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----
-title: std::call_once bug in Visual Studio 2012/2013
-layout: post
-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.
----
-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:
-
-```
-#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:
-
-```
-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:
-
-```
-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.
-
-```
-#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:
-
-```
-#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/gh-pages/src/posts/std_call_once_bug_in_visual_studio_2012_2013
-
-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.