[lug] Polling a file under linux

Ed Hill ed at eh3.com
Mon Apr 21 12:16:40 MDT 2003

On Mon, 2003-04-21 at 11:52, Rob Judd wrote:
> 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?

If you're writing a program thats essentially a daemon, then by all
means you should stick to standard behavior and use signals.

And I don't see how "it becomes infeasible to use the above method" as
you mention.  Per your description, you have one program running,
right?  Then you just kill -HUP that program to force a re-read of the
config file.  And if you have multiple such programs running then you
"killall -HUP progname".  How does that not scale?

In the case of /etc/resolv.conf, please see "man 7 gethostbyname" and
the related functions for details.   Most programs only effectively
"read" it once when they start or once for each time that the
information is needed.  Most programs don't assume that the network
addresses are dynamically changing underneath them.  When they bump into
errors they *may* try to get a new address or, more likely, they'll just
quit and/or report an error.  And they certainly *DON'T* directly parse
the /etc/resolv.conf file, they call the standard sys/socket.h functions
to get the DNS mappings.

good luck,

Edward H. Hill III, PhD 
Post-Doctoral Researcher
Division of ESE, Colorado School of Mines, Golden, CO 80401
Email: ed at eh3.com  ehill at mines.edu
Phone: 303-273-3483
URLs:  http://cesep.mines.edu/people/hill.htm  http://eh3.com
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