Re: Data Race problem

I have the following very simple multi-thread program:

#include <thread>

#include <mutex>

int var = 0;

void incr()

{

   std::mutex m;

   m.lock();

   var++;

   m.unlock();

}

int main()

{

   std::thread t{incr};

   std::mutex m;

   m.lock();

   ++var;

   m.unlock();

   t.join();

   return 0;

}

This is really simple program, and I think,

I made a mutual exclusion, when two threads

are writing shared variable (var).

But, when I run valgrind cmd , it turns out there is

data race. I used the following commands under Linux (GCC 4.7.0):

$ g++ -std=c++11 -pthread -pedantic -Wall simple_data_race.c++

$ valgrind -q --tool=helgrind ./a.out

Here it is the <abbreviated> valgrind messages:

<Message>

...

Possible data race during write of size 8 at <address> by thread #1

...

This conflicts with a previous write of size 8 by thread #2

...

</Message>

I struggled a few hours with such a simple code.

Please shed some light.