cpp multi thread std::lock_guard,mutex

#include <atomic>
#include <chrono>
#include <cmath>
#include <condition_variable>
#include <ctime>
#include <fstream>
#include <functional>
#include <future>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <mutex>
#include <queue>
#include <random>
#include <sstream>
#include <thread>
#include <uuid/uuid.h>
#include <vector>

std::string get_time_now()
{
    std::chrono::time_point<std::chrono::high_resolution_clock> now = std::chrono::high_resolution_clock::now();
    time_t raw_time = std::chrono::high_resolution_clock::to_time_t(now);
    struct tm tm_info = *localtime(&raw_time);
    std::stringstream ss;
    std::chrono::seconds seconds = std::chrono::duration_cast<std::chrono::seconds>(now.time_since_epoch());
    std::chrono::milliseconds mills = std::chrono::duration_cast<std::chrono::milliseconds>(now.time_since_epoch());
    std::chrono::microseconds micros = std::chrono::duration_cast<std::chrono::microseconds>(now.time_since_epoch());
    std::chrono::nanoseconds nanos = std::chrono::duration_cast<std::chrono::nanoseconds>(now.time_since_epoch());
    ss << std::put_time(&tm_info, "%Y%m%d%H%M%S")
       << std::setw(3) << std::setfill('0') << std::to_string(mills.count() - seconds.count() * 1000) << ","
       << std::setw(3) << std::setfill('0') << std::to_string(micros.count() - mills.count() * 1000)
       << std::setw(3) << std::setfill('0') << std::to_string(nanos.count() - micros.count() * 1000);
    return ss.str();
}

char *uuid_value = (char *)malloc(40);
char *get_uuid()
{
    uuid_t new_uuid;
    uuid_generate(new_uuid);
    uuid_unparse(new_uuid, uuid_value);
    return uuid_value;
}


std::mutex _mtx;
std::map<int, std::string> _mtx_map;
void fill_mtx_map(const int &len)
{
    std::lock_guard<std::mutex> _lock_guard(_mtx);
    static int num = 0;
    for (int i = 0; i < len; i++)
    {
        std::string temp_str = get_uuid();
        _mtx_map.insert(std::pair<int, std::string>(++num, temp_str));        
    }
}

void mt_fill_mtx_map(const int &x, const int &y, const int &z, const int &w)
{
    std::thread t1(fill_mtx_map, std::cref(x));
    std::thread t2(fill_mtx_map, std::cref(y));
    std::thread t3(fill_mtx_map, std::cref(z));
    std::thread t4(fill_mtx_map, std::cref(w));
    t1.join();
    t2.join();
    t3.join();
    t4.join();
    std::cout << "Size =" << _mtx_map.size() << std::endl;
    int idx = 0;
    auto itr = _mtx_map.begin();
    while (itr != _mtx_map.end())
    {
        while (++idx % 100000 == 0)
        {
            std::cout<<"Idx:"<<idx<<"," << "key:" << itr->first << ",value:" << itr->second << std::endl;
        }
        ++itr;
    }
    std::cout << get_time_now() << ", finish in lock_guard mutex of multi threads" << std::endl;
}

int main(int args, char **argv)
{
    mt_fill_mtx_map(atoi(argv[1]), atoi(argv[2]), atoi(argv[3]), atoi(argv[4]));
}

g++ -std=c++2a -I. *.cpp -o h1 -luuid -lpthread

./h1 100000000 10000000 100000000 10000000