16.4.c

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

#define MAXADDRLEN  256
#define BUFLEN      128
#define MAXSLEEP    128

int connect_retry(int sockfd, const struct sockaddr *addr, socklen_t alen)
{
    int     nsec;

    for (nsec = 1; nsec <= MAXSLEEP; nsec <<= 1) {
        if (connect(sockfd, addr, alen) == 0) {
            return 0;
        }

        if (nsec <= MAXSLEEP / 2) {
            sleep(nsec);
        }
    }

    return -1;
}

void print_uptime(int sockfd)
{
    int     n;
    char    buf[BUFLEN];

    while ((n = recv(sockfd, buf, BUFLEN, 0)) > 0) {
        write(STDOUT_FILENO, buf, n);
    }

    if (n < 0) {
        printf("recv error\n");
    }
}

int main(int argc, char *argv[])
{
    struct addrinfo     *ailist, *aip;
    struct addrinfo     hint;
    int                 sockfd, err;

    if (argc != 2) {
        printf("usage: ruptime hostname\n");
        exit(-1);
    }

    hint.ai_flags    = 0;
    hint.ai_family   = 0;
    hint.ai_socktype = SOCK_STREAM;
    hint.ai_protocol = 0;
    hint.ai_addrlen  = 0;
    hint.ai_canonname = NULL;
    hint.ai_addr     = NULL;
    hint.ai_next     = NULL;

    if ((err = getaddrinfo(argv[1], "ruptime", &hint, &ailist)) != 0) {
        printf("getaddrinfo error: %s\n", gai_strerror(err));
        exit(-1);
    }

    for (aip = ailist; aip != NULL; aip = aip->ai_next) {
        if ((sockfd = socket(aip->ai_family, SOCK_STREAM, 0)) < 0) {
            err = errno;
        }

        if (connect_retry(sockfd, aip->ai_addr, aip->ai_addrlen) < 0) {
            err = errno;
        } else {
            print_uptime(sockfd);
            
            return 0;
        }
    }

    fprintf(stderr, "can't connect to %s: %s\n", argv[1], strerror(errno));

    return 1;
}