【转】c标准库源代码之strncmp

/* Compare no more than N characters of 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 

STRNCMP (const char *s1, const char *s2, size_t n) 

{ 

  unsigned reg_char c1 = '/0'; 

  unsigned reg_char c2 = '/0'; 

   

  if (n >= 4) 

    { 

      size_t n4 = n >> 2; 

      do 

    { 

      c1 = (unsigned char) *s1++; 

      c2 = (unsigned char) *s2++; 

      if (c1 == '/0' || c1 != c2) 

        return c1 - c2; 

      c1 = (unsigned char) *s1++; 

      c2 = (unsigned char) *s2++; 

      if (c1 == '/0' || c1 != c2) 

        return c1 - c2; 

      c1 = (unsigned char) *s1++; 

      c2 = (unsigned char) *s2++; 

      if (c1 == '/0' || c1 != c2) 

        return c1 - c2; 

      c1 = (unsigned char) *s1++; 

      c2 = (unsigned char) *s2++; 

      if (c1 == '/0' || c1 != c2) 

        return c1 - c2; 

    } while (--n4 > 0); 

      n &= 3; 

    } 

   

  while (n > 0) 

    { 

      c1 = (unsigned char) *s1++; 

      c2 = (unsigned char) *s2++; 

      if (c1 == '/0' || c1 != c2) 

    return c1 - c2; 

      n--; 

    } 

   

  return c1 - c2; 

}

源代码这么高大上，上学时写的练习题代码爆弱了。