c++ - Why is this not thread-safe?

Question

Why is this not thread-safe? From my understanding, the result should be a string of 100s.

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

int main()
{
    std::vector<int*> values(100, new int(0));
    std::vector<std::thread> threads;
    std::vector<std::mutex> mutexes(100);
    for (int i = 0; i < 100; i++)
    {
        threads.emplace_back([](int* v, std::mutex* m)
        {
            for (int j = 0; j < 100; j++)
            {
                std::lock_guard<std::mutex> guard(*m);
                (*v)++;
            }
        }, values[i], &mutexes[i]);
    }
    for (auto& t : threads) t.join();
    for (int i = 0; i < 100; i++)
    {
        std::lock_guard<std::mutex> guard(mutexes[i]);
        std::cout << (*(values[i])) << ' ';
    }
    std::cout << std::endl;
}

Result:

9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863 9863

question from:https://stackoverflow.com/questions/65933150/why-is-this-not-thread-safe

Answer 1

You need just a single mutex. With 100 mutexes each thread will immediately lock its own mutex instead of waiting for other threads so no sync happens.