Number of async/futures in C++11

Question

I'm trying a program:

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

int foo() {
  return 0;
}

int main(int argc, char* argv[]) {
  for (auto i = 0L; i < 10000; ++i) {
    auto f = std::async(foo);
    f.get();
  }
  return 0;
}

Compiler VS11 x64.

Build:

cl /EHsc /Zi async.cpp && async

For me this program crashes. I suspect, there is a limit in the number of futures physically running at the same time. If I reduce the number of iterations to a couple of orders, it works.

So, two questions:

1. Is there a limit of actually running futures in C++11?

2. Why does this code crash at all? If I explicitly do "get()" immediately after "async()", it must complete the future before the next iteration, which means only one future runs at a time.

UPDATE

I've simplified the code to:

#include <future>

int main(int argc, char* argv[]) {
  for (auto i = 0L; i < 1000000; ++i) {
    auto f = std::async([](){ return 0; });
    f.get();
  }
  return 0;
}

And it still crashes for me. It doesn't throw, I've checked that. But now I have a visible stack trace:

async.exe!_Mtx_unlock(_Mtx_internal_imp_t * * mtx) Line 229  C++
async.exe!std::_Mtx_unlockX(_Mtx_internal_imp_t * * _Mtx) Line 84  C++
async.exe!std::_Mutex_base::unlock() Line 47  C++
async.exe!std::unique_lock<std::mutex>::~unique_lock<std::mutex>() Line 284  C++
async.exe!std::_Associated_state<int>::_Set_value(int && _Val, bool _At_thread_exit) Line 358  C++
async.exe!std::_Packaged_state<int __cdecl(void)>::_Call_immediate() Line 569  C++
async.exe!std::_Async_state<int>::`std::U_Nil::ain::ain'::`3'::<lambda_A200A86DFF9A63A1>::operator()() Line 700  C++
async.exe!??$_ApplyX@X@?$_Callable_obj@V<lambda_A200A86DFF9A63A1>@?2???$?0V?$_Bind@$0A@XV<lambda_23AC5A2FBB53FD4D>@?5?main@U_Nil@std@@U23@U23@U23@U23@U23@U23@@std@@@?$_Async_state@H@std@@QEAA@$$QEAV?$_Bind@$0A@XV<lambda_23AC5A2FBB53FD4D>@?5?main@U_Nil@std@@U23@U23@U23@U23@U23@U23@@3@@Z@$0A@@std@@QEAAXXZ() Line 420  C++
async.exe!?_Do_call@?$_Func_impl@U?$_Callable_obj@V<lambda_A200A86DFF9A63A1>@?2???$?0V?$_Bind@$0A@XV<lambda_23AC5A2FBB53FD4D>@?5?main@U_Nil@std@@U23@U23@U23@U23@U23@U23@@std@@@?$_Async_state@H@std@@QEAA@$$QEAV?$_Bind@$0A@XV<lambda_23AC5A2FBB53FD4D>@?5?main@U_Nil@std@@U23@U23@U23@U23@U23@U23@@3@@Z@$0A@@std@@V?$allocator@V?$_Func_class@XU_Nil@std@@U12@U12@U12@U12@U12@U12@@std@@@2@XU_Nil@2@U42@U42@U42@U42@U42@U42@@std@@UEAAXXZ() Line 217  C++
async.exe!std::_Func_class<void,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil>::operator()() Line 486  C++
async.exe!`Concurrency::details::_MakeVoidToUnitFunc'::`3'::<lambda_25D33530A43E1C90>::operator()() Line 1056  C++
async.exe!std::_Callable_obj<`Concurrency::details::_MakeVoidToUnitFunc'::`3'::<lambda_25D33530A43E1C90>,0>::_ApplyX<Concurrency::details::_Unit_type>() Line 420  C++
async.exe!std::_Func_impl<std::_Callable_obj<`Concurrency::details::_MakeVoidToUnitFunc'::`3'::<lambda_25D33530A43E1C90>,0>,std::allocator<std::_Func_class<Concurrency::details::_Unit_type,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil> >,Concurrency::details::_Unit_type,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil>::_Do_call() Line 217  C++
async.exe!std::_Func_class<Concurrency::details::_Unit_type,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil>::operator()() Line 486  C++
async.exe!`Concurrency::details::_Task_impl<Concurrency::details::_Unit_type>::_ScheduleTask'::`3'::<lambda_7D9BCD859405C05B>::operator()() Line 325  C++
async.exe!Concurrency::details::_PPLTaskHandle<`Concurrency::details::_Task_impl<Concurrency::details::_Unit_type>::_ScheduleTask'::`3'::<lambda_7D9BCD859405C05B> >::operator()() Line 72  C++
async.exe!Concurrency::details::_UnrealizedChore::_InvokeBridge<Concurrency::details::_PPLTaskHandle<`Concurrency::details::_Task_impl<Concurrency::details::_Unit_type>::_ScheduleTask'::`3'::<lambda_7D9BCD859405C05B> > >(Concurrency::details::_PPLTaskHandle<`Concurrency::details::_Task_impl<Concurrency::details::_Unit_type>::_ScheduleTask'::`3'::<lambda_7D9BCD859405C05B> > * _PChore) Line 4190  C++
async.exe!Concurrency::details::_UnrealizedChore::_UnstructuredChoreWrapper(Concurrency::details::_UnrealizedChore * pChore) Line 275  C++
async.exe!Concurrency::details::_PPLTaskChore::_DeletingChoreWrapper(Concurrency::details::_UnrealizedChore * pChore) Line 78  C++
async.exe!Concurrency::details::InternalContextBase::ExecuteChoreInline(Concurrency::details::WorkItem * pWork) Line 1600  C++
async.exe!Concurrency::details::InternalContextBase::Dispatch(Concurrency::DispatchState * pDispatchState) Line 1704  C++
async.exe!Concurrency::details::FreeThreadProxy::Dispatch() Line 191  C++
async.exe!Concurrency::details::ThreadProxy::ThreadProxyMain(void * lpParameter) Line 173  C++
kernel32.dll!0000000076df652d()  Unknown
ntdll.dll!0000000076f2c521()  Unknown

and threads:

Unflagged       1864    0   Worker Thread   ntdll.dll thread    ntdll.dll!0000000076f518ca  Normal
Unflagged       10964   0   Main Thread Main Thread async.exe!do_signal Normal
Unflagged       7436    0   Worker Thread   ntdll.dll thread    ntdll.dll!0000000076f52c1a  Normal
Unflagged       10232   0   Worker Thread   async.exe!Concurrency::details::ThreadProxy::ThreadProxyMain    async.exe!Concurrency::details::ThreadProxy::SuspendExecution   Normal
Unflagged   >   10624   0   Worker Thread   async.exe!Concurrency::details::ThreadProxy::ThreadProxyMain    async.exe!_Mtx_unlock   Normal
Unflagged       4756    0   Worker Thread   async.exe!Concurrency::details::ThreadProxy::ThreadProxyMain    async.exe!Concurrency::details::ThreadProxy::SuspendExecution   Normal
Unflagged       11100   0   Worker Thread   async.exe!Concurrency::details::ThreadProxy::ThreadProxyMain    async.exe!Concurrency::details::InternalContextBase::WaitForWork    Normal
Unflagged       6440    0   Worker Thread   async.exe!Concurrency::details::ThreadProxy::ThreadProxyMain    async.exe!std::vector<std::pair<void (__cdecl*)(void * __ptr64),void * __ptr64>,std::allocator<std::pair<void (__cdecl*)(void * __ptr64),void * __ptr64> > >::_Tidy Normal

I'm using VS 11.0.40825.2 PREREL.

Answer 1

1. Obviously there are implementation limits, just like there's a limit to how big an array can be. std::async can signal an error 'resource_unavailable_try_again' if the launch policy is lauch::async and the system couldn't start a new thread. But you're not getting this error.

2. The program shouldn't crash and doesn't for me (VS11 x64, release build, same source and command line).

   I believe that even without .get() the program would not have more than one asynchronous operation going at once. You assign the future to a local variable the future is destroyed each loop iteration, forcing the asynchronous operation to complete before you start another one in the next loop iteration.