printing duplications when using threads c++

Question

I'm trying to write a basic code for my bigger program. The original code suppose to write on a txt file some results of my calculations but here I changed it to write the numer 1 (to simplify the code). The problem is that I dont get the number of ones I suppose to get... instede of 1000 prints of 1 I get a bigger and random number of 1 each running...

What is the problem with my code?

(I'm using windows 10, codeblock workspace, I'm writing the code in c++)

The code:

    #include <iostream>
    #include <thread>
    #include <vector>
    #include <fstream>
    using namespace std;
    ofstream myfile;
    void doTask()
    {

       myfile << "1\n";
     }

    void f()
    {
        vector<thread> threads;
        for(int i = 0; i < 10; ++i)
        {
            threads.push_back(thread(doTask));
        }

        for(int j=0; j<10; j++) threads[j].join();
        threads.erase(threads.begin(), threads.end());
     }




     int main()
     {
      myfile.open("a.txt");
      for(int i=0; i<100; i++) f();
      myfile.close();
      return 0;
     }

Thank you all!

Answer 1

Make ofstream thread safe. Following will do.

#include <iostream>
#include <thread>
#include <vector>
#include <fstream>
#include <mutex>
using namespace std;
ofstream myfile;
std::mutex myMutex;

void doTask()
{
    myMutex.lock();
    myfile << "1\n";
    myMutex.unlock();
}

void f()
{
    vector<thread> threads;
    for(int i = 0; i < 10; ++i)
    {
        threads.push_back(thread(doTask));
    }

    for(int j=0; j<10; j++) threads[j].join();
    threads.erase(threads.begin(), threads.end());
}




int main()
{
    myfile.open("a.txt");
    for(int i=0; i<100; i++) f();
    myfile.close();
    return 0;
}