当使用两个不同的地址调用时，inet_ntoa会给出相同的结果

Question

//
char ip1[] = "127.0.0.1";
char ip2[] = "211.100.21.179";
printf("ip1: %s\nip2: %s\n", ip1, ip2);

// 
long l1 = inet_addr(ip1);
long l2 = inet_addr(ip2);
printf("ip1: %ld\nip2: %ld\n", l1, l2);

//
struct in_addr addr1, addr2;
memcpy(&addr1, &l1, 4);
memcpy(&addr2, &l2, 4);
printf("%u\n", addr1.s_addr);
printf("%u\n", addr2.s_addr);

//
printf("%s\n", inet_ntoa(addr1));
printf("%s\n", inet_ntoa(addr2));

//
printf("%u,%s\n", addr1.s_addr, inet_ntoa(addr1));
printf("%u,%s\n", addr2.s_addr, inet_ntoa(addr2));
printf("%s <--> %s\n", inet_ntoa(addr1), inet_ntoa(addr2));

The output is:

ip1: 127.0.0.1
ip2: 211.100.21.179
ip1: 16777343
ip2: 3004523731
16777343
3004523731
127.0.0.1
211.100.21.179
16777343,127.0.0.1
3004523731,211.100.21.179
211.100.21.179 <--> 211.100.21.179    // why the same??

I know printf parse arg from right to left or vise versa is platform-dependent, but why the output is the same value, please help to explain.

Answer 1

From the Linux man pages : https://linux.die.net/man/3/inet_ntoa

The inet_ntoa() function converts the Internet host address in, given in network byte order, to a string in IPv4 dotted-decimal notation. The string is returned in a statically allocated buffer, which subsequent calls will overwrite.

It seems the two calls the inet_nota() share a buffer, so calling the function twice overwrites whats in the buffer and replaces it with the next call, thus you get the same output

Answer 2

inet_ntoa() uses an internal buffer to convert an address to a string. Every time you call it it rewrites the same buffer with the new address. So both calls to inet_ntoa() are returning the same pointer, and thus printf() prints the same string twice. inet_ntoa() does this so that you don't have to free() a string every time you call it. If you want your output to look the way you expect you can do this:

 printf("%s <--> ", inet_ntoa(addr1))
 printf("%s\n", inet_ntoa(addr2));

That way printf() will have printed the first address before the second call to inet_ntoa() overwrites it.