Printf调用与std::thread混淆，但std::cout没有问题

当使用printf (stdio.h)时，"启动两个线程n"answers"输入一个数字n"混合在一起(类似于" steantert b a th thnureabernadsn")，但在std::cout (iostream)中不会发生这种情况。我怀疑这与std::thread有关，但我仍然是多线程编程的新手。

我真的不想使用iostream，因为它使程序超大!

我使用的是mingw32g++ 4.9.2，用g++ -o foreback foreback.cpp -O2 -std=c++11编译。(虽然我的电脑是64位的，但是我发现mingw-w64产生的程序大约是mingw32的两倍大，所以我没有使用它)

//#define IOS
#ifdef IOS
#include <iostream>
#endif
#include <stdio.h>
#include <thread>
#include <atomic>
#include <windows.h> // for Sleep()
std::atomic<int> atom(1);
void foreground() {
    int c = 1;
    while (c) {
#ifdef IOS
        std::cout << "Enter a number: ";
        std::cin >> c;
#else
        printf("Enter a number: ");
        scanf("%d", &c);
#endif
        atom.store(c, std::memory_order_relaxed);
    }
}
void background() {
    FILE *out = fopen("foreback.txt", "w");
    int c = 1;
    while (c) {
        fprintf(out, "%d", c);
        c = atom.load(std::memory_order_relaxed);
        Sleep(500);
    }
    fclose(out);
}
int main() {
    std::thread f(foreground), b(background);
#ifdef IOS
    std::cout << "Start both threads.n";
#else
    printf("Start both threads.n");
#endif
    f.join();
    b.join();
#ifdef IOS
    std::cout << "End of both threads.n";
#else
    printf("End of both threads.n");
#endif
    return 0;
}