Question

我正在尝试执行executionTime指定的确切时间的循环。我正在使用ctime来获得该时间，我需要它在几毫秒的精度范围内（我相信是）。一旦该循环运行了指定的执行时间，它就会中断。

我的问题是执行时间为0.5，正在打印的结果为1。经过进一步测试，看来我的程序将执行时间四舍五入为最接近的整数。因此，对于4.5 executionTime，要打印的执行时间为5.0000000。我的代码如下。我不确定此错误来自何处。

#include <time.h> 
#include <stdlib.h> 
#include <stdio.h> 
#include <unistd.h> 

int main() 
{
    float  executionTime = 4.5; 
    int i;
    float  timeDifference = -1;  
    time_t t1;
    time_t t2;

    t1 = time(0);

    for (i = 0; i < 1000000000; i++) 
    {
        t2 = time(0); 

        timeDifference = difftime(t2, t1); 

        if (timeDifference >= executionTime) 
        { 
            break; 
        }

    }

    printf("time difference is: %f\n", timeDifference); 

    return 0; 

}

尝试使用clock_gettime的新版本。这个版本的问题是循环内出于某种原因达到执行时间时永不中断。

#include <time.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>

#define BILLION 1E9 

int main()
{
    float  executionTime = 4.5;
    int i;
    float  elapsedTime = -1;

    struct timespec start;
    struct timespec stop;

    //Get starting time 
    clock_gettime(CLOCK_REALTIME, &start);

    for (i = 0; i < 1000000000; i++)
    {
        //Get current time
        clock_gettime(CLOCK_REALTIME, &stop);

        elapsedTime = ((stop.tv_sec - start.tv_sec) + (stop.tv_nsec - start.tv_nsec)) / BILLION;

        if (elapsedTime >= executionTime)
        {
            break;
        }
    }

    printf("time difference is: %f\n", elapsedTime);

    return 0;

}

Answer 1

time()返回最接近的秒，而difftime()返回两者的差。因此，基本上该函数计算到整数的差。为了获得更准确的估算，您可以使用clock()

time_t的目的是测量日历时间

int main() 
{
    float  executionTime = 1.3; 
    int i;
    double  timeDifference = 1.0;  
    clock_t t1;
    clock_t t2;

    t1 = clock();

    for (i = 0; i < 1000000000; i++) 
    {
        timeDifference = (double)(clock() - t1) / CLOCKS_PER_SEC;

        if (timeDifference >= executionTime) 
        { 
            break; 
        }

    }

    printf("time difference is: %.9g\n", timeDifference); 

    return 0; 

}

Answer 2

clock_gettime可用于获得更高的准确性。
调用delay并设置一个大于0的值来设置延迟时间，然后使用-1来检查经过的时间。
在main中调用clock_gettime会得到main中已用的时间。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <time.h>

#define BILLION  1E9

int delay ( double seconds)
{
    static double usec = 0.0;
    double elapsed = 0.0;
    static struct timespec start;
    struct timespec stop;

    if ( 0.0 <= seconds) {
        usec = seconds;
        clock_gettime ( CLOCK_REALTIME, &start);
        return 0;
    }
    clock_gettime ( CLOCK_REALTIME, &stop);
    elapsed = difftime ( stop.tv_sec, start.tv_sec);
    elapsed += ( stop.tv_nsec - start.tv_nsec) / BILLION;
    if ( ( elapsed < usec)) {
        return 1;
    }
    return 0;
}

int main() {

    double  executionTime = 4.5;
    int i;
    double  timeDifference = -1;
    struct timespec end;
    struct timespec begin;

    clock_gettime ( CLOCK_REALTIME, &begin);
    delay( executionTime);//call to set time

    for (i = 0; i < 1000000000; i++)
    {
        if ( !delay( -1)) {//call to check elapsed time
            break;
        }
    }
    clock_gettime ( CLOCK_REALTIME, &end);
    timeDifference = difftime ( end.tv_sec, begin.tv_sec);
    timeDifference += ( end.tv_nsec - begin.tv_nsec) / BILLION;

    printf("time difference is: %f\n", timeDifference);

    return 0;
}

ctime无法正确获得循环开始/结束的时差

2 个答案: