在Visual Studio中，当与std :: async一起使用时，不会调用"thread_local"变量析构函数，这是一个错误吗? [英] In Visual Studio, `thread_local` variables' destructor not called when used with std::async, is this a bug?

问题描述

以下代码

#include <iostream>
#include <future>
#include <thread>
#include <mutex>

std::mutex m;

struct Foo {
    Foo() {
        std::unique_lock<std::mutex> lock{m};
        std::cout <<"Foo Created in thread " <<std::this_thread::get_id() <<"\n";
    }

    ~Foo() {
        std::unique_lock<std::mutex> lock{m};
        std::cout <<"Foo Deleted in thread " <<std::this_thread::get_id() <<"\n";
    }

    void proveMyExistance() {
        std::unique_lock<std::mutex> lock{m};
        std::cout <<"Foo this = " << this <<"\n";
    }
};

int threadFunc() {
    static thread_local Foo some_thread_var;

    // Prove the variable initialized
    some_thread_var.proveMyExistance();

    // The thread runs for some time
    std::this_thread::sleep_for(std::chrono::milliseconds{100}); 

    return 1;
}

int main() {
    auto a1 = std::async(std::launch::async, threadFunc);
    auto a2 = std::async(std::launch::async, threadFunc);
    auto a3 = std::async(std::launch::async, threadFunc);

    a1.wait();
    a2.wait();
    a3.wait();

    std::this_thread::sleep_for(std::chrono::milliseconds{1000});        

    return 0;
}

在macOS中编译并运行宽度的叮当声

clang++ test.cpp -std=c++14 -pthread
./a.out

获得结果

Foo Created in thread 0x70000d9f2000
Foo Created in thread 0x70000daf8000
Foo Created in thread 0x70000da75000
Foo this = 0x7fd871d00000
Foo this = 0x7fd871c02af0
Foo this = 0x7fd871e00000
Foo Deleted in thread 0x70000daf8000
Foo Deleted in thread 0x70000da75000
Foo Deleted in thread 0x70000d9f2000

已编译并在Visual Studio 2015 Update 3中运行:

Foo Created in thread 7180
Foo this = 00000223B3344120
Foo Created in thread 8712
Foo this = 00000223B3346750
Foo Created in thread 11220
Foo this = 00000223B3347E60

不调用析构函数.

这是bug还是未定义的灰色区域?

P.S.

如果最后的睡眠std::this_thread::sleep_for(std::chrono::milliseconds{1000});时间不够长，有时您可能看不到全部3条删除"消息.

使用std::thread而不是std::async时，在两个平台上都将调用析构函数，并且将始终打印所有3条删除"消息.

解决方案

介绍性提示:我现在已经学到了很多有关此内容，因此重新编写了答案.感谢@ super，@ M.M和@DavidHaim和@NoSenseEtAl(使我)走上了正确的轨道.

tl; dr 微软对std::async的实现是不符合标准的，但是它们有其​​原因，并且一旦您正确理解它们的所作所为实际上就会有用.

对于那些不想要的人来说，编写std::async的直接替换替代品并不是很困难，该替代品在所有平台上的工作方式都相同.我在此处发布了一个.

哇，最近这些天打开的情况如何，我喜欢，请参见: cppreference 的意思是(强调和删除线):

模板函数async异步运行函数f(可选地潜在在单独的线程中，该线程可能是线程池的一部分).

但是， C ++标准表示:

如果在policy中设置了launch::async，则[std::async]调用[f函数] 就像在新的执行线程中一样 ...

那么哪个是正确的?正如OP所发现的，这两个语句具有非常不同的语义.当然，正如clang和gcc所示，该标准是正确的，那么Windows的实现为何有所不同?就像很多东西一样，它可以追溯到历史.

(古老的)链接该MM疏的意思是:

... Microsoft以std::async]的实现> PPL (并行模式库)... [并且]我可以理解那些公司急于修改规则并通过std::async访问这些库的渴望，尤其是在它们可以显着提高性能的情况下…… /p>

...当使用launch_policy::async.进行调用时，Microsoft希望更改std::async的语义.我认为在随后的讨论中几乎排除了这一点...(如果要了解更多信息，请阅读链接，非常值得).

PPL基于Windows对

Compiled and run width clang in macOS:

clang++ test.cpp -std=c++14 -pthread
./a.out

Got result

Foo Created in thread 0x70000d9f2000
Foo Created in thread 0x70000daf8000
Foo Created in thread 0x70000da75000
Foo this = 0x7fd871d00000
Foo this = 0x7fd871c02af0
Foo this = 0x7fd871e00000
Foo Deleted in thread 0x70000daf8000
Foo Deleted in thread 0x70000da75000
Foo Deleted in thread 0x70000d9f2000

Compiled and run in Visual Studio 2015 Update 3:

Foo Created in thread 7180
Foo this = 00000223B3344120
Foo Created in thread 8712
Foo this = 00000223B3346750
Foo Created in thread 11220
Foo this = 00000223B3347E60

Destructor are not called.

Is this a bug or some undefined grey zone?

P.S.

If the sleep std::this_thread::sleep_for(std::chrono::milliseconds{1000}); at the end is not long enough, you may not see all 3 "Delete" messages sometimes.

When using std::thread instead of std::async, the destructors get called on both platform, and all 3 "Delete" messages will always be printed.

解决方案

Introductory Note: I have now learned a lot more about this and have therefore re-written my answer. Thanks to @super, @M.M and (latterly) @DavidHaim and @NoSenseEtAl for putting me on the right track.

tl;dr Microsoft's implementation of std::async is non-conformant, but they have their reasons and what they have done can actually be useful, once you understand it properly.

For those who don't want that, it is not too difficult to code up a drop-in replacement replacement for std::async which works the same way on all platforms. I have posted one here.

Edit: Wow, how open MS are being these days, I like it, see: https://github.com/MicrosoftDocs/cpp-docs/issues/308

---

Let's being at the beginning. cppreference has this to say (emphasis and strikethrough mine):

The template function async runs the function f asynchronously (potentially optionally in a separate thread which may be part of a thread pool).

However, the C++ standard says this:

If launch::async is set in policy, [std::async] calls [the function f] as if in a new thread of execution ...

So which is correct? The two statements have very different semantics as the OP has discovered. Well of course the standard is correct, as both clang and gcc show, so why does the Windows implementation differ? And like so many things, it comes down to history.

The (oldish) link that M.M dredged up has this to say, amongst other things:

... Microsoft has its implementation of [std::async] in the form of PPL (Parallel Pattern Library) ... [and] I can understand the eagerness of those companies to bend the rules and make these libraries accessible through std::async, especially if they can dramatically improve performance...

... Microsoft wanted to change the semantics of std::async when called with launch_policy::async. I think this was pretty much ruled out in the ensuing discussion ... (rationale follows, if you want to know more then read the link, it's well worth it).

And PPL is based on Windows' built-in support for ThreadPools, so @super was right.

So what does the Windows thread pool do and what is it good for? Well, it's intended to manage frequently-sheduled, short-running tasks in an efficient way so point 1 is don't abuse it, but my simple tests show that if this is your use-case then it can offer significant efficiencies. It does, essentially, two things

• It recycles threads, rather than having to always start a new one for each asynchronous task you launch.
• It limits the total number of background threads it uses, after which a call to std::async will block until a thread becomes free. On my machine, this number is 768.

So knowing all that, we can now explain the OP's observations:

1. A new thread is created for each of the three tasks started by main() (because none of them terminates immediately).

2. Each of these three threads creates a new thread-local variable Foo some_thread_var.

3. These three tasks all run to completion but the threads they are running on remain in existence (sleeping).

4. The program then sleeps for a short while and then exits, leaving the 3 thread-local variables un-destructed.

I ran a number of tests and in addition to this I found a few key things:

• When a thread is recycled, the thread-local variables are re-used. Specifically, they are not destroyed and then re-created (you have been warned!).
• If all the asynchonous tasks complete and you wait long enough, the thread pool terminates all the associated threads and the thread-local variables are then destroyed. (No doubt the actual rules are more complex than that but that's what I observed).
• As new asynchonous tasks are submitted, the thread pool limits the rate at which new threads are created, in the hope that one will become free before it needs to perform all that work (creating new threads is expensive). A call to std::async might therefore take a while to return (up to 300ms in my tests). In the meantime, it's just hanging around, hoping that its ship will come in. This behaviour is documented but I call it out here in case it takes you by surprise.

Conclusions:

1. Microsoft's implementation of std::async is non-conformant but it is clearly designed with a specific purpose, and that purpose is to make good use of the Win32 ThreadPool API. You can beat them up for blantantly flouting the standard but it's been this way for a long time and they probably have (important!) customers who rely on it. I will ask them to call this out in their documentation. Not doing that is criminal.

2. It is not safe to use thread_local variables in std::async tasks on Windows. Just don't do it, it will end in tears.

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