There is a trick you can use with gcc that is a non-standard C
construct where you can use ‘goto’ and give it a variable containing
the address of a label. You then create an array of label address and
you can then dynamically index that array to jump to various
locations. I’ve seen it used as an optimization technique, and you
can also have more control over the program flow, though it is using
the infamous keyword, Essentially though it end up looking pretty much
like a switch-case.
That is really odd about that gcc bug. It’s not like I’ve never seen
them, but 99.9% of the time when I thought I had found a compiler bug
in C/C++, it turns out to be something else. (hafl the time one of
those things that disappears with a “clean/rebuild all”) I remember
the Arduino /AVR/gcc linker used to have a bug related to 8-bit AVR
chips that had more than 64KB of flash memory, such as the ATMega
1284. Those chips address by 16 bit words not bytes, so 128kb of flash
is accessible without trick likes far pointers, but the linker would
mess up the address calculations sometimes I think for interrupt
handlers or functions called by interrupt handlers that crossed the
64kb boundary.
*From: *Huan Truong via TriEmbed <mailto:[email protected]>
*Sent: *Tuesday, June 30, 2020 12:48 PM
*To: *Brian <mailto:[email protected]>
*Cc: *Triangle Embedded Computing Discussion
<mailto:[email protected]>
*Subject: *Re: [TriEmbed] Hacking a fake vintage radio (with Arduino +
Pi 0)
Oh yeah, that explains my issue. I definitely ran into that issue
where I have checked and had no reason to believe I was doing
something wrong, yet, when I evacuated each switch to a function, the
switch worked correctly. But neither scoping with an anonymous scope
nor renaming the variables work.
The reason I used the switch was that I read on stackoverflow at one
point and someone said that we should use switches instead of elseifs
when we have a lot of cases. Then, using switches, the compiler will
be able to (at some point) create a lookup table for you so it's
faster. I doubt that was what happening at least on the Arduino case.
You'll need a giant lookup table which the uCs don't have memory for.
I suspect that in a lot of cases, using switches is probably just as
slow as using elseifs. Now as I see that it is so buggy, I probably
will not use switches, at least on Arduino.
Cheers,
- Huan.
On Tue, Jun 30, 2020 at 9:23 AM Brian via TriEmbed
<[email protected]> wrote:
>
> Side note:
>
> The arduino compiler has bugs in how it handles switch statements. I've
> run into situations lately where the order of the case statements matter
> (which it never should); cases are completely ignored, etc.
>
> I believe it may be tied to the use of local scoping within a case,
e.g.:
>
> switch(thing) {
> case 1:
> {
> // stuff with case-local scope
> }
> break;
> }
>
> Syntactically- and semantically-correct code has proven to generate
> incorrect runtime results.
>
> I haven't had time/motivation to submit a bug report, but I should do
> that. At any rate, a potential workaround is to reorder your cases.
>
> -B
>
> On 6/24/20 9:51 PM, Huan Truong via TriEmbed wrote:
> > Thanks Pete!
> >
> > I feel like there was something really mysterious about the switch
statement. Even if I pasted the whole blocks of code of each function
I would have called to the {} inside a case, the code still wouldn’t
work. That baffled me by a mile.
> >
> > But yeah, I spent way too much time on the project that I’m
comfortable with the idea of not understanding some of it now. The
watchdog timer code was baffling too.
> >
> > Please excuse my typos, sent from phone.
> >
> > On Jun 24, 2020, at 10:14 AM, Pete Soper via TriEmbed
<[email protected]> wrote:
> >
> > What a beautifully presented adventure. Loved reading it. And
when you say a problem "could be bad" you make your point. :-) (meant
as a "find Waldo" exercise for alert readers)
> >
> > Hadn't heard of "kev" or any other Arduino emulator for that
matter. That aspect was interesting too.
> >
> > The other issue with redeclaration of the vars local to the switch
statement is that they literally don't exist outside it, so
communicating their values outside the block would be difficult. :-)
In general, every {} defines a local scope in C/C++ and you can
declare variables inside that scope but they cease to be defined
outside the scope. The scope outside any {} (aka "global") or vars
declared "static" can avoid this issue but not the redefine issue.
> >
> > Thanks for sharing this!
> >
> > Pete
> >
> >
> >> On 6/24/20 12:43 PM, Huan Truong via TriEmbed wrote:
> >> This has taken me way more time than I thought, but finishing this
> >> retrofit is a big achievement for me. It's really silly and serves
> >> exactly no purpose other than RE'ing something no one cares about. So
> >> I just want to share for some shits and giggles.
> >>
> >>
http://www.tnhh.net/posts/adventures-hacking-fake-vivitar-vintage-radio.html
> >>
> >>
> >>
> >>
> >>
> >
> > _______________________________________________
> > Triangle, NC Embedded Computing mailing list
> >
> > To post message: [email protected]
> > List info:
http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
> > TriEmbed web site: http://TriEmbed.org
> > To unsubscribe, click link and send a blank message:
mailto:[email protected]?subject=unsubscribe
> >
> >
> > _______________________________________________
> > Triangle, NC Embedded Computing mailing list
> >
> > To post message: [email protected]
> > List info:
http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
> > TriEmbed web site: http://TriEmbed.org
> > To unsubscribe, click link and send a blank message:
mailto:[email protected]?subject=unsubscribe
> >
>
>
> _______________________________________________
> Triangle, NC Embedded Computing mailing list
>
> To post message: [email protected]
> List info:
http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
> TriEmbed web site: http://TriEmbed.org
> To unsubscribe, click link and send a blank message:
mailto:[email protected]?subject=unsubscribe
>
--
Huan Truong
www tnhh.net / twitter @huant
_______________________________________________
Triangle, NC Embedded Computing mailing list
To post message: [email protected]
List info: http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
TriEmbed web site: http://TriEmbed.org
To unsubscribe, click link and send a blank message:
mailto:[email protected]?subject=unsubscribe
_______________________________________________
Triangle, NC Embedded Computing mailing list
To post message: [email protected]
List info: http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
TriEmbed web site: http://TriEmbed.org
To unsubscribe, click link and send a blank message:
mailto:[email protected]?subject=unsubscribe
