procfs question

procfs question

[yenganti pradeep]

Hi,

I've created a new entry under /proc, to make tests.

I've defined an static int var=0;

Then I link my proc entry read function to a function
that only performs this:

int length;
length=sprintf(page,"Value %d",var++);

return length;

But when I cat/vi the file continuosly I get:

Value 0
Value 3
Value 6

etc...

Why is this three numbers increment? 

Thanks
Pradeep





		
__________________________________________________________ 
Yahoo! India Matrimony: Find your partner now. Go to http://yahoo.shaadi.com

Re: procfs question

[Phillip Susi]

yenganti pradeep wrote:
> length=sprintf(page,"Value %d",var++);
<snip>
> Why is this three numbers increment? 

Ummm... because you are incrementing it?  var++ means increment var.

Re: procfs question

[Paulo Marques]

yenganti pradeep wrote:
> Hi,

Hi,

> I've created a new entry under /proc, to make tests.
> 
> I've defined an static int var=0;
> 
> Then I link my proc entry read function to a function
> that only performs this:
> 
> int length;
> length=sprintf(page,"Value %d",var++);
> 
> return length;
> 
> But when I cat/vi the file continuosly I get:
> 
> Value 0
> Value 3
> Value 6
> 
> etc...
> 
> Why is this three numbers increment? 

'cat' will issue a read for more bytes than your function provides. As 
this read isn't fully satisfied it will issue another read for the rest 
at a different offset, etc. So your function gets called several times.

Just do a 'strace' on 'cat' to see what 'cat' really does. For more 
details search for the thread 'procfs uglyness caused by "cat"'.

Your read function really shouldn't have side effects...

-- 
Paulo Marques - www.grupopie.com

Pointy-Haired Boss: I don't see anything that could stand in our way.
            Dilbert: Sanity? Reality? The laws of physics?