Building and Installing Software Packages for Linux

13. Fifth Example: XmDipmon

Bullwinkle: Hey Rocky, watch me pull a rabbit out of my hat.
Rocky:      But that trick never works.
Bullwinkle: This time for sure.
            Presto!
            Well, I'm gettin' close.
Rocky:      And now it's time for another special feature.
            --- "Rocky and His Friends"

XmDipmon is a nifty little application that displays a button showing the status of an Internet connection. It flashes and beeps when the connection is broken, as is all too often the case in on rural telephone systems. Unfortunately, XmDipmon works only with dip, making it useless for those people, the majority, who use chat to connect.

Building XmDipmon is not a problem. XmDipmon links to the Motif libraries, but it builds and works fine with Lesstif. The challenge is to alter the package to work when using chat. This involves actually tinkering with the source code, and necessarily requires some programming knowledge.

        "When xmdipmon starts up, it checks for a file called /etc/dip.pid
         (you can let it look at another file by using the -pidfile
         command line option).  This file contains the PID of the dip
         deamon (dip switches itself into deamon mode once it has
         established a connection)."
                       --- from the XmDipmon README file

Using the -pidfile option, the program can be directed to check for a different file upon startup, one that exists only during a successful chat login. The obvious candidate is the modem lock file. We could therefore try invoking the program with xmdipmon -pidfile /var/lock/LCK..ttyS3 (this assumes that the modem is on com port #4, ttyS3). This only solves part of the problem, however. The program continually monitors the dip daemon, and we need to change this so it instead polls a process associated with chat or ppp.

There is only a single source file, and fortunately it is well-commented. Scanning the xmdipmon.c file, we find the getProcFile function, whose header description reads as follows.

/*****
* Name:                 getProcFile
* Return Type:  Boolean
* Description:  tries to open the /proc entry as read from the dip pid file.
<snip>
*****/

We are hot on the trail now. Tracing into the body of the function...

                        /* we watch the status of the real dip deamon */
                        sprintf(buf, "/proc/%i/status", pid);
                        procfile = (String)XtMalloc(strlen(buf)*sizeof(char)+1);
                        strcpy(procfile, buf);
                        procfile[strlen(buf)] = '\0';

The culprit is line 2383:

                        sprintf(buf, "/proc/%i/status", pid);
                                      ^^^^^^^^^^^^^^^^^^^^^

This checks whether the dip daemon process is running . So, how can we change this to monitor the pppd daemon instead?

Looking at the pppd manpage:

FILES
       /var/run/pppn.pid (BSD or Linux), /etc/ppp/pppn.pid (others)
                     Process-ID for pppd process on ppp interface unit n.

Change line 2383 in xmdipmon.c to:

                        sprintf(buf, "/var/run/ppp0.pid" );

Rebuild the revised package. No problems with the build. Now test it with the new command line argument. It works like a charm. The little blue button indicates when a ppp connection to the ISP has been established, and flashes and beeps when the connection is broken. Now we have a fully functional chat monitor.

XmDipmon can be downloaded from Ripley Linux Tools.