IP地址点分十进制与网络字节序整数的转化

博客http://blog.csdn.net/unimen/article/details/6844456给出相关实现，我根据C库的代码，也给出一个：

#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;	
}