c++ - 使用 boost 线程库打印

Question

所以我开始写一个程序来解析然后处理大量的文本。我已经设置了一个类，其中包含作为 boost 线程运行的方法。到目前为止，这些线程中的每一个都只是打印一些文本语句然后返回。代码编译并运行没有任何错误。但是，打印出的文本不一致。这是意料之中的，因为线程是并行运行的，所以我尝试使用互斥锁来协调输出的使用。但是，我显然做错了什么，因为输出仍然不一致。除此之外，一些输出被打印了两次，我无法将其解释为未能正确编码互斥锁。下面是我的代码:

/* 
 * File:   ThreadParser.h
 * Author: Aaron Springut
 *
 * Created on Feburary 2, 2012, 5:13 PM
 */

#ifndef THREADPARSER_H
#define THREADPARSER_H

#include <string.h>
#include <iostream>
#include <boost/thread/thread.hpp>
#include <boost/thread/mutex.hpp>
#include <boost/bind.hpp>
#include "FileWordCounter.h"

class ThreadParser {

    public:

    ThreadParser();
    ThreadParser(std::string fileName);



    private:

    //mutex for cout
    boost::mutex coutMut;  

    std::string dataFile;
    FileWordCounter fCounter;

    //threads
    void parseFile();
    void processSearches();



};

#endif     


/* 
 * File:   ThreadParser.cpp
 * Author: Aaron Springut
 *
 * Created on Feburary 2, 2012, 5:13 PM
 */


 #include "ThreadParser.h"

 using namespace std;


 ThreadParser::ThreadParser(){

    double buyNum = 0;
    buyNum = buyNum * 100;
    cout << "Percentage of people buying: "<< buyNum <<"%"<<endl;

 }

 ThreadParser::ThreadParser(string fileName){

    dataFile = fileName;
    double buyNum = 0;

    //create the mutex and aquire a lock on it for this thread

    boost::mutex::scoped_lock(coutMut);

    boost::thread parseThread(boost::bind(&ThreadParser::parseFile, this));
    boost::thread processSearches(boost::bind(&ThreadParser::processSearches,this));

    buyNum = buyNum * 100;
    cout << "Percentage of people buying: "<< buyNum <<"%"<<endl;


 }


 void ThreadParser::parseFile(){

    boost::mutex::scoped_lock(coutMut);
    cout << "parseFileThreadLaunch"<<endl;
    return;

 }


 void ThreadParser::processSearches(){

    boost::mutex::scoped_lock(coutMut);
    cout << "processSearchesLaunch"<<endl;
    return;

 }

作为这里出错的示例，运行此程序的两个输出:

Percentage of people buying: parseFileThreadLaunch
processSearchesLaunch
0%

好吧，cout 不是线程安全的，我在互斥体上做错了。

Percentage of people buying: parseFileThreadLaunch
0%
processSearchesLaunch
processSearchesLaunch

这就迷惑了，最后一行怎么打印了两次？这是 cout 不是线程安全的结果吗？或者，我是否遗漏了全局的一部分。

编辑:该类在主函数中的调用方式如下:

string fileName("AOLData.txt");
cout << fileName<< endl;

ThreadParser tp(fileName);

Answer 1

boost::mutex::scoped_lock(coutMut);

这并不像您认为的那样。这会创建一个临时对象，它会立即被销毁，从而释放锁。就像 int(3); 一样。

你想要:

boost::mutex::scoped_lock sl(moutMut);

这将创建一个对象 sl，它持有锁直到它超出范围。