Unix system programming: get a network identifier to be passed to getaddrinfo

Question

I am following on a book the code in C to build a server using system calls.

The main function is the following:

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);
}

My problem is when getting the host name with gethostname and then using it with getaddrinfo .

Running the code on OSX I get a name such as pippo's-MacBook-pro.local memorized in the host char pointer variable. Passing this to getaddrinfo results in the error: nodename nor servname provided, or not known .

I was expecting the gethostname to return a local IP or a local network identifier (even localhost would be good for learning). I doubt such name can be used to identify a (local) server without proper settings on the machine (plus I cannot remember the book saying anything about setting the host name).

How can I get a network identifier (such as the local IP) to be passed to getaddrinfo ?

If I would like to use gethostname what changes or settings should be performed?

CODE

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 : containing the demonize and setcloexec functions. In daemonize function I did not close file descriptors for debugging.

#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));
}

error functions I used

/* 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

There is a relatively simple typo in the getaddrinfo call

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);
}

The code is looking for the address of "host" . I would also replace "ruptime" with NULL since ruptime isn't an entry in /etc/services . See getaddrinfo(3) for more detail.