16-16.c

#include <errno.h>
#include <netdb.h>
#include <sys/socket.h>
#include "apue.h"

#define BUFLEN 128
/*
extern int connect_retry(int, int, int, const struct sockaddr *, socklen_t);
*/
#define MAXSLEEP 128
int connect_retry(int domain, int type, int protocol,
                  const struct sockaddr *addr, socklen_t alen) {
    int numsec, fd;

    for (numsec = 1; numsec <= MAXSLEEP; numsec <<= 1) {
        if ((fd = socket(domain, type, protocol)) < 0) {
            return (-1);
        }

        if (connect(fd, addr, alen) == 0) {
            return (fd);
        }
        close(fd);

        if (numsec <= MAXSLEEP / 2) {
            sleep(numsec);
        }
    }
    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) {
        err_sys("recv error");
    }
}

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

    if (argc != 2) {
        err_quit("usage: ruptime hostname");
    }
    memset(&hint, 0, sizeof(hint));
    hint.ai_socktype = SOCK_STREAM;
    hint.ai_canonname = NULL;
    hint.ai_addr = NULL;
    hint.ai_next = NULL;
    if ((err = getaddrinfo(argv[1], "ruptime", &hint, &ailist)) != 0) {
        err_quit("getaddrinfo error: %s", gai_strerror(err));
    }
    for (aip = ailist; aip != NULL; aip = aip->ai_next) {
        if ((sockfd = connect_retry(aip->ai_family, SOCK_STREAM, 0,
                                    aip->ai_addr, aip->ai_addrlen)) < 0) {
            err = errno;
        } else {
            print_uptime(sockfd);
            exit(0);
        }
    }
    err_exit(err, "can't connect to %s", argv[1]);
}