GNU LIBC源代码学习之strcmp

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