The memory increased and eventually bad allocation.

#pragma once
#pragma comment(lib,"rpcrt4.lib")
#define _CTR_SECURE_NO_WARNINGS

#include <chrono>
#include <ctime>
#include <iostream>
#include <fstream>
#include <sstream>
#include <Windows.h>

using namespace std;
 
char* uuidValue;

char* getUuid()
{
    UUID newUUID;
    UuidCreate(&newUUID);
    UuidToStringA(&newUUID, (RPC_CSTR*)&uuidValue);
    return uuidValue;
}

void logFile(string fileName, int loops)
{
    fstream wFile(fileName, ios::app);
    if (!wFile.is_open())
    {
        cout << "Create or open " << fileName << " failed!" << endl;
        return;
    }

    uint64_t num = 0;
    stringstream ss;
    chrono::time_point<chrono::high_resolution_clock> startTime, endTime;
    for (int i = 0;i < loops;i++)
    {
        startTime = chrono::high_resolution_clock::now();
        ss = stringstream(); 
        for (int j = 0;j < 1000000;j++)
        {
            uuidValue = (char*)malloc(40);
            ss << ++num << "," << getUuid() << endl;
            free(uuidValue);
        }
        wFile << ss.str();
        ss.str();
        if (!wFile.good())
        {
            cout << num << ",write failed!" << endl;
            break;
        }
        cout << uuidValue << endl; 
        endTime = chrono::high_resolution_clock::now();
        cout << num << ","
            << chrono::duration_cast<chrono::seconds>(endTime - startTime).count() << " seconds,"
            << chrono::duration_cast<chrono::milliseconds>(endTime - startTime).count() << " milliseconds,"
            << chrono::duration_cast<chrono::milliseconds>(endTime - startTime).count() << " microseconds,"
            << chrono::duration_cast<chrono::nanoseconds>(endTime - startTime).count() << " nanoseconds!!!" << endl << endl;
    }
    
    wFile.close();
    cout << num << ",finished in " << __FUNCTION__ << endl;    
}

int main(int agrs, char** argv)
{
    logFile(argv[1], atoi(argv[2]));
}