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Why do we need to use boost::asio::io_service::work?

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There is an example of using boost::asio.

  1. Why does this example use the boost::asio::io_service::work ?
  2. And why is srv.run (); not called to perform tasks in the threads?
int main()
{
    boost::asio::io_service srv;
    boost::asio::io_service::work work(srv);
    boost::thread_group thr_grp;
    thr_grp.create_thread(boost::bind(&boost::asio::io_service::run, &srv));
    thr_grp.create_thread(boost::bind(&boost::asio::io_service::run, &srv));

    srv.post(boost::bind(f1, 123));
    srv.post(boost::bind(f1, 321));
    //sync

    srv.post(boost::bind(f2, 456));
    srv.post(boost::bind(f2, 654));
    //sync

    srv.stop();
    thr_grp.join();
}

Update: What is the difference between the poll and run, when io_service is used without io_service::work?

int main()
{
    boost::asio::io_service srv;
    //boost::asio::io_service::work work(srv);
    std::vector<boost::thread> thr_grp;

    srv.post(boost::bind(f1, 123));
    srv.post(boost::bind(f1, 321));
    //sync

    srv.post(boost::bind(f2, 456));
    srv.post(boost::bind(f2, 654));
    //sync

    // What is the difference between the poll and run, when io_service without work?
    thr_grp.emplace_back(boost::bind(&boost::asio::io_service::poll, &srv));// poll or run?
    thr_grp.emplace_back(boost::bind(&boost::asio::io_service::run, &srv));// poll or run? 

    srv.stop();
    for(auto &i : thr_grp) i.join();

    int b;
    std::cin >> b;

    return 0;
}
like image 514
Alex Avatar asked Jun 17 '13 20:06

Alex


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What does io_service run do?

io_service::run (1 of 2 overloads) Multiple threads may call the run() function to set up a pool of threads from which the io_service may execute handlers. All threads that are waiting in the pool are equivalent and the io_service may choose any one of them to invoke a handler.

What is boost io_service?

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What is io_ service?

Asio's io_service is the facilitator for operating on asynchronous functions. Once an async operation is ready, it uses one of io_service 's running threads to call you back. If no such thread exists it uses its own internal thread to call you. Think of it as a queue containing operations.


1 Answers

When the io_service::run method is called without a work object, it will return right away. Typically, that is not the behavior most developers are looking for. There are of course some exceptions, but most developers are looking to specify a thread to handle all of the asynchronous processing and don't want that thread to exit until told to do so. That is what your code example does.

The io_service::run method is specified as a delegate or function pointer in the create_thread methods. So, when the thread is created from the create_thread method it will call the io_service::run method and it passes the io_service object as an argument. Typically one io_service object can be used with multiple socket objects.

The stop method is usually called when shutting down the application or when communication between all clients/servers is no longer required and it is not anticipated that any new connections will need to be initiated.

like image 154
Bob Bryan Avatar answered Sep 22 '22 08:09

Bob Bryan