asio在handle級(jí)上提供了多線程保護(hù)鎖，命名為asio::strand,strand(繩、線之一股, 線, 繩, 串, 海濱, 河岸)顧名思義序列化,這個(gè)命名可能和asio的工作原理有關(guān).asio的內(nèi)部維護(hù)了一個(gè)隊(duì)列,當(dāng)異步請(qǐng)求的狀態(tài)轉(zhuǎn)移為完成時(shí),service會(huì)調(diào)用對(duì)應(yīng)的線程對(duì)應(yīng)的handle.如果在多線程方式下,可能有多個(gè)handle訪問共享變量.而strand把這一handle針對(duì)這一變量的的操作序列為不可分割的一段,這樣另外一段針對(duì)這一變量的的操作就必須等該操作完成后才能開始.這樣就起到了互斥信號(hào)的作用.
詳見下面的例子:
#include
#include
#include
#include
#include
class printer
{
public:
printer(boost::asio::io_service& io)
: strand_(io),
timer1_(io, boost::posix_time::seconds(1)),
timer2_(io, boost::posix_time::seconds(1)),
count_(0)
{
timer1_.async_wait(strand_.wrap(boost::bind(&printer::print1, this)));
timer2_.async_wait(strand_.wrap(boost::bind(&printer::print2, this)));
}
~printer()
{
std::cout << "Final count is " << count_ << "\n";
}
void print1()
{
if (count_ < 10)
{
std::cout << "Timer 1: " << count_ << "\n";
++count_;
timer1_.expires_at(timer1_.expires_at() + boost::posix_time::seconds(1));
timer1_.async_wait(strand_.wrap(boost::bind(&printer::print1, this)));
}
}
void print2()
{
if (count_ < 10)
{
std::cout << "Timer 2: " << count_ << "\n";
++count_;
timer2_.expires_at(timer2_.expires_at() + boost::posix_time::seconds(1));
timer2_.async_wait(strand_.wrap(boost::bind(&printer::print2, this)));
}
}
private:
boost::asio::strand strand_;
boost::asio::deadline_timer timer1_;
boost::asio::deadline_timer timer2_;
int count_;
};
int main()
{
//啟動(dòng)兩個(gè)線程
//asio的每個(gè)線程必須調(diào)用io_service::run,這個(gè)有點(diǎn)類似com的coinitlize
boost::asio::io_service io;
printer p(io);
boost::thread t(boost::bind(&boost::asio::io_service::run, &io));
io.run();
t.join();
return 0;
}
這個(gè)例子的最大不同就是
timer1_.async_wait(strand_.wrap(boost::bind(&printer::print1, this)));
這一行.我們知道
timer1_.async_wait(boost::bind(&printer::print1, this));
啟動(dòng)了一個(gè)時(shí)鐘請(qǐng)求,而print1和print2函數(shù)訪問了共享變量cout_,而要互斥地訪問該變量,我們只需要用strand把該這兩個(gè)handle包裝一下(wrap).這樣一來兩個(gè)線程就可以安全的在屏幕輸出各自的信息(當(dāng)然,這樣一來,輸出的信息時(shí)間的嚴(yán)格性就無法保證).如果去掉該wrap,我們將看到屏幕的輸出是無序的,而且變量的值也會(huì)出現(xiàn)變小的現(xiàn)象.
asio的strand當(dāng)然可以用在自己的函數(shù)內(nèi)部,同時(shí)它也是跨平臺(tái)的互斥量噢.