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

[Craig Ringer <craig@2ndquadrant.com>]

On Mon, 11 Jan 2021 at 01:21, Frank Ch. Eigler via Systemtap <
systemtap@sourceware.org> wrote:

>
> 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]
>

I never found that especially confusing, but I can see how the message
might be improved.

- WARNING: confusing usage, consider @entry($a->$)
+ WARNING: value of  $a is captured at function entry and will not reflect
any changes made since; consider explicitly using @entry($a->$)

or something like that?

Or a subhead hint in "man stapprobes" like "See also [man stapprobes]
heading MORE ON RETURN PROBES".

-- 
 Craig Ringer                   http://www.2ndQuadrant.com/
 2ndQuadrant - PostgreSQL Solutions for the Enterprise