2006-03-28 15:34:52

by yenganti pradeep

Subject: procfs question

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






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2006-03-28 16:15:46

by Phillip Susi

Subject: Re: procfs question

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.

2006-03-28 16:28:05

by Paulo Marques

Subject: Re: procfs question

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...

--
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