sync.cpp

#include<fstream>
#include<iostream>
#include<thread>
#include<string>
#include<cstdio>
void write(std::string filename, std::string message){
	std::ofstream f;
	f.open(filename, std::ofstream::out | std::ofstream::app);
	for (auto i = 0; i < 10; i++) {
		f << filename << " " << message << std::endl;
		f.flush();
	}
}

int numlines(std::string filename) {
	std::ifstream f{filename};
	std::string token;
	int counter;
	for (counter=0;std::getline(f, token);counter++){}
	return counter;
}

constexpr int q = 30;

int main() {
	constexpr int nl = q * 10;
	for (int i = 0; i < 1<<20; i++){
		std::thread ths[q];
		for (int j = 0; j < q; j++)
		{
			ths[j] = std::thread{write, "h.txt", std::string{(char)(65+j)}};
		}
		for (int j = 0; j < q; j++)
		{
			ths[j].join();
		}
	if (numlines("h.txt") != nl ){
			std::cout << "Not thread safe!" << std::endl;
			}
	else {
		std::cout << i << std::endl;
	}
	std::remove("h.txt");
	}
	return 0;
}