[lug] Polling a file under linux

[Rob Judd]

On Mon, 21 Apr 2003, Ed Hill wrote:

> On Mon, 2003-04-21 at 10:57, Rob Judd wrote:
> > I have a C/C++ application that has to connect to an IP address every few
> > minutes/seconds (exact timing is not yet clear).  The IP address may
> > change as the application is running, and the exact address is stored in
> > /etc/myapp.config
> >
> > Since this file may change at any time I have to check it before each
> > connection.  It seems to me that my options are
> >
> > 1. read the contents of the file before every connection
> > 2. stat the file before every connection
> >
> > both of these are rather expensive, especially if connections are made
> > several times a second.
> >
> > Is there a better way to poll a file to see when it changes?  (This is in
> > C or C++)
>
>
> The program that you describe sounds like a typical daemon--something
> thats running more-or-less continually and has a config file.  The

Yep, that's the general situation.

> ages-old Un*x solution for this problem is to *NOT* poll the config
> file.  Instead, the program or user that has modified the config file
> must then send a signal ("kill -HUP [PID]") to the daemon to tell it to
> re-read the config file.  The daemon has previously registered one or
> more signal handlers that then (re-)read the config file and (re-)set
> any connections, etc. while keeping any needed state.
>
> This solves two problems:
>
>   1) you don't waste lots of time/effort polling, and
>
>   2) the potential *RACE CONDITION* of having a partially-edited
>      config file is (largely) avoided.
>
> Think about it.  The second problem is non-trivial.  While a config file
> is being edited, it could easily contain incomplete information.

This is a great suggestion, thanks.  However, it brings to mind the
/etc/resolv.conf file.  This is (potentially) used by several
applications, so it becomes infeasible to use the above method.  Does
anyone know how applications use that file?

Thanks, Rob