C++ write batch files via filstream

#include <assert.h>
#include <atomic>
#include <chrono>
#include <fstream>
#include <iomanip>
#include <iostream>
#include <mutex>
#include <numeric>
#include <thread>
#include <unistd.h>
#include <uuid/uuid.h>
#include <vector>

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::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)
    {
        auto seconds = std::chrono::duration_cast<std::chrono::seconds>(now.time_since_epoch());
        auto mills = std::chrono::duration_cast<std::chrono::milliseconds>(now.time_since_epoch());
        auto micros = std::chrono::duration_cast<std::chrono::microseconds>(now.time_since_epoch());
        auto nanos = std::chrono::duration_cast<std::chrono::nanoseconds>(now.time_since_epoch());
        ss << std::setw(3) << std::setfill('0') << (mills.count() - seconds.count() * 1000)
           << std::setw(3) << std::setfill('0') << (micros.count() - mills.count() * 1000)
           << std::setw(3) << std::setfill('0') << (nanos.count() - micros.count() * 1000);
    }
    return ss.str();
}

void write_file(const std::string &file_name, const int &loops)
{
    std::fstream w_file(file_name, std::ios::app);
    if (!w_file.is_open())
    {
        std::cout << "Create or open " << file_name << " failed!" << std::endl;
        return;
    }

    static std::uint64_t num = 0;
    std::stringstream ss;

    for (int i = 0; i < loops; i++)
    {
        for (int j = 0; j < 1000000; j++)
        {
            ss << ++num << "," << get_uuid_value() << std::endl;
        }
        w_file << ss.str();
        ss = std::stringstream();
        std::cout << get_time_now(true) << ",num:" << num << ",loop:" << i + 1 << std::endl;
        if (!w_file.good())
        {
            std::cout << get_time_now(true) << ",num:" << num << ",loop:" << i + 1 << ",write failed!" << std::endl;
            break;
        }
    }
    w_file.close();
    std::cout << get_time_now(true) << ",num:" << num << ",finish in " << __FUNCTION__ << std::endl;
}

int main(int args, char **argv)
{
    try
    {
        write_file(argv[1], atoi(argv[2]));
    }
    catch (const std::exception &e)
    {
        std::cerr << e.what() << '\n';
    }
}

Compile

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

Run

./h1 log.txt 100000