Bug and question

Bug and question

[Sebastien Decugis]

Hi all,

--- bug ---
Running the latest snapshot posted on this mailing-list (on saturday),
I've run into a bug: when using the "substr" function, 'man stapfuncs'
tells that the third argument is an offset, but it appears to be an
absolute position.

As an example we have:

str = "abcdef"
sstr = substr(str, 2, 3)   

here, sstr is "c" whereas it should be "cde" according to man.

--- question ---
I want to access a structure field of a parameter which is a char table,
and I want to display it as a string. Is it possible? Here is what I
want to do:

* kernel code:

struct net_device {
    char name[LEN];
    ...
}

int dev_put(struct net_device *dev)
{
   ...
}

* stap code:
probe kernel.function("dev_put") {
    print("device name: " . $dev->name . "\n")
}


This doesn't work yet, stap complains that $dev->name is of type
'long'... Is there a workaround? I think I only get the 'name' address
yet (can be displayed if casted to a string). How can I get the string?

Thanks in advance, and SystemTap is a great tool :)
Seb.

-- 
Sebastien Decugis
Bull S.A.

Re: Bug and question

[Daniel P. Berrange]

On Wed, Oct 12, 2005 at 11:42:33AM +0200, Sebastien Decugis wrote:
> I want to access a structure field of a parameter which is a char table,
> and I want to display it as a string. Is it possible? Here is what I
> want to do:

Yes, but you need to use either 'user_string' or 'kernel_string' functions
to convert into the systemtap runtime string object. If you don't do this
you'll just get back the pointer as a long.
 
> probe kernel.function("dev_put") {
>     print("device name: " . $dev->name . "\n")
> }

Try changing it to look like:

  probe kernel.function("dev_put") {
      print("device name: " . kernel_string($dev->name) . "\n")
  }

Regards,
Dan.
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Re: Bug and question

[Sebastien Decugis]

Great, thank you! :)


On Wed, 2005-10-12 at 11:09 +0100, Daniel P. Berrange wrote:
> On Wed, Oct 12, 2005 at 11:42:33AM +0200, Sebastien Decugis wrote:
> > I want to access a structure field of a parameter which is a char table,
> > and I want to display it as a string. Is it possible? Here is what I
> > want to do:
> 
> Yes, but you need to use either 'user_string' or 'kernel_string' functions
> to convert into the systemtap runtime string object. If you don't do this
> you'll just get back the pointer as a long.
>  
> > probe kernel.function("dev_put") {
> >     print("device name: " . $dev->name . "\n")
> > }
> 
> Try changing it to look like:
> 
>   probe kernel.function("dev_put") {
>       print("device name: " . kernel_string($dev->name) . "\n")
>   }
> 
> Regards,
> Dan.
-- 
Sebastien Decugis
Bull S.A.