如何得知某段代码的运行时间

   之前，我的同学问了我一个算法题，由于这个题是要通过提交代码然后在线测试的，有运行时间的限制。我想应该有办法把某段代码的运行时间计算出来，当然现在某些IDE（集成开发环境）已经提供了这个功能，但是我猜它只是计算进程开始至结束的时间，如果我们需要更精确，精确到某段代码的运行时间的话，我们可以在代码中加入相应的代码就可以得到这个运行时间了。很多时候可以用于比较两（几）个算法的效率。

 

   下面给出两个比较方便的方法在C语言中的使用（C++的更多本文不讨论）

// count_time.c

// 求2000000内素数和，并计算所用时间(精确)
#include <stdio.h>
#include <math.h>
#include <sys/time.h> // struct timeval 的定义
#include <time.h> // clock的定义

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

    printf("2000000内素数和为：\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);
}

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

    gettimeofday( &start, NULL );//第二个参数不为空则用于返回时区结果，这里不需要

    //* 在start与end之间运行需要计时的代码
    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);
}

// 计时方法2
void count_runtime2()
{

    /* 使用下面的clock获取，更接近cpu运行时间 ,标准C更通用*/
    clock_t clock_start = clock();

    prime_number();// 需要计时的代码

    clock_t clock_end = clock();

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

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

    return 0;
}
/******************************************************************************/
/* man 手册中对gettimeofday函数的说明：
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 手册中对clock函数的说明：
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.
……
******************************************************************************/