gethostbyname() function returns empty buffer - c

Question

I am new to internet programming, and I am trying to use the gethostbyname() function. When I input a string such as "www.yahoo.com" to gethostbyname function it works fine, but when I input a char array, it will always return an empty buffer.
char hostname[100];
struct hostent* h;
gethostname(hostname, sizeof hostname );
printf("Hostname: %s\n", hostname);
h = gethostbyname(hostname);
Any idea how to solve this?

Answer 1

Your server can't resolve itself. The most common way of "fixing" this is to put its own name into its hostfile. While this is a good idea for various reasons, the underlying problem really should be fixed.
The DNS search list should normally be set to the domainname that contains the hostname -or- the hostname should be fully qualified itself.
DNS should be correctly set up for the host.
This makes it not really a C problem at all, but a server configuration problem. Off it goes then.

Answer 2

WSADATA wsaData;
int error;
if ((error = WSAStartup(MAKEWORD(1, 1), &wsaData)) !=0)
{
printf("Error %d in WSAStartup, result will fail\n",error);
}
char hostname[100];
struct hostent* h;
gethostname(hostname, sizeof hostname );
printf("Hostname: %s\n", hostname);
h = gethostbyname(hostname);

Answer 3

In the linux programmers manual the function has the following declaration:
struct hostent *gethostbyname(const char *name);
This means the parameter must be a char array (or string in layman terms). A quoted string such as "yahoo.com" can be used directly when calling the function.
The following code is a working example on how gethostbyname works:
#include <stdio.h>
#include <string.h>
#include <netdb.h>
int main(){
struct hostent* h=gethostbyname("yahoo.com");
printf("Hostname: %s\n", h->h_name);
printf("Address type #: %d\n", h->h_addrtype);
printf("Address length: %d\n", h->h_length);
char text[50]; // allocate 50 bytes (a.k.a. char array)
strcpy(text,"bing.ca"); //copy string "bing.ca" to first 7 bytes of the array
h=gethostbyname(text); //plug in the value into the function. text="bing.ca"
printf("Hostname: %s\n", h->h_name);
printf("Address type #: %d\n", h->h_addrtype);
printf("Address length: %d\n", h->h_length);
return 0;
}
I called it twice. Once for yahoo.com and once for bing.ca and I retrieved the hostname, the address type number and the address length (which is the number of bytes required to store the IP).
For calling the bing address, I allocated a char array, filled it with a string then passed that char array as a parameter to the function.

Answer 4

One good reason why it's returning NULL is because the hostname what you are passing is not correct.
Sometimes even doing hostname -v will not give the correct host name.
Try following:
cat /etc/hosts
this will show you output as:
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
::1 localhost localhost.localdomain localhost6 localhost6.localdomain
That 'localhost' next to 127.0.0.1 in the output above is your host name. This will work perfectly with gethostbyname.