how do you exec a native app in (presumably) the libs folder of the APK?

[aguaviva]

Excited by your lib I started writing an app that 'rsync' files that have been modified so my phone data gets backed up to my server.

All works great, I compiled rsync and put it in the lib folder, but I cannot run it :) Do you have any thoughts on how this could be done? I am super close :)

Cheers

[cnlohr]

I am unfamiliar with how this works myself, but I do know I was once able to compile an app with a main() and everything, and execute it off of the external data partition. Not sure if that still works.

[aguaviva]

I figured out how to access the libs folder

1. add android:extractNativeLibs="true" to the application in your manifest
2. get the path with this snippet:

int GetLibDir(char *out) 
{
    JavaVM* java_vm = g_App->activity->vm;
    JNIEnv* java_env = nullptr;

    jint jni_return = java_vm->GetEnv((void**)&java_env, JNI_VERSION_1_6);
    if (jni_return == JNI_ERR)
        return -1;

    jni_return = java_vm->AttachCurrentThread(&java_env, nullptr);
    if (jni_return != JNI_OK)
        return -2;

    jclass contextClass = java_env->GetObjectClass(g_App->activity->clazz);
    if (contextClass == nullptr)
        return -3;
    
    const jmethodID getApplicationContextMethod =
        java_env->GetMethodID(contextClass, "getApplicationContext", "()Landroid/content/Context;");
    const jobject contextObject =
        java_env->CallObjectMethod(g_App->activity->clazz, getApplicationContextMethod);
    
    const jmethodID getApplicationInfoMethod = java_env->GetMethodID(
        contextClass, "getApplicationInfo", "()Landroid/content/pm/ApplicationInfo;");
    const jobject applicationInfoObject =
        java_env->CallObjectMethod(contextObject, getApplicationInfoMethod);
    const jfieldID nativeLibraryDirField = java_env->GetFieldID(
        java_env->GetObjectClass(applicationInfoObject), "nativeLibraryDir", "Ljava/lang/String;");
    const jobject nativeLibraryDirObject =
        java_env->GetObjectField(applicationInfoObject, nativeLibraryDirField);
    const jmethodID getBytesMethod = java_env->GetMethodID(
        java_env->GetObjectClass(nativeLibraryDirObject), "getBytes", "(Ljava/lang/String;)[B");
    const auto bytesObject = static_cast<jbyteArray>(java_env->CallObjectMethod(
        nativeLibraryDirObject, getBytesMethod, java_env->NewStringUTF("UTF-8")));
    const size_t length = java_env->GetArrayLength(bytesObject);
    const jbyte* const bytes = java_env->GetByteArrayElements(bytesObject, nullptr);
    const std::string libDir(reinterpret_cast<const char*>(bytes), length);
    strcpy(out, libDir.c_str());
    return 0;
}

[cnlohr]

I believe I misunderstood your goals... This feels very ... unusual ... What purpose would you want to do this for?

[aguaviva]

That is useful for embedding native android binaries in the APK and running them.

For example I just crated an app that when detects you took a picture it will upload it using rsync & ssh to a server https://github.com/aguaviva/Syncy (warning it is highly experimental but it works on my phone :) )

[cnlohr]

Wow, that's janky. I would never have thought to even try it!

[aguaviva]

Why janky? It is just an experiment and, not that bad, the great thing is that I fully developed it on Linux (without any attempt at making it portable, I used threads, semaphores, fork, exec, inotify,...) and then it just works on android (just small issues like this one only).

[cnlohr]

It's fair I just never thought about that as a reasonable solution.

[cnlohr]

If you wanted to write this up, I'd totes accept a PR. Or if you made this another repo based on this one?