?

條件變量?

如果線程之間執行順序上有依賴關系，可使用條件變量（Condition variables）。?

可以到boost官網中參考條件變量（Condition variables）的使用。?

條件變量必須和互斥量配合使用，等待另一個線程重某個事件的發生（滿足某個條件），然后線程才能繼續執行。

共有兩種條件變量對象condition_variable, condition_variable_any，一般情況下使用condition_variable_any。?

條件變量的使用方式：?
擁有條件變量的線程先鎖定互斥量，然后循環檢查某個條件，如果條件不滿足，那么就調用條件變量的成員函數wait()等待直到條件滿足。其他線程處理條件變量要求的條件，當條件滿足時調用它的成員函數notify_one()或者notify_all()，以通知一個或者所有正在等待條件的變量的線程停止等待繼續執行。?

例子：生產--消費模型。?

緩沖區buffer使用了兩個條件變量cond_put和cond_get，分別用于處理put動作和get動作，如果緩沖區滿則cond_put持續等待，當cond_put得到通知 (緩沖區不滿)時線程寫入數據，然后通知cond_get條件變量可以獲取數據。cond_get的處理流程與cond_put類似。?

C++代碼??

#include?<boost/thread.hpp>??

#include?<boost/thread/mutex.hpp>??

??

#include?<iostream>??

#include?<stack>??

??

using?namespace?std;??

??

boost::mutex?io_mu;??

??

class?buffer??

{??

private:??

????boost::mutex?mu;?//?互斥量，配合條件變量使用??

????boost::condition_variable_any?cond_put;?//?寫入條件變量??

????boost::condition_variable_any?cond_get;?//?讀取條件變量??

??

????stack<int>?stk;?//?緩沖區對象??

????int?un_read,?capacity;??

????bool?is_full()?//?緩沖區滿判斷??

????{??

????????return?un_read?==?capacity;??

????}??

????bool?is_empty()??//?緩沖區空判斷??

????{??

????????return?un_read?==?0;??

????}??

??

public:??

????buffer(size_t?n)?:?un_read(0),?capacity(n){}??//?構造函數??

????void?put(int?x)??//?寫入數據??

????{??

????????{?//?開始一個局部域??

????????????boost::mutex::scoped_lock?lock(mu);?//鎖定互斥量??

????????????while?(?is_full()?)?//?檢查緩沖區是否滿??

????????????{??

????????????????{?//?局部域，鎖定cout輸出一條信息??

????????????????????boost::mutex::scoped_lock?lock(io_mu);??

????????????????????cout?<<?"full?waiting..."?<<?endl;??

????????????????}??

????????????????cond_put.wait(mu);?//?條件變量等待??

????????????}?//?條件變臉滿足，停止等待??

????????????stk.push(x);?//?壓棧，寫入數據??

????????????++un_read;??

????????}?//?解鎖互斥量，條件變量的通知不需要互斥量鎖定??

????????cond_get.notify_one();?//?通知可以讀取數據??

????}??

??

????void?get(int?*x)?//?讀取數據??

????{??

????????{?//?局部域開始??

????????????boost::mutex::scoped_lock?lock(mu);?//?鎖定互斥量??

????????????while?(is_empty())?//?檢查緩沖區是否空??

????????????{??

????????????????{??

????????????????????boost::mutex::scoped_lock?lock(io_mu);??

????????????????????cout?<<?"empty?waiting..."?<<?endl;??

????????????????}??

????????????????cond_get.wait(mu);?//?條件變量等待??

????????????}??

????????????--un_read;??

????????????*x?=?stk.top();?//?讀取數據??

????????????stk.pop();?//?彈棧??

????????}??

????????cond_put.notify_one();?//?通知可以寫入數據??

????}??

};??

??

buffer?buf(5);?//?一個緩沖區對象??

void?producter(int?n)?//?生產者??

{??

????for?(int?i?=?0;?i?<?n;?++i)??

????{??

????????{??

????????????boost::mutex::scoped_lock?lock(io_mu);??

????????????cout?<<?"put?"?<<?i?<<?endl;??

????????}??

????????buf.put(i);?//?寫入數據??

????}??

}??

??

void?consumer(int?n)?//?消費者??

{??

????int?x;??

????for?(int?i?=?0;?i?<?n;?++i)??

????{??

????????buf.get(&x);?//?讀取數據??

????????boost::mutex::scoped_lock?lock(io_mu);??

????????cout?<<?"get?"?<<?x?<<?endl;??

????}??

}??

??

int?main()??

{??

????boost::thread?t1(producter,?20);?//?一個生產者線程??

????boost::thread?t2(consumer,?10);?//?兩個消費者線程??

????boost::thread?t3(consumer,?10);??

??

????t1.join();??

????t2.join();??

????t3.join();??

??

????return?0;??

}??

運行結果：?
empty waiting...?
put 0?
empty waiting...?
put 1?
put 2?
get 1?
get 2?
get 0?
empty waiting...?
empty waiting...?
put 3?
put 4?
put 5?
put 6?
put 7?
get 6?
get 7?
get 5?
get 4?
get 3?
empty waiting...?
put 8?
empty waiting...?
put 9?
put 10?
put 11?
get 9?
get 11?
get 8?
empty waiting...?
put 12?
put 13?
put 14?
put 15?
put 16?
put 17?
full waiting...?
get 10?
get 16?
put 18?
full waiting...?
get 17?
get 15?
get 14?
get 13?
get 12?
get 18?
empty waiting...?
put 19?
get 19?

轉載于:https://www.cnblogs.com/fire909090/p/6801543.html

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