Shared Variables

If more than one thread is sharing a variable, you have to coordinate the access. That’s the job for mutexes and locks in C++.

Data race

A data race

A data race is a state in which at least two threads access a shared data at the same time, and at least one of the threads is a writer. Therefore the program has undefined behaviour.

You can observe very well the interleaving of threads if a few threads write to std::cout. The output stream std::cout is, in this case, the shared variable.

Unsychronised writing to std::cout

// withoutMutex.cpp 
...
#include <thread>
...

using namespace std;

struct Worker{
  Worker(string n):name(n){};
  void operator() (){
    for (int i= 1; i <= 3; ++i){
      this_thread::sleep_for(chrono::milliseconds(200));
      cout << name << ": " << "Work " << i << endl;
    }
  }
private:
  string name; 
};

thread herb= thread(Worker("Herb"));
thread andrei...