log4cpp (linux): can't write message into logfile

I use cpp under Linux and I want to use the log4cpp.

I have tried to use it under windows with vs2013 and it worked very well. Now I am working under Linux and I got a problem:
It doesn't work with file. Here is my test code:

int main(int argc, char* argv[])
{
    fstream logFile;
    logFile.open("log", std::ios::app);
    log4cpp::OstreamAppender* osAppender = new log4cpp::OstreamAppender("osAppender", &logFile);
    // I tried cout as below and it worked, but if I tried as above with a file, it didn't work anymore.
    // I mean the "log" file could be created but the message can't be written down in the file. The file is always empty.
    //log4cpp::OstreamAppender* osAppender = new log4cpp::OstreamAppender("osAppender", &cout);

    osAppender->setLayout(new log4cpp::BasicLayout());

    log4cpp::Category& root = log4cpp::Category::getRoot();
    root.addAppender(osAppender);
    root.setPriority(log4cpp::Priority::DEBUG);
    root.error("Hello log4cpp in aError Message!");
    root.warn("Hello log4cpp in aWarning Message!");
    log4cpp::Category::shutdown();

    cout<<"test";

    return 0;
}

I ran this test code many times, I got no error and the program finished successfully because I can see "test" at the console. But the file is always empty.

By the way, sudo chmod +777 log has been done. So it couldn't be the problem of permission.

The problem is here:

 fstream logFile;
 logFile.open("log", std::ios::app);

What I need is:
If there is no file, create it and write message in it;
If there has been the file, apprend the message in it.

To achieve this goal, we have two ways:

 ofstream logFile;
 logFile.open("log", std::ios::app);

OR

 fstream logFile;
 logFile.open("log", std::ios::app | std::ios::out);