Programming :: Script To Find Local IP - Output Unknown Host

I want to find out my IP and I test this code from beej tutorial:

Code:
char hostname[128];
int i;
struct hostent *he;
struct in_addr **addr_list;
struct in_addr addr;

gethostname(hostname, sizeof hostname);
printf("My hostname: %s
", hostname);
he = gethostbyname(hostname);
if (he == NULL) { // do some error checking
herror("gethostbyname"); // herror(), NOT perror()
return 1;
} // print information about this host:
printf("Official name is: %s
", he->h_name);
printf("IP address: %s
", inet_ntoa(*(struct in_addr*)he->h_addr));
printf("All addresses: ");
addr_list = (struct in_addr **)he->h_addr_list;

for(i = 0; addr_list[i] != NULL; i++) {
printf("%s ", inet_ntoa(*addr_list[i]));
}
printf("
");
return 0;

But my output is this:
Code:
My hostname: vBx
gethostbyname: Unknown host

I heard that it works in windows, why isn't working in linux?