Re: It's confused that local variable's value not change in function.return probe.

[fche@redhat.com (Frank Ch. Eigler)]

David McRay wrote:

> It's confused that local variable's value not change in function.return
> probe. [...]
>>>>> running stap <<<<
> gcc tests/TestStap.c -O0 -o build/TestStapC -ggdb -gdwarf
>
> # stap tracker.stp -c ./build/TestStapC
> WARNING: confusing usage, consider @entry($a->$) in .return probe:
> identifier '$a$' at scripts/track_cplusplus/track_c.stp:11:28
>  source:   printf("return a: %s\n", $a$)

The warning message is on topic.  If in a .return probe, you access a
general $context variable, this refers to a saved snapshot of the
variable as evaluated at function-entry time.  See also [man stapprobes]
If you wish to look at the fields of a->* -after- the function returns,
you can use something like:

    @cast(@entry(&$a),"Co")$
instead of
    $a$
which works by saving only the address of $a, and 
dereferencing/pretty-printing it at return time.

... though we should be able to automate that, in the future, down to
    @entry(&$a)$
with a little better dwarf type inference.


- FChE