比較两个字符串

我的代码块

#include <string.h>

int my_strcmp(const char* s1,const char * s2)

{

 if((s1==NULL)||(s2==NULL))

  return 0;

 while(1)

 {

  if((*s1=='\0')||(*s2=='\0'))

   break;

  if(*s1>*s2)

   return 1;

  if(*s1<*s2)

   return -1;

  s1++;

  s2++;

 }

 //if(*s1==*s2=='\0')

 // return 0;

 if(*s1=='\0'&&*s2=='\0')

  return 0;

 if(*s1>*s2)

  return 1;

 else

  return -1;

}

int main()

{

 char* a="abcdef";

 char* b="abcdef";

 my_strcmp(a,b);

 return 0;

}

这里，上面的方法。不可靠，没有明白的内在逻辑。

看看源代码

/* Copyright (C) 1991-2015 Free Software Foundation, Inc.

   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or

   modify it under the terms of the GNU Lesser General Public

   License as published by the Free Software Foundation; either

   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,

   but WITHOUT ANY WARRANTY; without even the implied warranty of

   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public

   License along with the GNU C Library; if not, see

   <http://www.gnu.org/licenses/>.  */

#include <string.h>

#undef strcmp

/* Compare S1 and S2, returning less than, equal to or

   greater than zero if S1 is lexicographically less than,

   equal to or greater than S2.  */

int

strcmp (const char *p1, const char *p2)

{

  const unsigned char *s1 = (const unsigned char *) p1;

  const unsigned char *s2 = (const unsigned char *) p2;

  unsigned char c1, c2;

  do

    {

      c1 = (unsigned char) *s1++;

      c2 = (unsigned char) *s2++;

      if (c1 == '\0')

return c1 - c2;

    }

  while (c1 == c2);

  return c1 - c2;

}

要比較s1和s2的每个字节的大小，则，两者比較，最简单的分类就是相等和不相等，在相等的时候继续，不相等时。马上计算返回。

至于

const unsigned char s1 = (const unsigned char ) p1;

const unsigned char s2 = (const unsigned char ) p2;

这样转换的妙处，未能体会