Unix 系統編程：獲取要傳遞給 getaddrinfo 的網絡標識符

Question

我正在關注一本書，用 C 語言編寫代碼以使用系統調用構建服務器。

main功能如下：

int main(int argc, char* argv[])
{
    printf("entered main\n");
    struct addrinfo *ailist, *aip, hint;
    int sockfd, err, n;
    char *host;
    if (argc != 1)
    {
        printf("usage: ruptimed\n");
        exit(1);
    }
    if ((n=sysconf(_SC_HOST_NAME_MAX))<0)
    {
        n = HOST_NAME_MAX;
    }
    if((host = malloc(n)) == NULL)
    {
        printf("malloc error\n");
        exit(1);
    }
    if (gethostname(host, n)<0)
    {
        printf("gethostname error\n");
        exit(1);
    }
    printf("host: %s\n", host);
    printf("Daemonizing\n");
    int res = daemonize("ruptimed");
    printf("%d\n", res);
    printf("Daemonized\n");
    memset(&hint, 0, sizeof(hint)); //set to 0 all bytes
    printf("hint initialized\n");
    hint.ai_flags = AI_CANONNAME;
    hint.ai_socktype = SOCK_STREAM;
    hint.ai_canonname = NULL;
    hint.ai_addr = NULL;
    hint.ai_next = NULL;
    printf("getting addresses\n");
    if((err = getaddrinfo(host, "ruptime", &hint, &ailist))!=0)
    {
        printf("error %s\n", gai_strerror(err));
        syslog(LOG_ERR, "ruptimed: getaddrinfo error %s", gai_strerror(err));
        exit(1);
    }
    printf("Got addresses\n");
    for (aip = ailist; aip!=NULL; aip = aip->ai_next)
    {
        if ((sockfd = initserver(SOCK_STREAM, aip->ai_addr, aip->ai_addrlen, QLEN))>=0)
        {
            printf("starting to serve\n");
            serve(sockfd);
            exit(0);
        }
    }
    exit(1);
}

我的問題是當使用gethostname獲取主機名然后將它與getaddrinfo一起使用時。

在 OSX 上運行代碼我得到了一個名字，比如pippo's-MacBook-pro.local記住在host字符指針變量中。 將此傳遞給getaddrinfo導致錯誤： nodename nor servname provided, or not known 。

我期待gethostname返回本地 IP 或本地網絡標識符（即使是 localhost 也有利於學習）。 我懷疑這樣的名稱是否可以用於在機器上沒有正確設置的情況下識別（本地）服務器（另外，我不記得這本書說過關於設置主機名的任何內容）。

如何獲取要傳遞給getaddrinfo的網絡標識符（例如本地 IP）？

如果我想使用gethostname應該執行哪些更改或設置？

代碼

server.c

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h> //_SC_HOST_NAME_MAX
#include<string.h>
#include<netdb.h> //Here are defined AF_INET and the others of the family
#include<syslog.h> //LOG_ERR
#include<errno.h> //errno
#include <sys/types.h>

#include"utilities.h"
#include "error.h"

#define BUFLEN 128
#define QLEN 10

#ifndef HOST_NAME_MAX
#define HOST_NAME_MAX 156
#endif

int initserver(int type, const struct sockaddr *addr, socklen_t alen, int qlen);
void serve(int sockfd);

int main(int argc, char* argv[])
{
    printf("entered main\n");
    struct addrinfo *ailist, *aip, hint;
    int sockfd, err, n;
    char *host;
    if (argc != 1)
    {
        printf("usage: ruptimed\n");
        exit(1);
    }
    if ((n=sysconf(_SC_HOST_NAME_MAX))<0)
    {
        n = HOST_NAME_MAX;
    }
    if((host = malloc(n)) == NULL)
    {
        printf("malloc error\n");
        exit(1);
    }
    if (gethostname(host, n)<0)
    {
        printf("gethostname error\n");
        exit(1);
    }
    printf("host: %s\n", host);
    printf("Daemonizing\n");
    int res = daemonize("ruptimed");
    printf("%d\n", res);
    printf("Daemonized\n");
    memset(&hint, 0, sizeof(hint)); //set to 0 all bytes
    printf("hint initialized\n");
    hint.ai_flags = AI_CANONNAME;
    hint.ai_socktype = SOCK_STREAM;
    hint.ai_canonname = NULL;
    hint.ai_addr = NULL;
    hint.ai_next = NULL;
    printf("getting addresses\n");
    if((err = getaddrinfo(host, "ruptime", &hint, &ailist))!=0)
    {
        printf("error %s\n", gai_strerror(err));
        syslog(LOG_ERR, "ruptimed: getaddrinfo error %s", gai_strerror(err));
        exit(1);
    }
    printf("Got addresses\n");
    for (aip = ailist; aip!=NULL; aip = aip->ai_next)
    {
        if ((sockfd = initserver(SOCK_STREAM, aip->ai_addr, aip->ai_addrlen, QLEN))>=0)
        {
            printf("starting to serve\n");
            serve(sockfd);
            exit(0);
        }
    }
    exit(1);
}

void serve(int sockfd)
{
    int clfd;
    FILE *fp;
    char buf[BUFLEN];
    set_cloexec(sockfd);
    for(;;)
    {
        /*After listen, the socket can receive connect requests. accept
        retrieves a connect request and converts it into a connection.
        The file returned by accept is a socket descriptor connected to the client that
        called connect, haing the same coket type and family type. The original
        soket remains available to receive otherconneion requests. If we don't care
        about client's identity we can set the second (struct sockaddr *addr)
        and third parameter (socklen_t *len) to NULL*/
        if((clfd = accept(sockfd, NULL, NULL))<0)
        {
            /*This generates a log mesage.
            syslog(int priority, const char *fformat,...)
            priority is a combination of facility and level. Levels are ordered from highest to lowest:
            LOG_EMERG: emergency system unusable
            LOG_ALERT: condiotin that must be fied immediately
            LOG_CRIT: critical condition
            LOG_ERR: error condition
            LOG_WARNING
            LOG_NOTICE
            LOG_INFO
            LOG_DEBUG
            format and other arguments are passed to vsprintf function forf formatting.*/
            syslog(LOG_ERR, "ruptimed: accept error: %s", strerror(errno));
            exit(1);
        }
        /* set the FD_CLOEXEC file descriptor flag */
        /*it causes the file descriptor to be automatically and atomically closed
         when any of the exec family function is called*/
        set_cloexec(clfd);
        /**pg. 542 Since a common operation is to create a pipe to another process
        to either read its output or write its input Stdio has provided popen and
        pclose: popen creates pipe, close the unused ends of the pipe,
        forks a child and call exec to execute cmdstr and
        returns a file pointer (connected to std output if "r", to stdin if "w").
        pclose closes the stream, waits for the command to terminate*/
        if ((fp = popen("/usr/bin/uptime", "r")) == NULL)
        {
            /*sprintf copy the string passed as second parameter inside buf*/
            sprintf(buf, "error: %s\n", strerror(errno));
            /*pag 610. send is similar to write. send(int sockfd, const void *buf, size_t nbytes, it flags)*/
            send(clfd, buf, strlen(buf),0);
        }
        else
        {
            /*get data from the pipe that reads created to exec /usr/bin/uptime */
            while(fgets(buf, BUFLEN, fp)!=NULL)
            {
                /* clfd is returned by accept and it is a socket descriptor
                connected to the client that called connect*/
                send(clfd, buf, strlen(buf), 0);
            }
            /*see popen pag. 542*/
            pclose(fp);
        }
        close(clfd);
    }
}


int initserver(int type, const struct sockaddr *addr, socklen_t alen, int qlen)
{
    int fd, err;
    int reuse = 1;
    if ((fd = socket(addr->sa_family, type, 0))<0)
    {
        return (-1);
    }
    if (setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(int))<0)
    {
        goto errout;
    }
    if(bind(fd, addr, alen)<0)
    {
        goto errout;
    }
    if (type == SOCK_STREAM || type == SOCK_SEQPACKET)
    {
        if(listen(fd, qlen)<0)
        {
            goto errout;
        }
    }
    return fd;
    errout:
        err = errno;
        close (fd);
        errno = err;
        return(-1);
}

utilities.c ：包含demonize和setcloexec功能。 在daemonize函數中，我沒有關閉用於調試的文件描述符。

#include "utilities.h"
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <syslog.h>
#include <sys/time.h>//getrlimit
#include <sys/resource.h>//getrlimit
#include <signal.h> //sigempyset , asigcation (umask?)
#include <sys/resource.h>
#include <fcntl.h> //O_RDWR
#include <stdarg.h>

#include "error.h"
int daemonize(const char *cmd)
{
    int fd0, fd1, fd2;
    unsigned int i;
    pid_t pid;
    struct rlimit       rl;
    struct sigaction    sa;
    /* *Clear file creation mask.*/
    umask(0);
    /* *Get maximum number of file descriptors. */
    if (getrlimit(RLIMIT_NOFILE, &rl) < 0)
    {
        err_quit("%s: can’t get file limit", cmd);
    }
    /* *Become a session leader to lose controlling TTY. */
    if ((pid = fork()) < 0)
    {
        err_quit("%s: can’t fork", cmd);
    }
    else if (pid != 0) /* parent */
    {
        exit(0); //the parent will exit
    }
    setsid();
    /* *Ensure future opens won’t allocate controlling TTYs. */
    sa.sa_handler = SIG_IGN;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = 0;
    if (sigaction(SIGHUP, &sa, NULL) < 0)
    {
        err_quit("%s: can’t ignore SIGHUP", cmd);
    }
    if ((pid = fork()) < 0)
    {
        err_quit("%s: can’t fork", cmd);
    }
    else if (pid != 0) /* parent */
    {
        exit(0);
    }
    /*
    *Change the current working directory to the root so
    * we won’t prevent file systems from being unmounted.
    */
    if (chdir("/") < 0)
    {
        err_quit("%s: can’t change directory to /", cmd);
    }
    /* Close all open file descriptors. */
    if (rl.rlim_max == RLIM_INFINITY)
    {
        rl.rlim_max = 1024;
    }
    printf("closing file descriptors\n");
    /*for (i = 0; i < rl.rlim_max; i++)
    {
        close(i);
    }*/
    /* *Attach file descriptors 0, 1, and 2 to /dev/null.*/
    //printf not working
    /*printf("closed all file descriptors for daemonizing\n");*/
    /*fd0 = open("/dev/null", O_RDWR);
    fd1 = dup(0);
    fd2 = dup(0);*/
    /* *Initialize the log file. Daemons do not have a controlling terminal so
    they can't write to stderror. We don't want them to write to the console device
    because on many workstations the control device runs a windowing system. They can't
    write on separate files either. A central daemon error-logging facility is required.
    This is the BSD. 3 ways to generate log messages:
    1) kernel routines call the log function. These messages can be read from /dev/klog
    2) Most user processes (daemons) call syslog to generate log messages. This causes
    messages to be sent to the UNIX domain datagram socket /dev/log
    3) A user process on this host or on other host connected to this with TCP/ID
    can send log messages to UDP port 514. Explicit network programmin is required
    (it is not managed by syslog.
    The syslogd daemon reads al three of log messages.

    openlog is optional since if not called, syslog calls it. Also closelog is optional
    openlog(const char *ident, int option, int facility)
    It lets us specify ident that is added to each logmessage. option is a bitmask:
        LOG_CONS tells that if the log message can't be sent to syslogd via UNIX
        domain datagram, the message is written to the console instead.
    facility lets the configuration file specify that messages from different
    facilities are to be handled differently. It can be specified also in the 'priority'
    argument of syslog. LOG_DAEMON is for system deamons
    */
    /*
    openlog(cmd, LOG_CONS, LOG_DAEMON);
    if (fd0 != 0 || fd1 != 1 || fd2 != 2)
    {*/
        /*This generates a log mesage.
        syslog(int priority, const char *fformat,...)
        priority is a combination of facility and level. Levels are ordered from highest to lowest:
        LOG_EMERG: emergency system unusable
        LOG_ALERT: condiotin that must be fied immediately
        LOG_CRIT: critical condition
        LOG_ERR: error condition
        LOG_WARNING
        LOG_NOTICE
        LOG_INFO
        LOG_DEBUG

        format and other arguments are passed to vsprintf function forf formatting.*/
        /*syslog(LOG_ERR, "unexpected file descriptors %d %d %d", fd0, fd1, fd2);
        exit(1);
    }*/
    return 0;
}

/*The function set the FD_CLOEXEC flag of the file descriptor already open that
is passed to as parameter. FD_CLOEXEC causes the file descriptor to be
automatically and atomically closed when any of the exec family function is
called*/
int set_cloexec(int fd)
{
    int val;
    /* retrieve the flags of the file descriptor */
    if((val = fcntl(fd, F_GETFD, 0))<0)
    {
        return -1;
    }
    /* set the FD_CLOEXEC file descriptor flag */
    /*it causes the file descriptor to be automatically and atomically closed
     when any of the exec family function is called*/
    val |= FD_CLOEXEC;
    return (fcntl(fd, F_SETFD, val));
}

我使用的錯誤函數

/* Fatal error unrelated to a system call.
* Print a message and terminate*/
void err_quit (const char *fmt, ...)
{
    va_list ap;
    va_start (ap, fmt);
    err_doit (0, 0, fmt, ap);
    va_end (ap);
    exit(1);
}

/*Print a message and return to caller.
*Caller specifies "errnoflag"*/
static void err_doit(int errnoflag, int error, const char *fmt, va_list ap)
{
    char buf [MAXLINE];
    vsnprintf (buf, MAXLINE-1, fmt, ap);
    if (errnoflag)
    {
        snprintf (buf+strlen(buf), MAXLINE-strlen(buf)-1, ": %s",
            strerror (error));
    }
    strcat(buf, "\n");
    fflush(stdout); /*in case stdout and stderr are the same*/
    fputs (buf, stderr);
    fflush(NULL); /* flushes all stdio output streams*/
}

Answer 1

getaddrinfo調用中有一個比較簡單的錯字

if((err = getaddrinfo("host", "ruptime", &hint, &ailist))!=0)
/*                    ^^^^^^   */
/* Should be the variable host */
{
    printf("error %s\n", gai_strerror(err));
    syslog(LOG_ERR, "ruptimed: getaddrinfo error %s", gai_strerror(err));
    exit(1);
}

該代碼正在尋找"host"的地址。 我還將用NULL替換"ruptime" ，因為ruptime不是/etc/services 中的條目。 有關更多詳細信息，請參閱getaddrinfo(3) 。