程式運行時間測試 - 使用libc 中 clock 函數

我們運行程式的時候，可以簡單使用clock函數測試程式的運行時間：（本示例中以微秒為單位輸出）

https://github.com/yaowenxu/Workplace/blob/master/timer/clocktimer.c

/**
 * Author: Yaowen Xu
 * Github: https://github.com/yaowenxu
 * Organization: 北航系統結構研究所
 * Date: 2019-08-18 11:59:54
 * LastEditTime: 2019-08-18 12:45:45
 * Description: 使用 C 語言標準庫函數clock來進行測試程式運行時間
 */
#include <time.h>
#include <stdio.h>
#include <math.h>

int str2int(char* str){
    char *p = str;
    int sum = 0;
    while (*p != '\0')
    {
        sum = sum*10 + (*p-'0');
        p++;
    }
    return sum;
}

int main(int argc, char* argv[]){
    clock_t start, stop;
    int def = 1000;
    if (argc == 2)
    {
        def = str2int(argv[argc-1]);
    }
    start = clock();
    for (int i = 0; i < def ; i++)
    {
        float tmp = sqrt(i);
    }
    stop =  clock();
    double total =  stop - start; // 使用運行的時間
    printf("Clocks: %.1f\n", total); // 總共使用的時鐘
    printf("Time: %.1f us\n", total*1e6/(CLOCKS_PER_SEC)); // 轉換運行時間為微秒
    return 0;
}

/*
 * Approximates the processor time used by the program, 
 * since the beginning of an implementation-defined time 
 * period that is related to the program invocation. 
 * To measure the time spent in a program, call the clock() 
 * function at the start of the program, and subtract its 
 * returned value from the value returned by subsequent calls 
 * to clock(). Then, to obtain the time in seconds, divide 
 * the value returned by clock() by CLOCKS_PER_SEC.
 * 
 * If you use the system() function in your program, do not 
 * rely on clock() for program timing, because calls to system() 
 * may reset the clock.
 * 
 * In a multithread POSIX C application, if you are creating threads 
 * with a function that is based on a POSIX.4a draft standard, 
 * the clock() function is thread-scoped.
 * 
 * Refer：https://www.ibm.com/support/knowledgecenter/en/SSLTBW_2.3.0/com.ibm.zos.v2r3.bpxbd00/clock.htm
*/

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