getnameinfo()

Hi,
as I understand, getnameinfo() is used for reverse lookup i.e. to get hostname from the IP address. So, what's wrong with the following code

Code:

#include <arpa/inet.h>
#include <netdb.h>

int main()
{
  struct sockaddr_in sa;
  inet_pton(AF_INET, "127.0.0.1", &sa.sin_addr);

  char node[NI_MAXHOST];
  int res = getnameinfo((struct sockaddr*)&sa, sizeof(sa), node, sizeof(node), NULL, 0, 0);
  if (res)
 {
    printf("error: %d\n", res);
    printf("%s\n", gai_strerror(res));
  }

  return 0;
}

it exits with the error ai_family not supported?

If I use getnameinfo() with the address obtained with getaddrinfo() for example with

Code:

int main()
{
  struct addrinfo* ailist;
  struct addrinfo* aip;
  struct addrinfo hint;
  struct sockaddr_in* sinp;
  const char* addr;
  char abuf[INET_ADDRSTRLEN];
	
  hint.ai_flags = AI_CANONNAME;
  hint.ai_family = 0;
  hint.ai_socktype = 0;
  hint.ai_protocol = 0;
  hint.ai_addrlen = 0;
  hint.ai_canonname = NULL;
  hint.ai_addr = NULL;
  hint.ai_next = NULL;
  const char* host = "localhost";
  const char* serv = "ftp";

  if (getaddrinfo(host, NULL, &hint, &ailist))
  {
    printf("error: getaddrinfo()\n");
    exit(1);
  }

  for (aip = ailist; aip != NULL; aip = aip->ai_next)
  {
    char buf[512];
    if (getnameinfo(aip->ai_addr, sizeof(struct sockaddr), buf, 512, NULL, 0, 0) == 0)
      printf("%s\n", buf);
  }
}

then resolving works fine.

Are you sure you've correctly initialized the sockaddr_in struct? You should probably set the sin_family field to AF_INET.

Thanks, that was the problem. Here's the code which works:

Code:

#include <arpa/inet.h>
#include <netdb.h>

int main()
{
  struct sockaddr_in sa;
  sa.sin_family = AF_INET;
  inet_pton(AF_INET, "127.0.0.1", &sa.sin_addr);

  char node[NI_MAXHOST];
  int res = getnameinfo((struct sockaddr*)&sa, sizeof(sa), node, sizeof(node), NULL, 0, 0);
  if (res)
  {
    printf("&#37;s\n", gai_strerror(res));
    exit(1);
  }
  printf("%s\n", node);

  return 0;
}