strange error when using printf and inet_ntoa to print ip address

Question

struct in_addr a,b,c,d;
if(inet_aton ("10.0.0.1", &a)!=-1 );
    printf("a:%s\n", inet_ntoa(a));
if(inet_aton ("10.0.0.2", &b)!=-1 )
    printf("b:%s\n", inet_ntoa(b));

It's no problem to print the to IP address:

a:10.0.0.1
b:10.0.0.2

However I found that:

inet_aton ("10.0.0.3", &c);
inet_aton ("10.0.0.4", &d);
printf("c:%s %s\n", inet_ntoa(c), inet_ntoa(d));
printf("d:%s\n", inet_ntoa(d));

It prints:

c:10.0.0.3 d:10.0.0.3
d:10.0.0.4

The strange things is that it prints the wrong IP of d at this line:

printf("c:%s %s\n", inet_ntoa(c), inet_ntoa(d));

I don't know why!

Answer 1

The manual for inet_ntoa says:

The string is returned in a statically allocated buffer, which subsequent calls will overwrite.

You have two functions in printf("c:%s %s\n", inet_ntoa(c), inet_ntoa(d)); that overwrite the same buffer.

Try if (inet_ntoa(c) == inet_ntoa(d)), you may be surprised by the result.