#include <iostream>
#include <thread>
#include <sstream>

void f1() {
    for(;;) {
        std::stringstream stream;
        stream << "f1" << std::endl;
        std::cout << stream.str();
    }
}

void f2() {
    for(;;) {
        std::stringstream stream;
        stream << "f1" << std::endl;
        std::cout << stream.str();
    }
}

int main() {
    std::thread t1(f1);
    std::thread t2(f2);

    t1.join(); t2.join();

    return 0;
}