套接字小疑问

CODE:

#include <stdio.h>          /* stderr, stdout */
#include <netdb.h>          /* hostent struct, gethostbyname() */
#include <arpa/inet.h>      /* inet_ntoa() to format IP address */
#include <netinet/in.h>     /* in_addr structure */

int main(int argc, char **argv) {
struct hostent *host;     /* host information */
struct in_addr h_addr;    /* Internet address */
if (argc != 2) {
  fprintf(stderr, "USAGE: nslookup <inet_address>\n");
  exit(1);
}
if ((host = gethostbyname(argv[1])) == NULL) {
  fprintf(stderr, "(mini) nslookup failed on '%s'\n", argv[1]);
  exit(1);
}
h_addr.s_addr = *((unsigned long *) host->h_addr_list[0]);
fprintf(stdout, "%s\n", inet_ntoa(h_addr));
exit(0);
}