$return in wildcarded void functions

[David Smith]

Frank Ch. Eigler wrote:
> Mike Mason <mmlnx@us.ibm.com> writes:
> 
>> I'd prefer something like the current preprocessing conditionals
>> [...] I assume it isn't a simple matter of extending the
>> preprocessing code since preprocessing is done before accessing the
>> dwarf info.
> 
> Indeed.  Plus preprocessor conditionals don't nest within the other
> nonterminals - they just enclose arbitrary token lists.
> 
> 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
> 
> While $?var would be a compile-time constant, by also providing the
> "faultless" $!var variant, there is no need to be clever within the
> translator.

Hmm, I'm missing something.  Why do we need $?var *and* $!var?  It seems 
like $?var would suffice.

-- 
David Smith
dsmith@redhat.com
Red Hat
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