dtrace discuss - Dec 2010 - Why can't I use stack as index to an array but I can for an aggregation?

I am aggregating using the stack function as the index:

 @s[stack()] = count();

This works great. However, I found that in addition to how many times
each stack appears, it would also be useful to know which stacks were
called shortly before the script exited. So I thought to make an
associative array with the same indexes, and store the timestamp the
last time they were set:

ts[stack()] = timestamp;

This gives me an error:
 tracing function stack( ) may not be called from a D expression (D
program context required)

Why can''t I use stack as the index to an associative array when I can
use it as the index to an aggregation? Am I doing something wrong? And
if I can''t use it, does anyone have a suggestion how to store the time
of the last call of each stack?
-- 
blu

It''s bad civic hygiene to build technologies that could someday be
used to facilitate a police state. - Bruce Schneier
-----------------------------------------------------------------------|
Brian Utterback - Solaris RPE, Oracle Corporation.
Ph:603-262-3916, Em:brian.utterback at oracle.com

On Fri, Dec 17, 2010 at 03:44:00PM -0500, Brian Utterback
wrote:
> I am aggregating using the stack function as the index:
> 
>  @s[stack()] = count();
> 
> This works great. However, I found that in addition to how many times
> each stack appears, it would also be useful to know which stacks were
> called shortly before the script exited. So I thought to make an
> associative array with the same indexes, and store the timestamp the
> last time they were set:
> 
> ts[stack()] = timestamp;
> 
> This gives me an error:
>  tracing function stack( ) may not be called from a D expression (D
> program context required)
> 
> Why can''t I use stack as the index to an associative array when I
can
> use it as the index to an aggregation? Am I doing something wrong? And
> if I can''t use it, does anyone have a suggestion how to store the
time
> of the last call of each stack?
While I don''t know the answer to your question, a workaround would be
to
use max() as the aggregation function...

Nico
--

>
> Why can''t I use stack as the index to an associative array when I
can
> use it as the index to an aggregation? Am I doing something wrong? And
> if I can''t use it, does anyone have a suggestion how to store the
time
> of the last call of each stack?

 If Bryan is monitoring this, he can provide a more authoritative answer,
but I believe the use of stack() -- normally a data recording action -- as a
key to an aggregation was a special case. The stack() action should,
perhaps, have produced an object that could then be used as a key for an
aggregation or associative array, or recorded with, say, the trace() action
(e.g. trace(stack());).

Arguably it''s a bug that you can''t use stack() in the way that
you
described. You should feel free to file it with Oracle or in illumos.

Adam

-- 
Adam Leventhal, Delphix                        http://dtrace.org/blogs/ahl
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