Re: getloadavg and proc filesystem

[anoop aryal <aaryal@foresightint.com>]

On Tuesday 04 October 2005 12:27 pm, sin wrote:
> Andrew Miehs wrote:
> > Actually, I am looking at /proc/stat and that is the problem...
> >
> > I looked at the implementation of getloadavg to see how 'real
> > programmers' used
> > the proc filesystem. I do not see how the implementation (in glibc)  is
> > protected
> > from the kernel changing the data underneath it while it is being 
> > read....
> >
> > ie: getloadavg open /proc/loadavg for O_RDONLY
> >     getloadavg reads the first 4 bytes
> >     kernel completely updates the file
> >     getloadavg reads the rest of file.....
> >
> > And as /proc/stat is bigger, the problem is likely to occur even more
> > often?!
> > Or am i just being stupid?
>
> Not a kernel guru, but to get that kind of situation, your program
> should run awfully slow. When you read from proc you read directly from
> memory and from what I know, when you open a file in proc it stays the
> same for you until you close it (but I might be wrong)

unless that's changed in 2.6, the file did change on you even when you had it 
open in 2.4.  i wrote a program once that did an 

open();
while(1) {
read();
seek(0);
sleep(5); 
}

to collect data from /proc and it worked just fine.

anoop.