2020-08-27

Pseudo C++ using cpp (the RevK macro)

I am not sure if this is evil, or genius, or both. Either way I take full credit.

Mainly for a library where we want to add extra optional options in the future, so want almost C++ style optional and tagged arguments to a function, but in normal C (because C++ is just evil all by itself).


Not quite as flexible as C++, as no defaults for missing arguments, but you can often live with that knowing they are zero or NULL.

Unless someone can cite a prior art - please call this the RevK macro.

P.S. I have updated my string decimal library to use this, and it is way neater!

Some explanation...

Normally a function call in C has a fixed set of arguments. Well, not quite, some can have variable arguments at the end, like printf(...) but that is handled in a special way and you have to know (usually from a format string) how many arguments and which type.

However, in some languages, like C++, you can have optional arguments which are pre-defined types and names, but you can stop early in the list. In C++ you can say what default these missing arguments have. You also have the option to leave some arguments our and "tag" some others.

So, the idea you can call myfunc("hello",flag2:1) is setting the first argument (s), and the third (flag2) and not specifying the middle one (flag1) which ends up zeroed. I can't set defaults but can expect unspecified to be zeroed.

Now, normally, if I had some function you call as func(a,b,c) and I want some extra option later on, I would have to either change to func(a,b,c,d) everywhere it is used in every program, even if people specify d as 0 or NULL,  or make a new separate function that takes the extra argument as an alternative, e.g. func2(a,b,c,d).

With this trick I am able to add this extra argument which is optional, knowing that if the extra argument is not specified it has a known value of 0/NULL.

The way the trick works is by using the standard C pre-processor which does text substitution, and expanding the full list of arguments (...) in the macro (as __VA_ARGS__) within a structure initialisation. Unlike function arguments, C has a syntax to initialise a structure which allows you to omit arguments and tag arguments. I am using that syntax in the function, so myfunc("hello",flag2:1) becomes a structure initialiser {"hello",flag2:1} and this structure is passed to the function.

Of course I could just used C++, but that comes with a lot of other baggage, and not something I am that keen on. It has its merits and works well for some applications.

💩

 I have been involved with SMS (i.e. text messaging) for a long time. I was even on the ETSI committees that designed GSM (not specifically SMS, sadly), and have been doing things with SMS for nearly 30 years in one way or another, including an SMS->fax/email gateway, and even the ETSI landline SMS module for asterisk. Now, at A&A, we have code to send and receive SMS via a variety of carriers and even a SIP a-law based ETSI landline SMS system.

The specification for SMS is a typical telecoms specification - very different to internet specifications where single bits packed in some small data header can subtly change the interpretation of some or all that follows. These specifications are normally very precise but absolutely horrid, in my view.

But where does the pile of poo come in, and how does it relate to a 30 year old specification for SMS? Well, you may be surprised, but SMS allows for 💩.

SMS are actually coded in the signalling used for calls, and so had limited space. There were actually only 140 bytes (or more correctly octets) of data for the text itself. As you may know SMS allow 160 characters, so this is achieved by packing a 7 bit alphabet in to the 140 bytes.

In fact SMS allows 4 ways the data can be coded, a 7 bit special alphabet, an 8 bit Latin-1 alphabet, and 16 bit unicode (allowing 70 characters). There are also ways to send one longer message in smaller parts. The SMS can also be raw data to be sent to a SIM rather than displayed. Had I written this I'd have used 2 bits to say which it is, but no, the specification uses a Data Coding Scheme which is complicated to say the least. Some times the coding is in 2 bits but others it is implied. It is not fun.

The 7 bit alphabet is sort of ASCII, but does allow some interesting characters - being a European spec it includes some accented characters and even some Greek letters.

Of course this also leaves out some key ASCII such as {, }, [ ], and does not even have € (which was added later). These are coded as two character sequences using ESC.

The 8 bit character set is just normal Latin 1, and the 16 bit is unicode. The unicode allows all unicode characters U+0000 to U+FFFF, but where is pile of poo? It is U+1F4A9 which is too big for 16 bits.

The way this is done is to use a little known trick called UTF-16. There are reserved 16 bit unicode characters U+D800 to U+DFFF. Using two such codes it is possible to encode U+10000 to U+10FFFF.

This means 💩 is actually coded as two 16 bit sequences, 0xD83D 0xDCA9 in SMS!

Why does this matter, I mean, who sends 💩 by SMS? As you can imagine, in the early 90's nobody had heard of 💩, and the best emojis we had were :-)

But we do care, honest, as we use it as a blue* M&M test for carriers we deal with. If they have enough attention to detail to handle a pile of poo they probably have the rest sewn up, technically. We are working with a new carrier for SMS messages, and I am pleased to say the unicode is working. They properly translate to/from UTF-8 coding in the messages we exchange (which is what we use internally). Unlike our previous carrier who could not cope. (* see comments)

We have seen a range of such failures, even the case where one carrier could not handle an @ symbol (presumably as it coded to 0x00 which is an end of string in languages like C). Thankfully that carrier was happy for us to send a raw hex TPDU for SMS, and hence allowing us to code any characters. Our SIP2SIM service has handled pile of poo since we launched it...

The end result is that, shortly, we will be handling a lot more SMS with unicode characters correctly, in most cases, both incoming and outgoing. Watch this space.

2020-08-15

Making a meme?

This is my attempt to make a meme.


Of course, there is a danger that it will be seen as insensitive. Tricky one. If anything I am having a go at ofqual. I do feel rather sad for students struggling with downgraded exam results this year. The whole situation is crazy.

I know there are plenty of people that say that they got poor results and did fine, and I know that a lot of companies would not worry about A-level grades when hiring someone, but it is a gateway to university. I vaguely remember my concern over results and whether it met the offers I got from universities. Wrong grades can ruin a promising carrier before it starts, especially in vocations like medicine. I really hope they fix this somehow.

As for making a meme - we will see. I did include a "deliberate mistake" so people can feel smug pointing it out - I think that is a feature that helps a meme happen. (well, one mistake, one that looks like a mistake but sort of isn't, so people can argue over it).

P.S. My keyboard broke, started constantly repeating keys - ones I went nowhere near with the screw driver. I think it is just getting it's own back at me.

2020-08-01

A simple flat tyre - but this is 2020, so no...

Really boring post for your today...

On cycling out of Bracknell town on one of the cycle paths (see, I do use them when they go where I want), I hit a pot hole. I have been back and looked since and it looks really innocuous, but it was very jarring and my first thought is that it will have killed my tyres.

Unsurprisingly, within half a mile, or so, I had a flat back tyre. Crap!

I got a lift back from Tescos, and later walked in (3 miles) with my cycle repair kit and pump. The puncture was obvious, and not that small, so I used the sandpaper thing on the rubber and applied a self adhesive (skabs) patch, pumped up and cycled home. Perfect, job done.

Next morning, tyre flat! I investigated and it was the patch, it had popped allowing air out the side. WTF? I patched again and it immediately popped when pumping up.

I figured that maybe the glue goes off, this repair kit was a few years old, so ordered more. When that arrived, patched, and the same!

So I ordered a different make of self adhesive patch this time.

Again, popped as soon as inflated. This is mental.

I figured it was on a seam in the tyre, so I carefully trimmed that flat with a scalpel blade and tried again, no joy.

OK, time to go old school. I ordered good old fashioned repair kit with the rubber patches and the rubber glue.

I have probably done hundreds of puncture repairs in my life, and never had this trouble.

To my surprise, that did not work either, WTF?

Just to be clear, and thanks for all of the helpful advice, I did apply glue and wait for it to dry before applying the patch. I also, on some attempts, applied glue to the patch, which I don't normally have to do.

I tried the large patch sideways to cover where it popped, no joy.

I even applied a patch on top of the patch where it popped, no joy.

As an almost last resort I even used some Loctite 480 which is especially for bonding rubber. Close, but still popped.

This really is getting beyond a joke. I have never had this much trouble with a simple patch to an inner tube in my life.

I think I now have six puncture repair kits.

I have ordered a new inner tube, and some would say that should have been step 1, or at most step 2. Well, yes, except this is the back wheel with hub brakes, hub gears, and enclosed chain guard, all of which need removing, and at least one cable needs unhooking (and hence re-fitting and adjusting) and to be honest that seemed like a lot of hassle. Hence trying the simple puncture repair.

I then had a brain wave... This puncture is not a usual puncture. Well, apart from now being a tear around 5mm long because of the number of patches I had removed, it was on the inside of the inner tube, i.e. facing the wheel. This fits with it being pinched when I went over a pot hole - after all the tyres I have are meant to be puncture resistant. So not the usual place to get a puncture, which would typically be from a spiky thing through the tyre and hence on the outside. In fact, it was almost certainly exactly on the part where the inner tube is not going to be smooth when inflated, but actually a step where the inside of the tyre is in the wheel. This may be the clue, and why it only popped when inflated ing the tyre (I could inflate quite a bit outside the tyre with no issue).

My fix! Well, for a start I used the Loctite to weld the tear shut anyway, and applied a rubber patch over that. The trick, though, was a plastic card (credit card sized) bent round on the inside of the tyre between the wheel and in inner tube. A real hack, but magically the tyre inflated, and I have managed to cycle round the block and no sign of it deflating yet.

Yay, sorted, and, bollocks, the front tyre is now flat. That seems to be a much smaller slow puncture which was actually simple to fix with a patch as normal (well, so far).

So yay. I do have a spare inner tube coming tomorrow, and I hope I don't need it.

I would stress that this has taken (I think) 4 days now, and so given my run of luck I fully expect to find both tries flat tomorrow, probably pecked by a crow or eaten by a squirrel or something...

Update: Using a card was certainly a clue, as it lasted a lot longer than anything else, but today (the next day) the back tyre is flat again - so fun with dismantling stuff when the inner tube arrives. FML.

... And someone has "borrowed" my Allen keys, arrrg!

Update: I have two inner tubes and new Allen keys. Yay. I figured I would change front one first. It literally exploded in the tyre at around 50psi. WTF?! So now waiting until tomorrow for another new inner tube. I did not have this on my 2020 bingo card.

Looks like it is a full moon at just before 5pm today - is that a bad sign I wonder?

Update: Finally, new inner tube fitted to the back. All working. Pain in the arse to take it all apart though.


P.S. I now find I put the front wheel back wrong and have been cycling with brakes partly on - I thought I was just unfit (which I am), but that was daft. Finally all sorted now.