C++ thread_local

#include <stdio.h>
#include <thread>
#include <mutex>

std::mutex mtx;

struct S
{
    S() {
        std::lock_guard<std::mutex> lock(mtx);
        printf("S() called i=%d\n", i);
    }
    int i = 0;
};

thread_local S gs;

void foo()
{
    std::lock_guard<std::mutex> lock(mtx);
    gs.i += 1;
    printf("foo  %p, %d\n", &gs, gs.i);
}

void bar()
{
    std::lock_guard<std::mutex> lock(mtx);
    gs.i += 2;
    printf("bar  %p, %d\n", &gs, gs.i);
}

int main()
{
    std::thread a(foo), b(foo);
    a.join();
    b.join();
}