使用C++中的线程报告计算进度

我正在编写一个通用的抽象类,以便能够根据需要报告任意数量的实例变量的状态.例如,考虑以下无用循环:

int a, b;
for (int i=0; i < 10000; ++i) {
    for (int j=0; j < 1000; ++j) {
        for (int k =0; k < 1000; ++k) {
            a = i;
            b = j;
        }
    }
}

能够看到a和b不必修改循环的值是很好的.在过去,我写过if语句,如下所示:

int a, b;
for (int i=0; i < 10000; ++i) {
    for (int j=0; j < 1000; ++j) {
        for (int k =0; k < 1000; ++k) {
            a = i;
            b = j;
            if (a % 100 == 0) {
                printf("a = %d\n", a);
            }
        }
    }
}

这将允许我看到a每100次迭代的值.但是,根据正在进行的计算,有时无法以这种方式检查进度.我们的想法是能够离开计算机,在给定时间后回来并检查您想要看到的任何值.

为此我们可以使用pthreads.以下代码有效,我发布它的唯一原因是因为我不确定我是否正确使用该线程,主要是如何关闭它.

首先让我们考虑文件"reporter.h":

#include 
#include 
#include 

void* run_reporter(void*);

class reporter {
public: 
    pthread_t thread;
    bool stdstream;
    FILE* fp;

    struct timespec sleepTime;
    struct timespec remainingSleepTime;

    const char* filename;
    const int sleepT;
    double totalTime;

    reporter(int st, FILE* fp_): fp(fp_), filename(NULL), stdstream(true), sleepT(st) {
        begin_report();
    }
    reporter(int st, const char* fn): fp(NULL), filename(fn), stdstream(false), sleepT(st) {
        begin_report();
    }
    void begin_report() {
        totalTime = 0;
        if (!stdstream) fp = fopen(filename, "w");
        fprintf(fp, "reporting every %d seconds ...\n", sleepT);
        if (!stdstream) fclose(fp);
        pthread_create(&thread, NULL, run_reporter, this);
    }
    void sleep() {
        sleepTime.tv_sec=sleepT;
        sleepTime.tv_nsec=0;
        nanosleep(&sleepTime, &remainingSleepTime);
        totalTime += sleepT;
    }
    virtual void report() = 0;
    void end_report() {
        pthread_cancel(thread);
        // Wrong addition of remaining time, needs to be fixed
        // but non-important at the moment.
        //totalTime += sleepT - remainingSleepTime.tv_sec;
        long sec = remainingSleepTime.tv_sec;
        if (!stdstream) fp = fopen(filename, "a");
        fprintf(fp, "reported for %g seconds.\n", totalTime);
        if (!stdstream) fclose(fp);
    }
};

void* run_reporter(void* rep_){
    reporter* rep = (reporter*)rep_;
    while(1) {
        if (!rep->stdstream) rep->fp = fopen(rep->filename, "a");
        rep->report();
        if (!rep->stdstream) fclose(rep->fp);
        rep->sleep();
    }
}

该文件声明了抽象类reporter,注意纯虚函数report.这是将打印消息的功能.每个记者都有自己的,thread并在reporter调用构造函数时创建线程.要reporter在我们无用的循环中使用该对象,我们现在可以:

#include "reporter.h"
int main() {
    // Declaration of objects we want to track
    int a = 0;
    int b = 0;
    // Declaration of reporter
    class prog_reporter: public reporter {
    public:
        int& a;
        int& b;
        prog_reporter(int& a_, int& b_):
            a(a_), b(b_),
            reporter(3, stdout)
        {}
        void report() {
            fprintf(fp, "(a, b) = (%d, %d)\n", this->a, this->b);
        }
    };
    // Start tracking a and b every 3 seconds
    prog_reporter rep(a, b);

    // Do some useless computation
    for (int i=0; i < 10000; ++i) {
        for (int j=0; j < 1000; ++j) {
            for (int k =0; k < 1000; ++k) {
                a = i;
                b = j;
            }
        }
    }
    // Stop reporting
    rep.end_report();
}

在编译此代码(没有优化标志)并运行它后,我获得:

macbook-pro:Desktop jmlopez$ g++ testing.cpp
macbook-pro:Desktop jmlopez$ ./a.out 
reporting every 3 seconds ...
(a, b) = (0, 60)
(a, b) = (1497, 713)
(a, b) = (2996, 309)
(a, b) = (4497, 478)
(a, b) = (5996, 703)
(a, b) = (7420, 978)
(a, b) = (8915, 78)
reported for 18 seconds.

这正是我想要它做的事情,然后我得到优化标志:

macbook-pro:Desktop jmlopez$ g++ testing.cpp -O3
macbook-pro:Desktop jmlopez$ ./a.out 
reporting every 3 seconds ...
(a, b) = (0, 0)
reported for 0 seconds.

这并不奇怪,因为编译器可能会重写我的代码,以便在更短的时间内给出相同的答案.我原来的问题是,如果我让循环更长,记者没有给我变量的值,例如:

for (int i=0; i < 1000000; ++i) {
    for (int j=0; j < 100000; ++j) {
        for (int k =0; k < 100000; ++k) {
            a = i;
            b = j;
        }
    }
}

使用优化标志再次运行代码后:

macbook-pro:Desktop jmlopez$ g++ testing.cpp -O3
macbook-pro:Desktop jmlopez$ ./a.out 
reporting every 3 seconds ...
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
(a, b) = (0, 0)
reported for 39 seconds.

问题:这个输出是由于优化标志修改了代码,它只是决定不更新变量直到最后？

主要问题:

在reporter方法中end_report我调用函数pthread_cancel.在阅读了以下答案后,它让我怀疑该功能的使用以及我如何终止报告线程.对于那些经验丰富的人来说pthreads,有没有明显的漏洞或使用thread我所做的潜在问题？