c++ condition_variable wait notify_one

#include <chrono>
#include <condition_variable>
#include <ctime>
#include <curl/curl.h>
#include <curl/easy.h>
#include <fstream>
#include <future>
#include <iostream>
#include <iomanip>
#include <memory>
#include <stdio.h>
#include <string>
#include <uuid/uuid.h>
#include <vector>

std::string get_time_now(bool is_exact = false)
{
    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;
    ss << std::put_time(&tm_info, "%Y%m%d%H%M%S");
    if (is_exact)
    {
        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());
        std::uint64_t mills_count = mills.count() - seconds.count() * 1000;
        std::uint64_t micros_count = micros.count() - mills.count() * 1000;
        std::uint64_t nanos_count = nanos.count() - micros.count() * 1000;
        ss << "_" << std::setw(3) << std::setfill('0') << std::to_string(mills_count)
           << std::setw(3) << std::setfill('0') << std::to_string(micros_count)
           << std::setw(3) << std::setfill('0') << std::to_string(nanos_count);
    }
    return ss.str();
}


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


std::condition_variable _cv;
std::mutex _mtx;
std::vector<std::string> _vec;
int _vec_len = -1;
int _idx = 0;
void fill_vector(const int &len)
{
    std::cout << get_time_now(true) << " in " << std::this_thread::get_id() << ",start in " << __FUNCTION__ << std::endl;
    for (int i = 0; i < len; i++)
    {
        _vec.push_back(get_uuid_value());
    }
    _vec_len = _vec.size();
    std::cout << "_vec.size()= " << _vec.size() << " in " << std::this_thread::get_id() << ",in " << __FUNCTION__ << ",line:" << __LINE__ << std::endl;
    std::cout << get_time_now(true) << " in " << std::this_thread::get_id() << ",finish in " << __FUNCTION__ << std::endl;
    _cv.notify_one();
}

void print_vector()
{
    std::cout << get_time_now(true) << " in " << std::this_thread::get_id() << ",start in " << __FUNCTION__ << std::endl;
    std::unique_lock<std::mutex> _lk(_mtx);
    _cv.wait(_lk, []
             { return _vec.size() == _vec_len; });

    std::cout << "_vec.size()= " << _vec.size() << " in " << std::this_thread::get_id() << ",in " << __FUNCTION__ << ",line:" << __LINE__ << std::endl;
    
    auto itr = _vec.begin();
    while (itr != _vec.end())
    {
        if (++_idx % 100000 == 0)
        {
            std::cout << _idx << "," << *itr << std::endl;
        }
        ++itr;
    }
    std::cout << get_time_now(true) << " in " << std::this_thread::get_id() << ",finish in " << __FUNCTION__ << std::endl;
}

void cv_mt_wait(const int &len)
{
    std::thread t1(fill_vector, std::cref(len));
    std::thread t2(print_vector); 
    t1.join();
    t2.join(); 
    std::cout << get_time_now(true) << ",finish in " << __FUNCTION__ << std::endl;
}

int main(int args, char **argv)
{ 
    cv_mt_wait(atoi(argv[1]));
    return 0;
}

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

./h1 3000000