Creating temporary files in Android with NDK

The best way we've found is to call Context.getCacheDir on startup, get its path with getAbsolutePath, then call a JNI function to store that path in a global. Any function that wants to create a temporary file simply appends a suitable temporary file name to that path.

If you really want to fetch it from JNI another alternative would be to pass in a Context to a JNI function and use a bunch of GetMethodID / CallObjectMethod stuff to call back into Java to getCacheDir, but the former approach is a lot simpler.

Unfortunately, there does not appear to be a more elegant solution at the moment.

Below is the GetMethodID / CallObjectMethod procedure that Ertebolle refers to. It is necessary if you are working with a pure native app (such as built by Visual Studio 2015) and cannot use java code.

std::string android_temp_folder( struct android_app *app ) {
    JNIEnv* env;
    app->activity->vm->AttachCurrentThread( &env, NULL );

    jclass activityClass = env->FindClass( "android/app/NativeActivity" );
    jmethodID getCacheDir = env->GetMethodID( activityClass, "getCacheDir", "()Ljava/io/File;" );
    jobject cache_dir = env->CallObjectMethod( app->activity->clazz, getCacheDir );

    jclass fileClass = env->FindClass( "java/io/File" );
    jmethodID getPath = env->GetMethodID( fileClass, "getPath", "()Ljava/lang/String;" );
    jstring path_string = (jstring)env->CallObjectMethod( cache_dir, getPath );

    const char *path_chars = env->GetStringUTFChars( path_string, NULL );
    std::string temp_folder( path_chars );

    env->ReleaseStringUTFChars( path_string, path_chars );
    app->activity->vm->DetachCurrentThread();
    return temp_folder;
}