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----
-title: std::call_once bug in Visual C++ 2012/2013
-excerpt: >
-  In this post I will describe a nasty bug I've stumbled upon in the C++
-  Standard Library implementation shipped with Microsoft Visual Studio
-  2012/2013.
-category: C++
-custom_css:
-  - syntax.css
----
-I've recently come across a nasty standard library bug in the implementation
-shipped with Microsoft Visual Studio 2012/2013.
-[StackOverflow was of no help], so I had to somehow report the bug to the
-maintainers.
-Oddly enough, Visual Studio's [Connect page] wouldn't let me report one,
-complaining about the lack of permissions, 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], 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 obvious standard library
-misbehaviours.
-
-[StackOverflow was of no help]: https://stackoverflow.com/questions/26477070/concurrent-stdcall-once-calls
-[Connect page]: https://connect.microsoft.com/VisualStudio
-[Stephan T. Lavavej]: http://nuwen.net/stl.html
-
-C++11 and singletons
---------------------
-
-Anyway, the story begins with me trying to implement the singleton pattern
-using C++11 facilities like this:
-
-```c++
-#include <mutex>
-
-template <typename Derived>
-class Singleton {
-public:
-    static Derived& get_instance() {
-        std::call_once(initialized_flag, &initialize_instance);
-        return Derived::get_instance_unsafe();
-    }
-
-protected:
-    Singleton() = default;
-    ~Singleton() = default;
-
-    static Derived& get_instance_unsafe() {
-        static Derived instance;
-        return instance;
-    }
-
-private:
-    static void initialize_instance() {
-        Derived::get_instance_unsafe();
-    }
-
-    static std::once_flag initialized_flag;
-
-    Singleton(const Singleton&) = delete;
-    Singleton& operator=(const Singleton&) = delete;
-};
-
-template <typename Derived>
-std::once_flag Singleton<Derived>::initialized_flag;
-```
-
-Neat, huh?
-Now other classes can inherit from `Singleton`, implementing the singleton
-pattern effortlessly:
-
-```c++
-class Logger : public Singleton<Logger> {
-private:
-    Logger() = default;
-    ~Logger() = default;
-
-    friend class Singleton<Logger>;
-};
-```
-
-Note that the [N2660] standard proposal isn't/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:
-
-```c++
-class Logger {
-public:
-    static Logger& get_instance() {
-        static Logger instance;
-        return instance;
-    }
-
-private:
-    Logger() = default;
-    ~Logger() = default;
-};
-```
-
-<div class="alert alert-info" markdown="1">
-
-The point is that the `Logger::get_instance` routine above wasn't thread-safe
-until C++11.
-Imagine what might happen if `Logger`'s constructor takes some time to
-initialize the instance.
-If a couple of threads then call `get_instance`, the first thread might begin
-the initialization process, making the other thread believe that the instance
-had already been intialized.
-This other thread might then return a reference to the instance which hasn't
-yet completed its initialization and is most likely unsafe to use.
-    
-Since C++11 includes the proposal mentioned above, this routine would indeed be
-thread-safe in C++11.
-Unfortunately, the compilers shipped with Visual Studio 2012/2013 don't/didn't
-implement this particular proposal, which caused me to look at
-`std::call_once`, which seemed to implement exactly what I needed.
-
-</div>
-
-[N2660]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2660.htm
-
-Problem
--------
-
-Unfortunately, matters became a bit more complicated when I tried to introduce
-two singletons, one having a dependency on the other.
-I had `Logger`, like in the example above, and some kind of a "master"
-singleton (let's call it `Duke`).
-`Duke`'s constructor was complicated and time-consuming, and definetely
-required some logging to be done.
-I thought that I could simply call `Logger::get_instance` inside `Duke`'s
-constructor, and everything looked fine at first glance.
-
-```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";
-        // It's a lot of work to be done.
-        std::this_thread::sleep_for(std::chrono::seconds{10});
-        Logger::get_instance() << "finishing Duke's initialization";
-    }
-
-    ~Duke() = default;
-
-    friend class Singleton<Duke>;
-};
-```
-
-Now, what happens if I have two threads, one using the `Duke` instance, and the
-other logging something?
-Like in this example:
-
-```c++
-#include <thread>
-
-namespace {
-
-void get_logger() {
-    entered(__FUNCTION__);
-    Logger::get_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;
-}
-```
-
-`entered` and `exiting` are utility functions to print timestamps.
-The implementation is included in the [complete code sample].
-{: .alert .alert-info }
-
-The first thread is supposed to have the total running time of 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 done in about 3 seconds
-required for the initialization of `Logger`.
-
-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 in case it was me who made a mistake.
-Unfortunately, the program behaves as expected when compiled using GCC:
-
-    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 it appears that the implementation of `std::call_once` shipped with Visual
-Studio 2012/2013 relies on some kind of a global lock, which causes even the
-simple example above to misbehave.
-
-The [complete code sample] to demonstrate the misbehaviour described above can
-be found in this blog's repository.
-
-[complete code sample]: {{ site.github.repository_url }}/tree/master/std_call_once_bug
-
-Resolution
-----------
-
-So, since I couldn't submit the bug via Visual Studio's [Connect page], 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
-before actually using any of them.
-In a few months, Mr. Lavavej replied to me that the bug has been fixed in
-Visual Studio 2015 RTM.
-I would like to thank him for the professionalism and responsibility he's
-shown.