GNU LIBC源碼學習之strcmp

標籤：源碼

比較兩個字串

My Code塊

#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;}

這裡，上面的方法，不可靠，沒有明確的內在邏輯，
看看源碼

/* 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.  */intstrcmp (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的每一個位元組的大小，則，兩者比較，最簡單的分類就是相等和不相等，在相等的時候繼續，不相等時，立即計算返回。

至於
const unsigned char s1 = (const unsigned char ) p1;
const unsigned char s2 = (const unsigned char ) p2;
這樣轉換的妙處，未能體會

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GNU LIBC源碼學習之strcmp