If you've got some multi-threaded code, you may have data races in it. Data races are hard to find and reproduce – usually they will not occur in testing but will fire once a month in production.

For example, you ask each of your two interns to bring you a bottle of beer. This will usually result in your getting two bottles (perhaps empty), but in a rare situation that the interns collide near the fridge, you may get fewer bottles.

 4 int bottles_of_beer = 0;
 5 void Intern1() { bottles_of_beer++; } // Intern1 forgot to use Mutex.
 6 void Intern2() { bottles_of_beer++; } // Intern2 copied from Intern1.
 7 int main() {
 8 // Folks, bring me one bottle of beer each, please.
 9 ClosureThread intern1(NewPermanentCallback(Intern1)),
10 intern2(NewPermanentCallback(Intern2));
11 intern1.SetJoinable(true); intern2.SetJoinable(true);
12 intern1.Start(); intern2.Start();
13 intern1.Join(); intern2.Join();
14 CHECK_EQ(2, bottles_of_beer) << "Who didn't bring me my beer!?";
15 }



Want to find data races in your code? Run your program under Helgrind!

$ helgrind path/to/your/program
Possible data race during read of size 4 at 0x5429C8
at 0x400523: Intern2() tott.cc:6
  by 0x400913: _FunctionResultCallback_0_0::Run() ...
  by 0x4026BB: ClosureThread::Run() ...
  ...
  Location 0x5429C8 has never been protected by any lock
  Location 0x5429C8 is 0 bytes inside global var "bottles_of_beer"
  declared at tott.cc:4



Helgrind will also detect deadlocks for you.

Helgrind is a tool based on Valgrind. Valgrind is a binary translation framework which has other useful tools such as a memory debugger and a cache simulator. Related TotT episodes will follow.

No beer was wasted in the making of this TotT.