[TriEmbed] Hacking a fake vintage radio (with Arduino + Pi 0)

Scott Hall scottghall1 at gmail.com
Wed Jul 1 08:13:23 CDT 2020


NO NO, DON'T USE GOTO !

Seriously though, this has been a behaviour of the Arduino compiler
compilation for a while -- its even mentioned in some Arduino books.  The
workaround is to have a single function called for each case label and to
put the statements desired with the function.  This gets optimized to a
jump table anyway, so its just a matter of doing this in practice.

On Wed, Jul 1, 2020 at 12:34 AM Jon Wolfe via TriEmbed <
triembed at triembed.org> wrote:

> Yep, GCC has a ton of extensions to C and C++, though they obviously need
> careful consideration when using, if it’s work the downside of making the
> code less portable.
>
>
>
> https://gcc.gnu.org/onlinedocs/gcc-5.2.0/gcc/C-Extensions.html
>
>
>
> One of my favorites is local functions.
>
>
>
>
>
>
>
> *From: *Rodney Radford <ncgadgetry at gmail.com>
> *Sent: *Tuesday, June 30, 2020 9:04 PM
> *To: *Jon Wolfe <jonjwolfe at anibit.com>
> *Cc: *Brian <triembed at undecidedgames.net>; Triangle Embedded Computing
> Discussion <triembed at triembed.org>
> *Subject: *Re: [TriEmbed] Hacking a fake vintage radio (with Arduino + Pi
> 0)
>
>
> I had never heard of the GCC label variable, so I had to google it... wow,
> I learned something new tonight!
>
>
> https://stackoverflow.com/questions/1777990/is-it-possible-to-store-the-address-of-a-label-in-a-variable-and-use-goto-to-jum
>
>
> On Tue, Jun 30, 2020 at 8:57 PM Jon Wolfe via TriEmbed <
> triembed at triembed.org> wrote:
>
>>
>>
>> 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 <triembed at triembed.org>
>> *Sent: *Tuesday, June 30, 2020 12:48 PM
>> *To: *Brian <triembed at undecidedgames.net>
>> *Cc: *Triangle Embedded Computing Discussion <triembed at triembed.org>
>> *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
>> <triembed at triembed.org> 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 <
>> triembed at triembed.org> 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
>> > >
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>>
>>
>> --
>>
>> Huan Truong
>> www tnhh.net / twitter @huant
>>
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-- 
Scott G. Hall
Raleigh, NC, USA
scottghall1 at gmail.com
*Although kindness is rarely a job, no matter what you do it's always an
option.*
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