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June 2009 C++ Standards Committee Mailing - New Working Draft and Concurrency Papers

Wednesday, 24 June 2009

The June 2009 mailing for the C++ Standards Committee was published today. This is the pre-meeting mailing for the upcoming committee meeting. Not only does it include the current C++0x working draft, but there are 39 other papers, of which 6 are concurrency related.

Concurrency-related papers

N2880: C++ object lifetime interactions with the threads API

This is a repeat of the same paper from the May 2009 mailing. It is referenced by several of the other concurrency papers.

N2888: Moving Futures - Proposed Wording for UK comments 335, 336, 337 and 338

This is the first of my papers in this mailing. The current working draft restricts std::unique_future to be MoveConstructible, but not MoveAssignable. It also restricts std::shared_future in a similar way, by making it CopyConstructible, but not CopyAssignable. This severely limits the potential use of such futures, so the UK panel filed comments on CD1 such as UK comment 335 which requested relaxation of these unnecessary restrictions. This paper to provide a detailed rationale for these changes, along with proposed wording.

N2889: An Asynchronous Call for C++

This is the first of two papers in the mailing that proposes a std::async() function for starting a task possibly asynchronously and returning a future for the result from that task. This addresses a need identified by UK comment 329 on CD1 for a simple way of starting a task with a return value asynchronously.

N2901: A simple async()

This is the second of the papers in the mailing that proposes a std::async() function for starting a task possibly asynchronously and returning a future for the result from that task.

The primary difference between the papers is the type of the returned future. N2889 proposes a new type of future (joining_future), whereas N2901 proposes using std::unique_future. There are also a few semantic differences surrounding whether tasks that run asynchronously aways do so on a new thread, or whether they may run on a thread that is reused for multiple tasks. Both papers provide a means for the caller to specify synchronous execution of tasks, or to allow the implementation to decide between synchronous execution and asynchronous execution on a case-by-case basis. N2901 also explicitly relies on the use of lambda functions or std::bind to bind parameters to a call, whereas N2889 supports specifying function arguments as additional parameters, as per the std::thread constructor (see my introduction to starting threads with arguments in C++0x).

Personally, I prefer the use of std::unique_future proposed in N2901, but I rather like the argument-passing semantics of N2889. I also think that the thread_local issues can be addressed by my proposal for that (N2907, see below). A final concern that I have is that calling the task inside future::get() can yield surprising behaviour, as futures can be passed across threads, so this may end up being called on another thread altogether. For synchronous execution, I would prefer invoking the task inside the std::async call itself, but delaying it to the get() does allow for work-stealing thread pools.

N2907: Managing the lifetime of thread_local variables with contexts

This is the second of my papers in this mailing. It proposes a potential solution to the lifetime-of-thread_local-variables issues from N2880 discussed in last month's blog post.

The idea is that you manage the lifetime of thread_local variables by constructing an instance of a new class std::thread_local_context. When the std::thread_local_context instance is destroyed, all thread_local variables created in that context are also destroyed. You can then construct a subsequent instance of std::thread_local_context, and create new thread_local instances in that new context. This means that you can reuse a thread for multiple unrelated tasks, without "spilling" thread_local variables from an earlier task into later tasks. It can also be used with a std::async() function to ensure that the thread_local variables are destroyed before the associated future is made ready.

N2917: N2880 Distilled, and a New Issue With Function Statics

This is Herb Sutter's take on the issues from N2880. He starts with a general discussion of the issue with detached threads and static destruction of globals, and then continues with a discussion of the issues surrounding the destruction of function-scoped thread_local variables. In particular, Herb focuses on something he calls Function thread_local statics poisoning thread_local destructors — if the destructor of a thread_local object x calls a function that itself uses a function-scope thread_local y, then the destructor of y might already have been called, resulting in undefined behaviour.

I found Herb's coverage of the issues surrounding detached threads dismissive of the idea that people could correctly write manual synchronization (e.g. using a std::condition_variable or a std::unique_future), even though this is common practice amongst those using POSIX threads (for example, in David Butenhof's Programming with POSIX Threads, he says "pthread_join is a convenience, not a rule", and describes examples using detached threads and condition variables to signal when the thread is done). I can see many possibilities for such usage, so as a consequence, I am personally in favour of his "solution" 1D: leave things as they are with regard to detached threads — it is already undefined behaviour to access something after its destruction.

However, the issue Herb raises with regard to order of destruction for thread_local variables is important, and not something that my std::thread_local_context proposal addresses. As Herb points out, the problem does exist with regard to function-local static variables already — thread_local just amplifies the problem. I am inclined to go with what POSIX threads does, and what boost::thread_specific_ptr does: make them "phoenix" variables that are re-initialized when accessed after destruction, and are thus added back onto the destructor list. This is Herb's solution 2B.

Other papers

Now that CD1 is out, and the committee is trying to get things pinned down for CD2, Concepts are getting a lot of discussion. There are therefore several papers on Concepts in the mailing. There are also papers on automatically defining move constructors, revised wording for lambdas, a proposal for unifying the function syntax, and several others. Check out the full list of papers for details.

Your comments

Do you have any comments on the papers (especially the concurrency ones, but if you have any comments on the others I'd like to know too)? Which std::async proposal do you prefer, or do you like aspects of both or neither? Do you think that thread_local_context objects combined with resurrecting thread_local objects on post-destruction access solves the thread_local issues?

Let me know by posting a comment.

Posted by Anthony Williams
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