Re: $return in wildcarded void functions

[Mike Mason <mmlnx at us dot ibm dot com>]

Frank Ch. Eigler wrote:
> I've thought about it a while, and it seems easiest as well as fairly
> general to support markup of the $variable use itself.  Since more
> elaborate script code may want to substitute a missing $value, and
> also know that such a substitution has been done, we'd probably need
> two variables:
>
> # probe module("uhci_hcd").function("*").return
> # {
> # if ($?return)
> #   printf ("%s <- %s: %s\n", thread_indent(-2), probefunc(), errno_str($!return))
> # else
> #     printf ("%s <- %s: void\n", thread_indent(-2), probefunc())
> # }
>
> So $?var would return true or false if the given expression was
> defined.  And $!var would return the value, if defined, or else fall
> back to 0.  (This could also cover a fault being encountered while
> $!var->field->array[99] pointer chains were gbeing followed.)
>
> So:
>    $?var==0 implies $!var==0
>    $?var==0 implies $var would have caused compile-time error
>    $?var==1 implies $!var equals what $var would have

I like this approach.  Could it also work for variables defined locally in probes (e.g. ?var and !var, no $)?  What about global variables?

I'm not crazy about returning 0 for a non-existent variable, but at least this method provides a way to avoid that.

Does $?var==1 imply that $var and $!var exist ?

Mike