struct hostent *gethostbyname(const char *name);

#include <netdb.h>

struct hostent {
  char  *h_name;            /* official name of host */
  char **h_aliases;         /* alias list */
  int    h_addrtype;        /* host address type */
  int    h_length;          /* length of address */
  char **h_addr_list;       /* list of addresses */
}
#define h_addr h_addr_list[0]  /* for backward compatibility */

char *h_name

This is the “official” name of the host. 

char **h_aliases

These are alternative names for the host, represented as a null-terminated vector of strings. 

int h_addrtype

This is the host address type; in practice, its value is always either AF_INET or AF_INET6, with the latter being used for IPv6 hosts. In principle other kinds of addresses could be represented in the database as well as Internet addresses; if this were done, you might find a value in this field other than AF_INET or AF_INET6. See Socket Addresses. 

int h_length

This is the length, in bytes, of each address. 

char **h_addr_list

This is the vector of addresses for the host. (Recall that the host might be connected to multiple networks and have different addresses on each one.) The vector is terminated by a null pointer. 

char *h_addr

This is a synonym for h_addr_list[0]; in other words, it is the first host address.

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define SERVPORT 80

int main(int argc, char **argv)
{
    int i, sockfd;
    struct hostent *host;
    struct sockaddr_in serv_addr;

    if ((host = gethostbyname(argv[1])) == NULL) {
        printf("gethostbyname error\n");
        exit(0);
    }

    printf("official name: %s\n\n", host->h_name);
    printf("address length: %d bytes\n\n", host->h_length);

    printf("host name alias: \n");
    for (i = 0; host->h_aliases[i]; i++) {
        printf("%s\n", host->h_aliases[i]);
    }
    printf("\naddress list: \n");

    for (i = 0; host->h_addr_list[i]; i++) {
        if ((sockfd = socket(AF_INET, SOCK_STREAM,0)) == -1) {
            printf("socket error\n");
            exit(0);
        }
        // 先清零，然后用struct sockaddr_in来填值
        bzero(&serv_addr, sizeof(serv_addr));
        serv_addr.sin_family = AF_INET;
        serv_addr.sin_port = htons(SERVPORT);
        /* h_addr_list[i]指向in_addr类型 */
        serv_addr.sin_addr = *((struct in_addr *)host->h_addr_list[i]);
        const char *ip = inet_ntoa(serv_addr.sin_addr);
        printf("connect to %s  ", ip);

        // 系统调用的时候，把sockaddr_in转换成sockaddr
        if (connect(sockfd, (struct sockaddr *)&serv_addr, \
                    sizeof(struct sockaddr)) == -1) {
            printf("error\n");
            exit(0);
        }
        printf("success\n");
    }

    return 0;
}

serv_addr.sin_addr = *((struct in_addr *)host->h_addr_list[i]);

#include <arpa/inet.h>
/* network to ascii */
char *inet_ntoa(struct in_addr in);
                          
/* ascii to network */
int inet_aton(const char *cp, struct in_addr *inp);

const char *inet_ntop(int af, const void *src, char *dst, socklen_t size);
                              
int inet_pton(int af, const char *src, void *dst);  /* presentation format to network */