Re: bind() to /dev/log failed

[Mansuri, Wasim (NSN - IN/Bangalore)]

I am working on some network server which has to monitor the os events and report it to some GUI. For that I want to read the /dev/log and process it and report it to the GUI.

So here my aim is not to write but I want to read the messages coming to /dev/log. I made some changes in the code which I pasted in my first post. And found out working with some limitation. My program has to come up before the syslogd comes up. And second is once I bind and start reading syslogd is not able to read anything with which I can not live with.

Now I want to know, Is there any possiblity that My program and syslogd both can listen and read the messages? I mean once I or syslogd reads message buffre for /dev/log does not emptied. I don't want to consume message from the /dev/log, I just want to read it.

If I summerise my question than, Is there any possibility to read the datagram socket whithout consuming data from the socket? I want to keep the data for other applications to read.

-----Original Message-----
From: Rick Stevens [mailto:[e-mail]] 
Sent: Thursday, July 22, 2010 9:52 PM
To: Community support for Fedora users
Subject: Re: bind() to /dev/log failed

On 07/21/2010 11:29 PM, Mansuri, Wasim (NSN - IN/Bangalore) wrote:
> Hello All,
>
> I want to listen to /dev/log. For that I am using the following code.
>
> Please let me know whats the issue in the following code snip. Code I am running with root user.
>
> Thanks, wasim
>
> if ((sockfd = socket(AF_UNIX,SOCK_STREAM, 0)) == -1)
> {
>     printf("Failed To open AF_UNIX Socket");
>     return -1;
> }
> sunx.sun_family = AF_UNIX;
> strncpy(sunx.sun_path, PR_PATH_LOG, sizeof(sunx.sun_path));
> /*if (connect(sockfd,&syslog_addr, sizeof(syslog_addr)) == -1) {    //Connect call goes successful, But after that I am not able to recv any data.*/
>   if (bind(sockfd, (struct sockaddr *)&sunx,sizeof(sunx.sun_family)+strlen(sunx.sun_path))<  0) { /* bind fails saying 98 : Address already in use Even if I kill syslogd*/
>    printf("Connect failed with the errno %s %d : %s\n",PR_PATH_LOG, errno,strerror(errno));
>    close(sockfd);
> }
>
> if(recv( sockfd, (void *) buff, 1024,0)<=0) /* recv never come out if I use connect even after I do logger */
> {
>        printf("Error in recv call... errno = %d : %s : %s\n", errno, strerror(errno),buff);
>        break;
> }

I can see a couple of things.  First, [r|ng]syslogd _listens_ to 
/dev/log, it is not the output.  If you want to log things, you write
to /dev/log and syslogd will log it.  This, for example, is how the
utility "logger(1)" works.  If you're trying to monitor what's being
fed _to_ [r|ng]syslogd, this probably won't work as syslogd has already
opened the socket for read.

Second, If you're going to do a bind(), you must do it before the
connect(), not after.

Third, a bind() is not necessary if you are a client--only if you're
the server.  You can bind() as a client if you wish to have a specific
address assigned to you.  However your code is trying to use the
already existing data for syslogd.  You'd need to create a unique
pathname to bind to you.

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