why that happens? Am I doing something wrong in passing the pointer or converting it to Java?

Here is the declaration of the native C++ method in the Java file.

private static native boolean readBytes(long pipeHandle, byte[] buffer, int bytesToRead, int timeoutMsecs);

This is where I’m calling the native method.

boolean b = readBytes(namedPipeHandle, buffer, bytesToRead, timeoutMsecs);
String a = new String(buffer);

The buffer I read after the call is all 0’s, even though it prints the right text in the native code!

Answer 1

Look up ReleaseByteArrayElements.