path: root/_posts/2015-07-03-std-call-once-bug-in-visual-studio-2012-2013.md

---
title: std::call_once bug in Visual Studio 2012/2013
layout: post
---

### Abstract

In this post, I will describe a neat bug I've stumbled upon in C++ Standard
Library implementation shipped with Microsoft Visual Studio 2012/2013.

### Licensing

This post, including code samples, is licenced under the terms of the MIT License.
See
[LICENSE.txt](https://github.com/egor-tensin/cpp_tips/blob/gh-pages/LICENSE.txt)
for details.

## Introduction

I've recently come across a nasty standard library bug in the implementation
shipped with Microsoft Visual Studio 2012/2013.
[StackOverflow](https://stackoverflow.com)
was of
[no help](https://stackoverflow.com/questions/26477070/concurrent-stdcall-once-calls),
so I had to somehow report the bug to the maintainers.
Oddly enough, Visual Studio's
[Connect page](https://connect.microsoft.com/VisualStudio)
wouldn't let me to report one, complaining that I supposedly had no right to do
so, even though I was logged in from my work account, associated with my Visual
Studio 2013 installation.

Fortunately, I've come across the personal website of this amazing guy,
[Stephan T. Lavavej](http://nuwen.net/stl.html),
who appears to be the chief maintainer of Microsoft's standard library
implementation.
He seems to be your go-to guy when it comes to standard library misbehaviour.

## C++11 and singletons

Anyway, I was designing a software system with a couple of singletons in it.
I was trying to do proper singletons using C++11 like this:

{% highlight c++ %}
#pragma once

#include <mutex>

template <typename DerivedT>
class Singleton
{
public:
    static DerivedT& get_instance()
    {
        std::call_once(initialized_flag, &initialize_instance);
        return DerivedT::get_instance_unsafe();
    }

protected:
    Singleton() = default;
    ~Singleton() = default;

    static DerivedT& get_instance_unsafe()
    {
        static DerivedT instance;
        return instance;
    }

private:
    static void initialize_instance()
    {
        DerivedT::get_instance_unsafe();
    }

    static std::once_flag initialize_flag;

    Singleton(const Singleton&) = delete;
    Singleton& operator=(const Singleton&) = delete;
};

template <typename DerivedT>
std::once_flag Singleton<DerivedT>::initialized_flag;
{% endhighlight %}

Neat, huh?
Now other classes can inherit `Singleton`, implementing the singleton pattern
with little work:

{% highlight c++ %}
class Logger : public Singleton<Logger>
{
private:
    Logger() = default;
    ~Logger() = default;

    friend class Singleton<Logger>;
};
{% endhighlight %}

Note that this was at the time when the
[N2660](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2660.htm)
standard proposal wasn't implemented in the compilers shipped with Visual Studio
2012/2013.
If it was, I wouldn't, of course, need to employ this `std::call_once` trickery,
and the implementation would be much simpler, i.e. something like this:

{% highlight c++ %}
class Logger
{
public:
    static Logger& get_instance()
    {
        static Logger instance;
        return instance;
    }

private:
    Logger() = default;
    ~Logger() = default;
};
{% endhighlight %}

<div class="alert alert-info">
<p>
The point is that the <code>Logger::get_instance</code> routine above wasn't
thread-safe until C++11.
Imagine what might happen if <code>Logger</code>s constructor takes some time to
initialize the instance.
If a couple of threads then call <code>get_instance</code>, the first thread
might begin the initialization process, making the other thread believe that it
doesn't need to initialize the instance anymore (goodbye, C++03).
The second thread might then return a reference to a not fully initialized (and
hence, unsafe to use) instance.</p>

<p>
Since C++11 includes the proposal for
<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2660.htm">"Dynamic Initialization and Destruction with Concurrency"</a>
mentioned above, this routine would indeed be thread-safe in C++11.
Unfortunately, the compilers shipped with Visual Studio 2012/2013 didn't
implement this particular proposal, which caused me to turn my eyes to
<code>std::call_once</code>, which seemed to implement exactly what I
wanted.
</p>
</div>

## The bug

Unfortunately, matters became a bit more complicated when I tried to have two
singleton classes.
I had `Logger`, like in the example above, and some kind of a "master" singleton
(let's call it `Duke`).
These two classes both inherited from `Singleton`, which I thought was nice.
`Duke`s constructor was heavy and complicated and definetely required some
logging to be done.
OK, I thought, I will simply call `Logger::get_instance` inside `Duke`s
constructor, and everything would be fine.

{% highlight c++ %}
#include <chrono>
#include <thread>

class Logger : public Singleton<Logger>
{
public:
    Logger& operator<<(const char* msg)
    {
        // Actual logging is stripped for brevity
        return *this;
    }

private:
    Logger()
    {
        // Opening log files, etc.
        std::this_thread::sleep_for(std::chrono::seconds{ 3 });
    }

    ~Logger() = default;

    friend class Singleton<Logger>;
};

class Duke : public Singleton<Duke>
{
private:
    Duke()
    {
        Logger::get_instance() << "started Duke's initialization";
        std::this_thread::sleep_for(std::chrono::seconds{ 10 });
        Logger::get_instance() << "finishing Duke's initialization";
    }

    ~Duke() = default;

    friend class Singleton<Duke>;
};
{% endhighlight %}

What would happen if I had two threads, one to do something with the `Duke`
instance, and the other to do something else, logging in process?
Like this:

{% highlight c++ %}
#include <ctime>

#include <iostream>
#include <sstream>
#include <thread>

namespace
{
    void entered(const char* f)
    {
        std::ostringstream oss;
        std::time_t tt = std::time(NULL);
        oss << "Entered " << f << " at " << std::ctime(&tt);
        std::cout << oss.str();
    }

    void exiting(const char* f)
    {
        std::ostringstream oss;
        std::time_t tt = std::time(NULL);
        oss << "Exiting " << f << " at " << std::ctime(&tt);
        std::cout << oss.str();
    }

    void get_logger()
    {
        entered(__FUNCTION__);
        Logger::get_instance() << "got the Logger instance";
        exiting(__FUNCTION__);
    }

    void get_duke()
    {
        entered(__FUNCTION__);
        Duke::get_instance();
        exiting(__FUNCTION__);
    }
}

int main()
{
    std::thread t1(&get_duke);
    std::thread t2(&get_logger);
    t1.join();
    t2.join();
    return 0;
}
{% endhighlight %}

The first thread is supposed to be executed in about 13 seconds, right?
Three seconds to initialize the `Logger` instance and ten to initialize the
`Duke` instance.
The second thread, similarly, is supposed to be executed in about 3 seconds
required for `Logger` initialization.

Weirdly, this program produces the following output when compiled using Visual
Studio 2013's compiler:

    Entered `anonymous-namespace'::get_duke at Fri Jul 03 02:26:16 2015
    Entered `anonymous-namespace'::get_logger at Fri Jul 03 02:26:16 2015
    Exiting `anonymous-namespace'::get_duke at Fri Jul 03 02:26:29 2015
    Exiting `anonymous-namespace'::get_logger at Fri Jul 03 02:26:29 2015

Isn't it wrong that the second thread actually took the same 13 seconds as the
first thread?
Better check with some other compiler.
Unfortunately, the program behaves as expected when compiled using GCC's
compiler:

    Entered get_logger at Fri Jul  3 02:27:12 2015
    Entered get_duke at Fri Jul  3 02:27:12 2015
    Exiting get_logger at Fri Jul  3 02:27:15 2015
    Exiting get_duke at Fri Jul  3 02:27:25 2015

So this is was the bug that broke everything for me.

## Resolution

So, since I couldn't submit the bug via Visual Studio's
[Connect page](https://connect.microsoft.com/VisualStudio),
I wrote to Mr. Lavavej directly, not hoping for an answer.
Amazingly, it took him less than a day to reply.
He told me he was planning to overhaul `std::call_once` for Visual Studio 2015.
Meanwhile, I had to stick to something else; I think I either dropped logging
from `Duke`s constructor or initialized all the singleton instances manually
upon program's startup.
In a few months, Mr. Lavavej replied to me (that's professionalism and
responsibility I lack) and wrote that the bug has been fixed in Visual Studio
2015 RTM.
Kudos to the amazing guy!