Pfthb

I have compiled the following dummy program under Linux using gcc 8.2.1:

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

struct Foo

 void start() 
 thread = std::thread(&Foo::run, this);
 

 void stop() 
 mutex.lock();
 done = true;
 mutex.unlock();

 thread.join();
 

 void run() 
 bool tmp;

 for (;;) 
 mutex.lock();
 tmp = done;
 mutex.unlock();

 if (tmp)
 break;
 
 

 std::thread thread;
 std::mutex mutex;
 bool done;
;

int main()

 Foo foo;

 std::cout << "starting...n";
 foo.start();

 std::cout << "stopping...n";
 foo.stop();

 std::cout << "donen";

If I subsequently run it under valgrind 3.14.0, I receive the following warning:

==30060== Thread 2:
==30060== Conditional jump or move depends on uninitialised value(s)
==30060== at 0x1095F3: Foo::run() (in /.../a.out)
==30060== by 0x109AAE: void std::__invoke_impl<void, void (Foo::*)(), Foo*>(std::__invoke_memfun_deref, void (Foo::*&&)(), Foo*&&) (in /.../a.out)
==30060== by 0x109771: std::__invoke_result<void (Foo::*)(), Foo*>::type std::__invoke<void (Foo::*)(), Foo*>(void (Foo::*&&)(), Foo*&&) (in /.../a.out)
==30060== by 0x10A012: decltype (__invoke((_S_declval<0ul>)(), (_S_declval<1ul>)())) std::thread::_Invoker<std::tuple<void (Foo::*)(), Foo*> >::_M_invoke<0ul, 1ul>(std::_Index_tuple<0ul, 1ul>) (in /.../a.out)
==30060== by 0x109FB8: std::thread::_Invoker<std::tuple<void (Foo::*)(), Foo*> >::operator()() (in /.../a.out)
==30060== by 0x109F8D: std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (Foo::*)(), Foo*> > >::_M_run() (in /.../a.out)
==30060== by 0x496A062: execute_native_thread_routine (thread.cc:80)
==30060== by 0x4894A9C: start_thread (in /usr/lib/libpthread-2.28.so)
==30060== by 0x4CD7A42: clone (in /usr/lib/libc-2.28.so)

I am not completely sure what is causing this, I have written this snippet in hopes of diagnosing a bug in a more complicated class (that I cannot post here) I am currently working on and which produces exceptions when calling the equivalent of Foo::stop(). Does the valgrind warning imply some serious misunderstanding of the C++ threading interface on my part? And assuming for a moment that Foo::run would actually do something useful, how could I fix this program while keeping Foo's interface the way it is?

What is the initial value of bool done; ? It is indeterminate (some garbage value), so your thread (run) can be stopped without calling stop method.

done must be initialized:

 //...
 std::mutex mutex;
 bool done = false; // <--