JNI AttachNativeThread always fails with -1 return value on Linux

Question

I'm writing a JNI wrapper around a native C++ library, but I'm having trouble attaching native threads to the JVM on Linux. Everything works fine on Windows, but when I call AttachNativeThread() on an Ubuntu machine, it always returns -1.

This is the way I'm getting JNIEnv pointers:

JNIEnv* Utils::getJNI() {
    JNIEnv* jni;

    int getEnvResult = FMODWrapper::jvm->GetEnv((void**) &jni, JNI_VERSION_1_8);

    if (getEnvResult == JNI_EDETACHED) {
        JavaVMAttachArgs attachArgs;

        attachArgs.version = JNI_VERSION_1_8;
        attachArgs.group = nullptr;

        std::stringstream newName;
        newName << "jni-attached-daemon-" << std::this_thread::get_id();

        attachArgs.name = (char*) newName.str().c_str();

        int attachResult = FMODWrapper::jvm->AttachCurrentThreadAsDaemon(ANDROID_VOIDPP_CAST &jni, &attachArgs);

        if(attachResult != 0) {
            std::cerr << "Failed to attach thread! " << attachResult << "(" << newName.str() << ")" << std::endl;

        }
    }

    return jni;
}

I know that the JVM pointer is not null, and again, the code works flawlessly on Windows. If it's not possible to attach native threads on Linux for some reason, I'm always open to alternatives: the thing I'm trying to achieve are basically callbacks into my Java code. An event happens in C++, for example a request for File open, and I need to be able to handle that in Java.

Update: My JVM versions, both java and javac, this is what I used to compile and run the program.

openjdk version "1.8.0_191"
OpenJDK Runtime Environment (build 1.8.0_191-8u191-b12-2ubuntu0.18.10.1-b12)
OpenJDK 64-Bit Server VM (build 25.191-b12, mixed mode)
javac 1.8.0_191

What is ANDROID_VOIDPP_CAST? A `void *` cast isn't required here. — user207421, Apr 22 '19 at 03:05

score 0 · Answer 1 · answered Apr 21 '19 at 20:24

0

Possibly your JVM on Linux is not Java 8 compatible. There is no reason to ask for JNI_VERSION_1_8 unless you use some Java 8 JNI features. Usually, JNI_VERSION_1_6 will suffice.

answered Apr 21 '19 at 20:24

Alex Cohn

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Wasn't it sadly, my JVM version is `OpenJDK 1.8.0_191`, using `JNI_VERSION_1_6` changed nothing. Didn't work with Oracle JVMs either – Kalányosi Ramszesz Apr 21 '19 at 20:27

score 0 · Answer 2 · answered Apr 22 '19 at 09:23

0

See JNI Linux segmentation fault. This is the same problem with the same FMODWrapper when switching from Windows to Linux.

The root cause could be the logger that is used in your wrapper. As Lóránt Viktor Gerber shows there, the logger is initialized with a local reference to java/lang/System. If this logger is somehow involved in Utils::getJNI(), it will fail.

Another suspicious part of your code is passing a temporary name to AttachCurrentThreadAsDaemon(). I would try to set it to nullptr to exclude any interference.

If all the above does not work, check if AttachCurrentThreadAsDaemon() (not as a daemon) still fails in your Linux environment.

answered Apr 22 '19 at 09:23

Alex Cohn

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9
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307

1

I've now narrowed it down some more, with your suggestions it works whith my _own_ threads, however, the ones that FMOD creates still fail to attach. – Kalányosi Ramszesz Apr 22 '19 at 14:00
Sadly, I have no idea where this FMOD comes from, and how its threads are created. – Alex Cohn Apr 22 '19 at 15:50
Probably the names, I did everything you said and new threads with `std::thread` now work, but the FMOD ones don't. – Kalányosi Ramszesz Apr 22 '19 at 23:19

JNI AttachNativeThread always fails with -1 return value on Linux

2 Answers2