getaddrinfo

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

int main(void)
{
    struct addrinfo hints;
    struct addrinfo *res;
    struct addrinfo *rp;
    int sockfd;

    memset(&hints, 0, sizeof(hints));
    hints.ai_family = AF_UNSPEC;      // IPv4 或 IPv6
    hints.ai_socktype = SOCK_STREAM;  // TCP

    int ret = getaddrinfo("127.0.0.1", "8888",
                          &hints, &res);
    if (ret != 0)
    {
        fprintf(stderr, "getaddrinfo: %s\n",
                gai_strerror(ret));
        return 1;
    }

    for (rp = res; rp != NULL; rp = rp->ai_next)
    {
        sockfd = socket(rp->ai_family,
                        rp->ai_socktype,
                        rp->ai_protocol);

        if (sockfd == -1)
            continue;

        if (connect(sockfd,
                    rp->ai_addr,
                    rp->ai_addrlen) == 0)
        {
            printf("connected\n");
            break;
        }

        close(sockfd);
    }

    freeaddrinfo(res);

    if (rp == NULL)
    {
        fprintf(stderr, "could not connect\n");
        return 1;
    }

    /* begin to communicate with server */

    close(sockfd);

    return 0;
}