IP地址与整型数之间的转换

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在Linux中关于IP和无符号整数之间的转换有两个比较好的函数:
int inet_aton(const char *cp, struct in_addr *inp);
char *inet_ntoa(struct in_addr in);
通过函数名就可以知道函数作用了.这里需要注意的是这两个函数操作的结果,或输入的整数都是网络字节序(和大端是一样的),与我们平常的intel机器上的整型数字节序是不一样的.
不过下面的代码中都没有转化成网络字节序.
下面的代码是不同的源代码中不同的实现.

// ip地址结构体  struct in_addr  {      unsigned int s_addr; // network byte order( big-endian)  };  // inet_aton将 点分十进制串cp 转换为一个网络字节顺序的32位整数 IP地址  // 例如将串cp "192.168.33.123 "  // 转为 1100 0000   1010 1000    0010 0001     0111 1011   // 成功返回1，出错返回0  // 转换成功，结果保存在结构体指针ap中  int inet_aton(const char *cp, struct in_addr *ap)  {      int dots = 0;      register u_long acc = 0, addr = 0;      do {              register char cc = *cp;              switch (cc)               {                  case '0':                  case '1':                  case '2':                  case '3':                  case '4':                  case '5':                  case '6':                  case '7':                  case '8':                  case '9':                      acc = acc * 10 + (cc - '0');                      break;                  case '.':                      if (++dots > 3)                      {                          return 0;                      }                      // Fall through                  case '/0':                      if (acc > 255)                       {                          return 0;                      }                      addr = addr << 8 | acc; // 这句是精华,每次将当前值左移八位加上后面的值                      acc = 0;                      break;                  default:                      return 0;              }      } while (*cp++) ;      // Normalize the address       if (dots < 3)      {          addr <<= 8 * (3 - dots) ;      }      //Store it if requested       if (ap)       {          ap->s_addr = htonl(addr);      }      return 1;      }

下面是OpenBSD内核中的代码:

/** * Check whether "cp" is a valid ascii representation * of an Internet address and convert to a binary address. * Returns 1 if the address is valid, 0 if not. * This replaces inet_addr, the return value from which * cannot distinguish between failure and a local broadcast address. */intinet_aton(const char *cp, struct in_addr *addr){    register u_int32_t val;    register int base, n;    register char c;    unsigned int parts[4];    register unsigned int *pp = parts;    c = *cp;    for (;;) {        /**         * Collect number up to ``.''.         * Values are specified as for C:         * 0x=hex, 0=octal, isdigit=decimal.         */        if (!isdigit(c))            return (0);        val = 0; base = 10;        if (c == '0') {            c = *++cp;            if (c == 'x' || c == 'X')                base = 16, c = *++cp;            else                base = 8;        }        for (;;) {            if (isascii(c) && isdigit(c)) {                val = (val * base) + (c - '0');                c = *++cp;            } else if (base == 16 && isascii(c) && isxdigit(c)) {                val = (val << 4) |                    (c + 10 - (islower(c) ? 'a' : 'A'));                c = *++cp;            } else                break;        }        if (c == '.') {            /**             * Internet format:             *  a.b.c.d             *  a.b.c   (with c treated as 16 bits)             *  a.b (with b treated as 24 bits)             */            if (pp >= parts + 3)                return (0);            *pp++ = val;            c = *++cp;        } else            break;    }    /**     * Check for trailing characters.     */    if (c != '\0' && (!isascii(c) || !isspace(c)))        return (0);    /**     * Concoct the address according to     * the number of parts specified.     */    n = pp - parts + 1;    switch (n) {    case 0:        return (0);     /** initial nondigit */    case 1:             /** a -- 32 bits */        break;    case 2:             /** a.b -- 8.24 bits */        if ((val > 0xffffff) || (parts[0] > 0xff))            return (0);        val |= parts[0] << 24;        break;    case 3:             /** a.b.c -- 8.8.16 bits */        if ((val > 0xffff) || (parts[0] > 0xff) || (parts[1] > 0xff))            return (0);        val |= (parts[0] << 24) | (parts[1] << 16);        break;    case 4:             /** a.b.c.d -- 8.8.8.8 bits */        if ((val > 0xff) || (parts[0] > 0xff) || (parts[1] > 0xff) || (parts[2] > 0xff))            return (0);        val |= (parts[0] << 24) | (parts[1] << 16) | (parts[2] << 8);        break;    }    if (addr)        addr->s_addr = htonl(val);    return (1);}

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