在helgrind中运行官方boos asio示例代码的http async_client示例时，会出现警告。来自文件boost/asio/detail/posix_event.hpp的以下代码似乎是引起警告的原因:
// Signal the event and unlock the mutex.
template <typename Lock>
void signal_and_unlock(Lock& lock)
{
BOOST_ASSERT(lock.locked());
signalled_ = true;
lock.unlock();
::pthread_cond_signal(&cond_); // Ignore EINVAL.
}
这是valgrind / helgrind的完整输出:
jcm@Ubuntu:~/samples/async-client/build$ valgrind --tool=helgrind ./async-client stackoverflow.com error.html
==2894== Helgrind, a thread error detector
==2894== Copyright (C) 2007-2011, and GNU GPL'd, by OpenWorks LLP et al.
==2894== Using Valgrind-3.7.0 and LibVEX; rerun with -h for copyright info
==2894== Command: ./async-client stackoverflow.com error.html
==2894==
==2894== ---Thread-Announcement------------------------------------------
==2894==
==2894== Thread #1 is the program's root thread
==2894==
==2894== ----------------------------------------------------------------
==2894==
==2894== Thread #1: pthread_cond_{signal,broadcast}: dubious: associated lock is not held by any thread
==2894== at 0x4C2C978: ??? (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==2894== by 0x4C2E5EA: pthread_cond_signal@* (in /usr/lib/valgrind/vgpreload_helgrind-amd64-linux.so)
==2894== by 0x43FCB2: void boost::asio::detail::posix_event::signal_and_unlock<boost::asio::detail::scoped_lock<boost::asio::detail::posix_mutex> >(boost::asio::detail::scoped_lock<boost::asio::detail::posix_mutex>&) (in /home/jcm/samples/async-client/build/async-client)
==2894== by 0x43B659: boost::asio::detail::task_io_service::wake_one_idle_thread_and_unlock(boost::asio::detail::scoped_lock<boost::asio::detail::posix_mutex>&) (in /home/jcm/samples/async-client/build/async-client)
==2894== by 0x43B68A: boost::asio::detail::task_io_service::wake_one_thread_and_unlock(boost::asio::detail::scoped_lock<boost::asio::detail::posix_mutex>&) (in /home/jcm/samples/async-client/build/async-client)
==2894== by 0x43B0B8: boost::asio::detail::task_io_service::post_immediate_completion(boost::asio::detail::task_io_service_operation*) (in /home/jcm/samples/async-client/build/async-client)
==2894== by 0x43DBFD: boost::asio::detail::resolver_service_base::start_resolve_op(boost::asio::detail::task_io_service_operation*) (in /home/jcm/samples/async-client/build/async-client)
==2894== by 0x442F86: void boost::asio::detail::resolver_service<boost::asio::ip::tcp>::async_resolve<boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> > >(std::shared_ptr<void>&, boost::asio::ip::basic_resolver_query<boost::asio::ip::tcp> const&, boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> >) (in /home/jcm/samples/async-client/build/async-client)
==2894== by 0x44187E: void boost::asio::ip::resolver_service<boost::asio::ip::tcp>::async_resolve<boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> > >(std::shared_ptr<void>&, boost::asio::ip::basic_resolver_query<boost::asio::ip::tcp> const&, boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> >&&) (in /home/jcm/samples/async-client/build/async-client)
==2894== by 0x44097E: void boost::asio::ip::basic_resolver<boost::asio::ip::tcp, boost::asio::ip::resolver_service<boost::asio::ip::tcp> >::async_resolve<boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> > >(boost::asio::ip::basic_resolver_query<boost::asio::ip::tcp> const&, boost::_bi::bind_t<void, boost::_mfi::mf2<void, client, boost::system::error_code const&, boost::asio::ip::basic_resolver_iterator<boost::asio::ip::tcp> >, boost::_bi::list3<boost::_bi::value<client*>, boost::arg<1> (*)(), boost::arg<2> (*)()> >&&) (in /home/jcm/samples/async-client/build/async-client)
==2894== by 0x43E05F: client::client(boost::asio::io_service&, std::string const&, std::string const&) (in /home/jcm/samples/async-client/build/async-client)
==2894== by 0x437018: main (in /home/jcm/samples/async-client/build/async-client)
==2894==
Response returned with status code 400
==2894==
==2894== For counts of detected and suppressed errors, rerun with: -v
==2894== Use --history-level=approx or =none to gain increased speed, at
==2894== the cost of reduced accuracy of conflicting-access information
==2894== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 89 from 29)
我似乎错了，互斥锁在发出条件变量信号之前就已解锁。如果是误报，请告诉我上述代码为什么正确。
最佳答案
假定lock是与pthread_mutex_t的匹配调用中使用的相同pthread_cond_(timed)wait对象的包装对象，大多数pthread实现将允许您以两种顺序发出信号和解锁，因为pthread_cond_(timed)wait被定义为在这种情况下解锁变量并获取互斥体作为原子操作-也就是说，只有在发出CV信号并且调用线程已获取互斥体之前，它不会成功返回。
互联网上的谣言表明，无论执行什么顺序，都不会有人使该操作失败，但是某些线程调度程序已在信号发生解锁后写入以达到最大效率；而其他线程调度器已被写入以在信号发生之前进行解锁以实现最大效率。 ，这意味着无论您如何写作，都不可能永远赢。就个人而言，如果我要维护有问题的代码，我将仅以使Helgrind满意为由来更改顺序。您并不需要遍历垃圾投诉来查找真正的比赛条件。
我在要获得时间机器的POSIX API机器程序列表中添加“pthread_cond_signal应该同时包含CV和互斥体，就像pthread_cond_wait一样”。