How to Use the getaddrinfo Function in C

1. Use the getaddrinfo Function to Host Name to IP Address in C
2. Use the getnameinfo Function to IP Address to Host Name in C

This article will demonstrate multiple methods about how to use the getaddrinfo function in C.

Use the getaddrinfo Function to Host Name to IP Address in C

getaddrinfo is part of the UNIX networking programming facilities, and it can convert network host information to the IP address and conversely. getaddrinfo is also a POSIX compliant function call, and it can do conversion regardless of the underlying protocols.

getaddrinfo takes four arguments,

1. The first can be the pointer to either the hostname or IPv4/IPv6 formatted address string.
2. The second argument can be a service name or a port number represented with the decimal integer.
3. The next two arguments are pointers to addrinfo structures. The first one is hints that specifies requirements to filter the retrieved socket structures, while the second one is the pointer, where the function will dynamically allocate a linked list of addrinfo structs.

Notice that, hints structure should be set with zeros and then should be its members assigned. ai_family member indicates the address family, e.g., IPv4 or IPv6 as AF_INET and AF_INET6, respectively. In this case, we are not interested in service name conversion and specify NULL as the second argument to the function. Finally, we call getnameinfo to translate the given sockaddr structures to the printable form.

#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>

int main(int argc, char const *argv[]) {
  struct addrinfo hints;
  struct addrinfo *res, *tmp;
  char host[256];

  memset(&hints, 0, sizeof(struct addrinfo));
  hints.ai_family = AF_INET;

  if (argc != 2) {
    fprintf(stderr, "%s string\n", argv[0]);
    exit(EXIT_FAILURE);
  }

  int ret = getaddrinfo(argv[1], NULL, &hints, &res);
  if (ret != 0) {
    fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(ret));
    exit(EXIT_FAILURE);
  }

  for (tmp = res; tmp != NULL; tmp = tmp->ai_next) {
    getnameinfo(tmp->ai_addr, tmp->ai_addrlen, host, sizeof(host), NULL, 0,
                NI_NUMERICHOST);
    puts(host);
  }

  freeaddrinfo(res);
  exit(EXIT_SUCCESS);
}

Sample Command:

./program localhost

Output:

127.0.0.1
127.0.0.1
127.0.0.1

Use the getnameinfo Function to IP Address to Host Name in C

The getnameinfo function is used in conjunction with getaddrinfo in this case, and it retrieves the hostnames for corresponding IP addresses. Notice that we process the user input from the first command-line argument and pass it as the getaddrinfo argument to retrieve the socketaddr structs. Finally, each structure can be translated to the hostname string. Since the getaddrinfo allocates dynamic memory to store the linked list in the fourth argument, this pointer should be freed by the user with the freeaddrinfo function call.

#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>

int main(int argc, char const *argv[]) {
  struct addrinfo hints;
  struct addrinfo *res, *tmp;
  char host[256];

  memset(&hints, 0, sizeof(struct addrinfo));
  hints.ai_family = AF_INET;

  if (argc != 2) {
    fprintf(stderr, "%s string\n", argv[0]);
    exit(EXIT_FAILURE);
  }

  int ret = getaddrinfo(argv[1], NULL, &hints, &res);
  if (ret != 0) {
    fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(ret));
    exit(EXIT_FAILURE);
  }

  for (tmp = res; tmp != NULL; tmp = tmp->ai_next) {
    getnameinfo(tmp->ai_addr, tmp->ai_addrlen, host, sizeof(host), NULL, 0, 0);
  }

  freeaddrinfo(res);
  exit(EXIT_SUCCESS);
}

Sample Command:

./program 127.0.0.1

Output:

localhost
localhost
localhost