asio 中strand的作用

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 print?

namespace
{
    // strand提供串行执行, 能够保证线程安全, 同时被post或dispatch的方法, 不会被并发的执行.
    // io_service不能保证线程安全
    boost::asio::io_service m_service;
    boost::asio::strand m_strand(m_service);
    boost::mutex m_mutex;

    void print(int id)
    {
        // boost::mutex::scoped_lock lock(m_mutex);
        static int count = 0;
        PRINT_DEBUG("id: " << boost::lexical_cast<std::string>(id));
        PRINT_DEBUG("count: " << boost::lexical_cast<std::string>(++count));
    }

    void ioRun1()
    {
        while(true)
        {
            m_service.run();
        }
    }

    void ioRun2()
    {
        while(true)
        {
            m_service.run();
        }
    }

    void strand_print1()
    {
        // PRINT_DEBUG("Enter print1");
        m_strand.dispatch(boost::bind(print, 1));
        // PRINT_DEBUG("Exit print1");
    }

    void strand_print2()
    {
        // PRINT_DEBUG("Enter print2");
        m_strand.post(boost::bind(print, 2));
        // PRINT_DEBUG("Exit print2");
    }

    void strand_print3()
    {
        // PRINT_DEBUG("Enter print3");
        m_strand.post(boost::bind(print, 3));
        // PRINT_DEBUG("Exit print3");
    }

    void strand_print4()
    {
        // PRINT_DEBUG("Enter print4");
        m_strand.post(boost::bind(print, 4));
        // PRINT_DEBUG("Exit print4");
    }

    // 将上面的m_strand换成m_service后，
    void service_print1()
    {
        // PRINT_DEBUG("Enter print1");
        m_service.dispatch(boost::bind(print, 1));
        // PRINT_DEBUG("Exit print1");
    }

    void service_print2()
    {
        // PRINT_DEBUG("Enter print2");
        m_service.post(boost::bind(print, 2));
        // PRINT_DEBUG("Exit print2");
    }

    void service_print3()
    {
        // PRINT_DEBUG("Enter print3");
        m_service.post(boost::bind(print, 3));
        // PRINT_DEBUG("Exit print3");
    }

    void service_print4()
    {
        // PRINT_DEBUG("Enter print4");
        m_service.post(boost::bind(print, 4));
        // PRINT_DEBUG("Exit print4");
    }
}

void test_strand()
{
    boost::thread ios1(ioRun1);
    boost::thread ios2(ioRun2);

    boost::thread t1(strand_print1);
    boost::thread t2(strand_print2);
    boost::thread t3(strand_print3);
    boost::thread t4(strand_print4);

    t1.join();
    t2.join();
    t3.join();
    t4.join();

    m_server.run();
}

void test_service()
{
    boost::thread ios1(ioRun1);
    boost::thread ios2(ioRun2);

    boost::thread t1(service_print1);
    boost::thread t2(service_print2);
    boost::thread t3(service_print3);
    boost::thread t4(service_print4);

    t1.join();
    t2.join();
    t3.join();
    t4.join();

    m_service.run();
}

test_strand的执行结果:

2013-01-05 17:25:34 626 [8228] DEBUG - id: 4
2013-01-05 17:25:34 631 [8228] DEBUG - count: 1
2013-01-05 17:25:34 634 [5692] DEBUG - id: 1
2013-01-05 17:25:34 637 [5692] DEBUG - count: 2
2013-01-05 17:25:34 640 [5692] DEBUG - id: 2
2013-01-05 17:25:34 642 [5692] DEBUG - count: 3
2013-01-05 17:25:34 646 [5692] DEBUG - id: 3
2013-01-05 17:25:34 649 [5692] DEBUG - count: 4

test_ioserivice的执行结果:

2013-01-05 17:26:28 071 [3236] DEBUG - id: 1
2013-01-05 17:26:28 071 [5768] DEBUG - id: 2
2013-01-05 17:26:28 071 [5108] DEBUG - id: 3
2013-01-05 17:26:28 076 [3236] DEBUG - count: 1
2013-01-05 17:26:28 079 [5768] DEBUG - count: 2
2013-01-05 17:26:28 083 [5108] DEBUG - count: 3
2013-01-05 17:26:28 087 [3236] DEBUG - id: 4
2013-01-05 17:26:28 099 [3236] DEBUG - count: 4

从结果可以看到, 在test_strand中print中两个打印函数成对执行, 在test_ioservice两个打印函数就没有线程安全可言了.
如果要保证test_ioservice同步, 就要加上mutex, 在代码中被注释的那句. 

注意从日志的线程号中可知: 真正执行print()是主线程, ios1, ios2, 而t1, t2, t3, t4线程只是往ioservice的队列中加入任务.