c++ async future get

#include <chrono>
#include <ctime>
#include <future>
#include <iomanip>
#include <iostream>
#include <sstream>
#include <string>
#include <unistd.h>
#include <uuid/uuid.h>

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::string get_num_and_uuid(const int&len)
{
    std::stringstream ss;
    for(int i=0;i<len;i++)
    {
        ss<<i+1<<","<<get_uuid()<<std::endl;
        sleep(1);
    }
    return ss.str();
}

void async_future_demo(const int &len)
{
    std::cout<<get_time_now()<<",start in "<<__FUNCTION__<<std::endl;
    std::future<std::string> fut=std::async(get_num_and_uuid,std::cref(len));
    std::cout<<get_time_now()<<" wait for the result!"<<std::endl;
    std::cout<<get_time_now()<<",the result is "<<fut.get()<<std::endl;
    std::cout<<get_time_now()<<",end in "<<__FUNCTION__<<std::endl;
}

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

Compile

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