字符串操作函数 strstr 实现

 /*
  The  strstr()  function finds the first occurrence of the substring needle 
  in the string haystack.  The terminating  '\0' characters are not compared.
 */
char *strstr_r(char *s1, char *s2)
{
   /*  s1 : hellwoleddlfjwo, s2: wol */
   const char *p1, *p2;
   if((*s2) == '\0') /* Early versions of Linux libc,strstr  would not allow an empty needle argument */
       return s1;


   while( (s1 = strchr(s1, *s2)) != NULL) {
       p1 = s1;     /* found first character of s2, see if the rest matches */
       p2 = s2;
       while((*p1++ == *p2++)) {
          if(*p1 == '\0')
              return s1; /* both strings ended together */
       }
       if(*p2 == '\0') {
          break;
       }
       s1++;
   }


    return s1;
}