多个DLL正在写入同一文本文件?

#include <iostream>
#include <fstream>
#include <windosws.h>

struct Mutex {
    Mutex () {
       h = ::CreateMutex(0, false, "{any-GUID-1247965802375274724957}");
    }
    ~Mutex () {
       ::CloseHandle(h);
    }        
    HANDLE h;
};

Mutex mutex; // GLOBAL mutex

void dll_write_func() {
    ::WaitForSingleObject(mutex.h, INFINITE);
    ////////////////
    // Write here
    std::ofstrem f("output.txt");
    f << "Test" << std::endl;
    ////////////////
    ::ReleaseMutex(mutex.h);
}

struct MutexLock {
    explicit MutexLock(Mutex & m) : m(m) {
        ::WaitForSingleObject(m.h, INFINITE);
    }
    ~MutexLock() {
         ::ReleaseMutex(m.h);
    }
    Mutex & m;
};

void dll_write_func2() {
    MutexLock mlock(mutex);

    // Write here
    std::ofstrem f("output.txt");
    f << "Auto mutex Release" << std::endl;
}