c++ argument changes in thread function

Issue

I'm writing an Android application which reads a text file in c++ (NDK).

I copy the file list from assets folder to /data/user/0/my-package/files/list where I can read with my native code, after the copy is done or if the destination file is exist, I do the following:

const char* fileName = env->GetStringUTFChars(_name, nullptr);
const char* filePath = env->GetStringUTFChars(_destination, nullptr);
if(copy_file(AAssetManager_fromJava(env, manager),fileName, filePath)){
    Reader *r = new Reader();
    log("Initializing reader with file: %s",filePath);
    thread t(&Reader::read, r, filePath);
    t.detach();// will crash if no detach, join() will block the UI thread.
}

And my Reader::read:

void Reader::read(const char* filePath){
    log("Reading from file: %s",filePath);
    ifstream infile(filePath);
    string line;
    while(infile>>line){
        // read logic ...
    }
}

I get different output, some times

Initializing reader with file: /data/user/0/my-package/files/list
Reading from file: /data/user/0/my-package/files/list

and everything works as expected. But some times I get

Initializing reader with file: /data/user/0/my-package/files/list
Reading from file: /data/user/0/my-package/files

see? It seems that the file path is cut somehow, and my ifstream is trying to read the directory files.

I was told that the argument filePath will be passed to the thread by value by default, and there's no other thread working with the variable, only thing that happens to filePath after the thread initialization, is:

  env->ReleaseStringUTFChars(_destination, filePath);

I have tried many different ways to pass the file path to new thread, if I pass the filePath like std::ref(filePath), I will get empty string in read function, because the variable is already cleared outside.

Any suggestion on this?

Solution

Calling ReleaseStringUTFChars before the thread has finished will be disastrous, since the memory that filePath points too will be freed. If you don't fully understand how pointers and memory handling works, I suggest using something like std::string instead of const char*, and pass that string to your function.

Answered By - Sven Nilsson