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-rw-r--r--_posts/2015-07-03-std-call-once-bug-in-visual-studio-2012-2013.md155
1 files changed, 65 insertions, 90 deletions
diff --git a/_posts/2015-07-03-std-call-once-bug-in-visual-studio-2012-2013.md b/_posts/2015-07-03-std-call-once-bug-in-visual-studio-2012-2013.md
index 19f6224..701c323 100644
--- a/_posts/2015-07-03-std-call-once-bug-in-visual-studio-2012-2013.md
+++ b/_posts/2015-07-03-std-call-once-bug-in-visual-studio-2012-2013.md
@@ -2,20 +2,11 @@
title: std::call_once bug in Visual Studio 2012/2013
layout: post
excerpt: >
- 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.
+ In this post I will describe a neat bug I've stumbled upon in the C++
+ Standard Library implementation shipped with Microsoft Visual Studio
+ 2012/2013.
---
-### 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.
-
-### License
-
-Distributed under the MIT License.
-See [LICENSE.txt] for details.
-
-[LICENSE.txt]: {{ site.github.repository_url }}/blob/gh-pages/LICENSE.txt
+{{ page.excerpt }}
Introduction
------------
@@ -24,9 +15,9 @@ 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 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.
+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
@@ -47,30 +38,30 @@ using C++11 facilities like this:
```
#include <mutex>
-template <typename DerivedT>
+template <typename Derived>
class Singleton
{
public:
- static DerivedT& get_instance()
+ static Derived& get_instance()
{
std::call_once(initialized_flag, &initialize_instance);
- return DerivedT::get_instance_unsafe();
+ return Derived::get_instance_unsafe();
}
protected:
Singleton() = default;
~Singleton() = default;
- static DerivedT& get_instance_unsafe()
+ static Derived& get_instance_unsafe()
{
- static DerivedT instance;
+ static Derived instance;
return instance;
}
private:
static void initialize_instance()
{
- DerivedT::get_instance_unsafe();
+ Derived::get_instance_unsafe();
}
static std::once_flag initialized_flag;
@@ -79,8 +70,8 @@ private:
Singleton& operator=(const Singleton&) = delete;
};
-template <typename DerivedT>
-std::once_flag Singleton<DerivedT>::initialized_flag;
+template <typename Derived>
+std::once_flag Singleton<Derived>::initialized_flag;
```
Neat, huh?
@@ -120,39 +111,39 @@ private:
};
```
-<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
-the instance has already been intialized.
-This other thread might then return a reference to the instance which hasn't
-completed its initialization and is most likely unsafe to use.</p>
+<div class="alert alert-info" markdown="1">
-<p>Since C++11 includes the proposal mentioned above, this routine would indeed
-be thread-safe in C++11.
+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 turn my eyes to
-<code>std::call_once</code>, which seems to implement exactly what I
-needed.</p>
+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
-The Bug
+Problem
-------
-Unfortunately, matters became a bit more complicated when I tried to have two
-singleton classes.
+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`).
-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.
+`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>
@@ -163,7 +154,7 @@ class Logger : public Singleton<Logger>
public:
Logger& operator<<(const char* msg)
{
- // Actual logging is stripped for brevity
+ // Actual logging is stripped for brevity.
return *this;
}
@@ -185,6 +176,7 @@ 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";
}
@@ -195,39 +187,19 @@ private:
};
```
-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:
+Now, what happens if I have two threads, one using the `Duke` instance, and the
+other logging something?
+Like in this example:
```
-#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";
+ Logger::get_instance();
exiting(__FUNCTION__);
}
@@ -241,20 +213,24 @@ namespace
int main()
{
- std::thread t1(&get_duke);
- std::thread t2(&get_logger);
+ std::thread t1{&get_duke};
+ std::thread t2{&get_logger};
t1.join();
t2.join();
return 0;
}
```
-The first thread is supposed to have to total running time of about 13 seconds,
-right?
+`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 executed in about 3 seconds
-required for `Logger` initialization.
+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:
@@ -266,9 +242,8 @@ Studio 2013's compiler:
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 the mistake.
-Unfortunately, the program behaves as expected when compiled using GCC's
-compiler:
+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
@@ -279,10 +254,10 @@ 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 the blog's repository.
+The [complete code sample] to demonstrate the misbehaviour described above can
+be found in this blog's repository.
-[complete code]: {{ site.github.repository_url }}/tree/gh-pages/src/posts/std_call_once_bug_in_visual_studio_2012_2013
+[complete code sample]: {{ site.github.repository_url }}/tree/gh-pages/src/posts/std_call_once_bug_in_visual_studio_2012_2013
Resolution
----------
@@ -292,9 +267,9 @@ 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!
+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.