Execute callback function on main thread from std::thread

I have a requirement of executing a callback function on exit of a std::thread and the callback function should be executed on the main thread.

On thread creation I need to detach the thread and cannot block the main loop execution for thread completion.

i tried using std::signal but that does not seem to execute callback function on the main thread

#include <thread>
#include <csignal>
#include <iostream>


std::thread::id main_thread_id;

void func2()
{
    for(int i = 0; i < 10000000; i++)
    {
        // do something
    }
}

void func()
{
    for(int i = 0; i < 10; i++)
    {
        func2();
    }
    std::raise(SIGUSR1);
}

void callback(int signal)
{
    std::cout << "SIGNAL: " << signal << "      THREAD ID:" << 
    std::this_thread::get_id() << std::endl;
    bool b = std::this_thread::get_id() == main_thread_id;
    std::cout << "IS EXECUTED ON MAIN THREAD: " << b << std::endl;
}

int main()
{
    main_thread_id = std::this_thread::get_id();
    std::cout << "MAIN THREAD ID: " << std::this_thread::get_id() << std::endl;
    std::signal(SIGUSR1, callback);
    std::thread t1(func);
    t1.detach();

    for(int i = 0; i < 20; i++)
    {
        func2();
    }

    if(t1.joinable())
        t1.join();
}

The result I get is that the callback function is not executed on main thread. Please suggest a way in which I can create a worker thread and call a callback function on main thread upon exit of the thread.

Thanks for the help