#include<assert.h>

int my_strcmp(const char* str1, const char* str2)
{
	assert(str1);//断言.
	assert(str2);
	while ((*str1 - *str2 == 0) && *str1 && *str2)
 //只有当str1与str2中对应字符相等,str1与str2都未遇到\0才可以进入循环.
//毕竟，遇到\0就标志着函数功能结束。
	{
		str1++;//移动位置，换下一个字符进行比较。
		str2++;
	}
	return (*str1 - *str2);
}

例图

?库中的实现（vs版）

?这个不禁让我想起了那个c语言大赛，比谁的代码********

strncmp

用法简介

int strncmp(const char* str1,const char* str2，size_t num)

它与strcmp唯一的区别就是你可以用num来限制进行比较的两个字符串的字符个数，即你来规定比较前几个就比较前几个。

小注意

当你所规定的字符个数超过了字符串的长度，并且这两个字符串还完全一样，那它会不会越界访问啊？don‘t worry, \0会在那里静静的看着你的。

?图例

?简单实现

#include<assert.h>

int my_strncmp(const char* str1,const char* str2,size_t num)
{
	assert(str1);
	assert(str2);
	if (num == 0)
	{
		return 0;
	}
	while ((*str1 - *str2 == 0) && *str1 && *str2)
	{
		if (--num)
		{
			str1++;
			str2++;
		}
		else
		{
			break;
		}
	}
	return (*str1 - *str2);
	
}

库中的实现（vs版）

这个就直接上传源码了

/***
*strncmp.c - compare first n characters of two strings
*
*       Copyright (c) Microsoft Corporation. All rights reserved.
*
*Purpose:
*       defines strncmp() - compare first n characters of two strings
*       for ordinal order.
*
*******************************************************************************/

#include <string.h>

#ifdef _M_ARM
    #pragma function(strncmp)
#endif

/***
*int strncmp(first, last, count) - compare first count chars of strings
*
*Purpose:
*       Compares two strings for ordinal order.  The comparison stops
*       after: (1) a difference between the strings is found, (2) the end
*       of the strings is reached, or (3) count characters have been
*       compared.
*
*Entry:
*       char *first, *last - strings to compare
*       unsigned count - maximum number of characters to compare
*
*Exit:
*       returns <0 if first < last
*       returns  0 if first == last
*       returns >0 if first > last
*
*Exceptions:
*
*******************************************************************************/

int __cdecl strncmp
(
    const char *first,
    const char *last,
    size_t      count
)
{
    size_t x = 0;

    if (!count)
    {
        return 0;
    }

    /*
     * This explicit guard needed to deal correctly with boundary
     * cases: strings shorter than 4 bytes and strings longer than
     * UINT_MAX-4 bytes .
     */
    if( count >= 4 )
    {
        /* unroll by four */
        for (; x < count-4; x+=4)
        {
            first+=4;
            last +=4;

            if (*(first-4) == 0 || *(first-4) != *(last-4))
            {
                return(*(unsigned char *)(first-4) - *(unsigned char *)(last-4));
            }

            if (*(first-3) == 0 || *(first-3) != *(last-3))
            {
                return(*(unsigned char *)(first-3) - *(unsigned char *)(last-3));
            }

            if (*(first-2) == 0 || *(first-2) != *(last-2))
            {
                return(*(unsigned char *)(first-2) - *(unsigned char *)(last-2));
            }

            if (*(first-1) == 0 || *(first-1) != *(last-1))
            {
                return(*(unsigned char *)(first-1) - *(unsigned char *)(last-1));
            }
        }
    }

    /* residual loop */
    for (; x < count; x++)
    {
        if (*first == 0 || *first != *last)
        {
            return(*(unsigned char *)first - *(unsigned char *)last);
        }
        first+=1;
        last+=1;
    }

    return 0;
}

讲真的，他这个大于4的时候，四个四个的比较，的确令我迷茫，希望哪位大佬可以指点一下。