Why does this hostent cause a segfault?

Question

struct hostent *hostName;

struct in_addr ipv4addr;

inet_pton(AF_INET, inet_ntoa(client.sin_addr), &ipv4addr);

hostName = gethostbyaddr(&ipv4addr, sizeof(ipv4addr), AF_INET);

printf("Host name: %s\n", hostName->h_name);

It segfaults on the last line. I looked up proper use of hostent, and the msdn docs show it being used EXACTLY like this. What would cause the segfault?

Maybe gethostbyaddre resturns NULL and dereferencing NULL causes a segfault; or hostName->h_name is NULL and then printf()'ing NULL segfaults. — , Apr 25 '12 at 20:43

score 1 · Accepted Answer · answered Apr 25 '12 at 20:43

The gethostbyaddr() function returns NULL in the case of an error, and I don't see you checking for that in your code. Attempting to dereference a NULL pointer would cause a segfault.

You need something like:

if (hostName == NULL) {
  printf("There was an error!\n");
  exit(1);
}

You may be able to use the herror() function to print out the actual error encountered by the resolver (although the man page indicates that herror() is obsolete).

Why does this hostent cause a segfault?

1 Answers1