点分十进制与网络字节序之间的相互转换

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 #include <sys/types.h>
 #include <netinet/in.h>
 #include <ctype.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <arpa/inet.h>

 /*
  * Ascii internet address interpretation routine.
  * The value returned is in network order.
  */
 unsigned int
 __inet_aton(const char *cp)
 {
     unsigned int val = 0;
     unsigned int tmp = 0;
     char c;
     int base = 10;
     int shift = 32;

     c = *cp;
     for (;;) {//可以给出一个任意长度的字符串；
         tmp = 0;
         for (;;) {//获得'.'分割的每一个字符串的数值；
             if (isdigit(c)) {
                 tmp = (tmp * base) + (c - '0');
                 c = *++cp;
             } else break;
         }

         shift -= 8;
         tmp <<= shift;
         val += tmp;

         if (c == '.') {
             c = *++cp;
         } else
             break;
     }

     /*
      * Check for trailing characters.
      */
     if (c != '\0' && (!isspace(c)))
         goto ret_0;

     return (htonl (val));//返回网络字节序
 ret_0:
     return (0);
 }

 /* The interface of this function is completely stupid, it requires a
    static buffer.  We relax this a bit in that we allow one buffer for
    each thread.  */
 static char buffer[18];//作为库函数，正式这个函数不能重入的原因；
 char *
 __inet_ntoa (unsigned int in)
 {
   unsigned char *bytes = (unsigned char *) &in;
   snprintf (buffer, sizeof (buffer), "%d.%d.%d.%d",
           bytes[0], bytes[1], bytes[2], bytes[3]);//网络字节序是小端表示，第一个是byte[0]；

   return buffer;
 }

 int main()
 {
     printf("ip :%lu\n",__inet_aton("192.168.1.105 "));
     printf("str:%s\n",__inet_ntoa(__inet_aton("192.168.1.105")));
     return 0;
 }