如何得知某段代碼的運行時間

    轉自：http://blog.chinaunix.net/uid-27034868-id-3371737.html     

    之前，我的同學問了我一個算法題，由於這個題是要通過提交代碼然後在線測試的，有運行時間的限制。我想應該有辦法把某段代碼的運行時間計算出來，當然現在某些IDE（集成開發環境）已經提供了這個功能，但是我猜它只是計算進程開始至結束的時間，如果我們需要更精確，精確到某段代碼的運行時間的話，我們可以在代碼中加入相應的代碼就可以得到這個運行時間了。很多時候可以用於比較兩（幾）個算法的效率。

 

   下面給出兩個比較方便的方法在C語言中的使用（C++的更多本文不討論）

點擊(此處)摺疊或打開

// count_time.c

// 求2000000內素數和，並計算所用時間(精確)
#include <stdio.h>
#include <math.h>
#include <sys/time.h> // struct
timeval 的定義
#include <time.h> // clock的定義

void prime_number()
{
    unsigned long m, k, i;//, n = 0;
    unsigned long sum = 0;

    printf("2000000內素數和爲：\n");
    for (m = 3; m < 2000000;m += 2) {
        k = (unsigned long)sqrt(m);
        for (i = 2; i <= k; i++) {
            if (m % i == 0)
                break;
        }
        if (i >= k + 1)
            sum += m;
    }
    printf("sum= %lld\n",sum+2);
}

// 計時方法一（ 在Unix/Linux下使用）
void count_runtime1()
{
    struct timeval start, end;

    gettimeofday( &start, NULL );//第二個參數不爲空則用於返回時區結果，這裏不需要

    //* 在start與end之間運行需要計時的代碼
    prime_number();

    gettimeofday( &end, NULL );

    unsigned int timeuse = 1000000 * ( end.tv_sec - start.tv_sec ) + \
                end.tv_usec -start.tv_usec;

    printf("timeuse: %d us(微妙)\n", timeuse);
}

// 計時方法2
void count_runtime2()
{

    /* 使用下面的clock獲取，更接近cpu運行時間 ,標準C更通用*/
    clock_t clock_start = clock();

    prime_number();// 需要計時的代碼

    clock_t clock_end = clock();

    printf("timesue: %ld us\n", clock_end - clock_start);
}

int main()
{
    count_runtime1();
    count_runtime2();

    return 0;
}
/******************************************************************************/
/* man 手冊中對gettimeofday函數的說明：
NAME
       gettimeofday, settimeofday - get / set time

SYNOPSIS
       #include <sys/time.h>

       int gettimeofday(struct timeval *tv, struct
timezone *tz);
       int settimeofday(const struct timeval *tv, const struct
timezone *tz);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       settimeofday(): _BSD_SOURCE

DESCRIPTION
       The functions gettimeofday() and settimeofday() can get and set     
the time as well as a timezone. The tv argument is a struct          meval (as
specified in <sys/time.h>):

           struct timeval {
               time_t tv_sec; // seconds
               suseconds_t tv_usec; // microseconds
           };
  …………
  RETURN VALUE
       gettimeofday() and settimeofday() return
0 for success, or -1 for fail?br
/>
       ure (in which case errno is set appropriately).

***************************************************************************************/

/* man 手冊中對clock函數的說明：
NAME
       clock - Determine processor time

SYNOPSIS
       #include <time.h>

       clock_t clock(void);

DESCRIPTION
       The clock() function returns an approximation of processor time     
used by the program.

RETURN VALUE
       The value returned is the CPU time used so far as a clock_t; to get     
the number of seconds used, divide by CLOCKS_PER_SEC. If the      processor time used is not available or its
value cannot be      represented, the
       function returns the value (clock_t) -1.

CONFORMING TO
       C89, C99, POSIX.1-2001. POSIX
requires that CLOCKS_PER_SEC equals
       1000000 independent of the actual resolution.
       int /usr/include/time.h
#define CLOCKS_PER_SEC 1000000l

NOTES
       The C standard allows for arbitrary values at the start of the program;subtract the value returned from a call to clock() at
the      start of the program to get maximum portability.
……
******************************************************************************/