Archive | Electronics

# The Specter of Undefined Behavior

If you’ve ever spoken to a programmer, and really got them on a roll, they may have said the words “undefined behavior” to you. Since you speak English, you probably know what each of those words mean, and can imagine a reasonable meaning for them in that order. But then your programmer friends goes on about “null-pointer dereferencing” and “invariant violations” and you start thinking about cats or football or whatever because you are not a programmer.

I often find myself being asked what it is that I do. Since I’ve spent the last few years working on my Computer Science degree, and have spent much of that time involved in programming language research, I often find myself trying to explain this concept. Unfortunately, when put on the spot, I usually am only able to come up with the usual sort of explanation that programmers use among themselves: “If you invoke undefined behavior, anything can happen! Try to dereference a null pointer? Bam! Lions could emerge from your monitor and eat your family!” Strictly speaking, while I’m sure some compiler writer would implement this behavior if they could, it’s not a good explanation for a person who doesn’t already kind of understand the issues at play.

Today, I’d like to give an explanation of undefined behavior for a lay person. Using examples, I’ll give an intuitive understanding of what it is, and also why we tolerate it. Then I’ll talk about how we go about mitigating it.

## Division By Zero

Here is one that most of us know. Tell me, what is `8 / 0`? The answer of course is “division by zero is undefined.” In mathematics, there are two sorts of functions: total and partial. A total function is defined for all inputs. If you say `a + b`, this can be evaluated to some result no matter what you substitute for `a` and `b`. Addition is total. The same cannot be said for division. If you say `a / b`, this can be evaluated to some result no matter what you substitute for `a` and `b` unless you substitute `b` with `0`. Division is not total.

If you go to the Wikipedia article for division by zero you’ll find some rationale for why division by zero is undefined. The short version is that if it were defined, then it could be mathematically proven that one equals two. This would of course imply that cats and dogs live in peace together and that pigs fly, and we can’t have that!

However, there is a way we can define division to be total that doesn’t have this issue. Instead of defining division to return a number, we could define division to return a set of numbers. You can think of a set as a collection of things. We write this as a list in curly braces. `{this, is, a, set, of, words}` I have two cats named Gatito and Moogle. I can have a set of cats by writing `{Gatito, Moogle}`. Sets can be empty; we call the empty set the null set and can write it as `{}` or using this symbol `∅`. I’ll stick with empty braces because one of the things I hate about mathematics is everybody’s insistence on writing in Greek.

So here is our new total division function:

``````totalDivide(a, b)
if (b does not equal 0)
output {a / b}
otherwise
output {}
``````

If you use `totalDivide` to do your division, then you will never have to worry about the undefined behavior of division! So why didn’t Aristotle (or Archimedes or Yoda or whoever invented division) define division like this in the first place? Because it’s super annoying to deal with these sets. None of the other arithmetic functions are defined to take sets, so we’d have to constantly test that the division result did not produce the empty set, and extract the result from the set. In other words: while our division is now total, we still need to treat division by zero as a special case. Let us try to evaluate `2/2 + 2/2` and `totalDivide(2,2) + totalDivide(2,2)`:

``````1: 2/2 + 2/2
2: 1 + 1
3: 2
``````

Even showing all my work, that took only 3 lines.

``````1: let {1} = totalDivide(2,2)
2: let {1} = totalDivide(2,2)
3: 1 + 1
4: 2
``````

Since you can’t add two sets, I had to evaluate `totalDivide` out of line, and extract the values and add them separately. Even this required my human ability to look at the denominator and see that it wasn’t zero for both cases. In other words, making division total made it much more complicated to work with, and it didn’t actually buy us anything. It’s slower. It’s easier to mess up. It has no real value. As humans, it’s fairly easy for us to look at the denominator, see that it’s zero, and just say “undefined.”

## Cartons of Eggs

I’m sure many of you have a carton of eggs in your fridge. Go get me the 17th egg from your carton of eggs. Some of you will be able to do this, and some of you will not. Maybe you only have a 12 egg carton. Maybe you only have 4 eggs in your 18 egg carton, and the 17th egg is one of the ones that are missing. Maybe you’re vegan.

A basic sort of construct in programming is called an “array.” Basically, this is a collection of the same sort of things packed together in a region of memory on your computer. You can think of a carton of eggs as an array of eggs. The carton only contains one sort of thing: an egg. The eggs are all packed together right next to each other with nothing in between. There is some finite amount of eggs.

If I told you “for each egg in the carton, take it out and crack it, and dump it in a bowl starting with the first egg”, you would be able to do this. If I told you “take the 7th egg and throw it at your neighbor’s house” you would be able to do this. In the first example, you would notice when you cracked the last egg. In the second example you would make sure that there was a 7th egg, and if there wasn’t you probably picked some other egg because your neighbor is probably a jerk who deserves to have his house egged. You did this unconsciously because you are a human who can react to dynamic situations. The computer can’t do this.

If you have some array that looks like this (array locations are separated by | bars | and * stars * are outside the array) ` ***|1|2|3|*** ` and you told the computer “for each location in the array, add 1 to the number, starting at the first location” it would set the first location to be 2, the second location to be 3, the third location to be 4. Then it would interpret the bits in the location of memory directly to the right of the third location as a number, and it would add 1 to this “number” thereby destroying the data in that location. It would do this forever because this is what you told the machine to do. Suppose that part of memory was involved in controlling the brakes in your 2010 era Toyota vehicle. This is obviously incredibly bad, so how do we prevent this?

The answer is that the programmer (hopefully) knows how big the array is and actually says “starting at location one, for the next 3 locations, add one to the number in the location”. But suppose the programmer messes up, and accidentally says “for the next 4 locations” and costs a multinational company billions of dollars? We could prevent this. There are programming languages that give us ways to prevent these situations. “High level” programming languages such as Java have built-in ways to tell how long an array is. They are also designed to prevent the programmer from telling the machine to write past the end of the array. In Java, the program will successfully write |2|3|4| and then it will crash, rather than corrupting the data outside of the array. This crash will be noticed in testing, and Toyota will save face. We also have “low level” programming languages such as C, which don’t do this. Why do we use low level programming languages? Let’s step through what these languages actually have the machine do for “starting at location one, for the next 3 locations, add one to the number in the location”: First the C program:

NOTE: location[some value] is shorthand for “the location identified by some value.” egg_carton[3] is the third egg in the carton. Additionally, you should read these as sequential instructions “first do this, then do that” Finally, these examples are greatly simplified for the purposes of this article.

``````1: counter = 1
2: location[counter] = 1 + 1
3: if (counter equals 3) terminate
4: counter = 2
5: location[counter] = 2 + 1
6: if (counter equals 3) terminate
7: counter = 3
8: location[count] = 3 + 1
9: if (counter equals 3) terminate
``````

Very roughly speaking, this is what the computer does. The programmer will use a counter to keep track of their location in the array. After updating each location, they will test the counter to see if they should stop. If they keep going they will repeat this process until the stop condition is satisfied. The Java programmer would write mostly the same program, but the program that translates the Java code into machine code (called a compiler) will add some stuff:

``````1: counter = 1
2: if (counter greater than array length) crash
3: location[counter] = 1 + 1
4: if (counter equals 3) terminate
5: counter = 2
6: if (counter greater than array length) crash
7: location[counter] = 2 + 1
8: if (counter equals 3) terminate
9: counter = 3
10: if (counter greater than array length) crash
11: location[count] = 3 + 1
12: if (counter equals 3) terminate
``````

As you can see, 3 extra lines were added. If you know for a fact that the array you are working with has a length that is greater than or equal to three, then this code is redundant.

For such a small array, this might not be a huge deal, but suppose the array was a billion elements. Suddenly an extra billion instructions were added. Your phone’s processor likely runs at 1-3 gigahertz, which means that it has an internal clock that ticks 1-3 billion times per second. The smallest amount of time that an instruction can take is one clock cycle, which means that in the best case scenario, the java program takes one entire second longer to complete. The fact of the matter is that “if (counter greater than array length) crash” definitely takes longer than one clock cycle to complete. For a game on your phone, this extra second may be acceptable. For the onboard computer in your car, it is definitely not. Imagine if your brakes took an extra second to engage after you push the pedal? Congressmen would get involved!

In Java, reading off the end of an array is defined. The language defines that if you attempt to do this, the program will crash (it actually does something similar but not the same, but this is outside the scope of this article). In order to enforce this definition, it inserts these extra instructions into the program that implement the functionality. In C, reading off the end of an array is undefined. Since C doesn’t care what happens when you read off the end of an array, it doesn’t add any code to your program. C assume you know what you’re doing, and have taken the necessary steps to ensure your program is correct. The result is that the C program is much faster than the Java program.

There are many such undefined behaviors in programming. For instance, your computer’s division function is partial just like the mathematical version. Java will test that the denominator isn’t zero, and crash if it is. C happily tells the machine to evaluate `8 / 0`. Most processors will actually go into a failure state if you attempt to divide by zero, and most operating systems (such as Windows or Mac OSX) will crash your program to recover from the fault. However, there is no law that says this must happen. I could very well create a processor that sends lions to your house to punish you for trying to divide by zero. I could define `x / 0 = 17`. The C language committee would be perfectly fine with either solution; they just don’t care. This is why people often call languages such as C “unsafe.” This doesn’t mean that they are bad necessarily, just that their use requires caution. A chainsaw is unsafe, but it is a very powerful tool when used correctly. When used incorrectly, it will slice your face off.

## What To Do

So, if defining every behavior is slow, but leaving it undefined is dangerous, what should we do? Well, the fact of the matter is that in most cases, the added overhead of adding these extra instructions is acceptable. In these cases, “safe” languages such as Java are preferred because they ensure program correctness. Some people will still write these sorts of programs in unsafe languages such as C (for instance, my own DMP Photobooth is implemented in C), but strictly speaking there are better options. This is part of the explanation for the phenomenon that “computers get faster every year, but [insert program] is just as slow as ever!” Since the performance of [insert program] we deemed to be “good enough”, this extra processing power is instead being devoted to program correctness. If you’ve ever used older versions of Windows, then you know that your programs not constantly crashing is a Good Thing.

This is fine and good for those programs, but what about the ones that cannot afford this luxury? These other programs fall into a few general categories, two of which we’ll call “real-time” and “big data.” These are buzzwords that you’ve likely heard before, “big data” programs are the programs that actually process one billion element arrays. An example of this sort of software would be software that is run by a financial company. Financial companies have billions of transactions per day, and these transactions need to post as quickly as possible. (suppose you deposit a check, you want those funds to be available as quickly as possible) These companies need all the speed they can get, and all those extra instructions dedicated to totality are holding up the show.

Meanwhile “real-time” applications have operations that absolutely must complete in a set amount of time. Suppose I’m flying a jet, and I push the button to raise a wing flap. That button triggers an operation in the program running on the flight computer, and if that operation doesn’t complete immediately (where “immediately” is some fixed, non-zero-but-really-small amount of time) then that program is not correct. In these cases, the programmer needs to have very precise control over what instructions are produced, and they need to make every instruction count. In these cases, redundant totality checks are a luxury that is not in the budget.

Real-time and big data programs need to be fast, so they are often implemented in unsafe languages, but that does not mean that invoking undefined behavior is OK. If a financial company sets your account balance to be `check value / 0`, you are not going to have a good day. If your car reads the braking strength from a location off to the right of the braking strength array, you are going to die. So, what do these sorts of programs do?

One very common method, often used in safety-critical software such as a car’s onboard computer is to employ strict coding standards. MISRA C is a set of guidelines for programming in C to help ensure program correctness. Such guidelines instruct the developer on how to program to avoid unsafe behavior. Enforcement of the guidelines is ensured by peer-review, software testing, and Static program analysis.

Static program analysis (or just static analysis) is the process of running a program on a codebase to check it for defects. For MISRA C, there exists tooling to ensure compliance with its guidelines. Static analysis can also be more general. Over the last year or so, I’ve been assisting with a research project at UCSD called Liquid Haskell. Simply put, Liquid Haskell provides the programmer with ways to specify requirements about the inputs and outputs of a piece of code. Liquid Haskell could ensure the correct usage of division by specifying a “precondition” that “the denominator must not equal zero.” (I believe that this actually comes for free if you use Liquid Haskell as part of its basic built-in checks) After specifying the precondition, the tool will check your codebase, find all uses of division, and ensure that you ensured that zero will never be used as the denominator.

It does this by determining where the denominator value came from. If the denominator is some literal (i.e. the number `7`, and not some variable `a` that can take on multiple values), it will examine the literal and ensure it meets the precondition of division. If the number is an input to the current routine, it will ensure the routine has a precondition on that value that it not be zero. If the number is the output from some other routine, it verifies that the the routine that produced the value has, as a “postcondition”, that its result will never be zero. If the check passes for all usages of division, your use of division will be declared safe. If the check fails, it will tell you what usages were unsafe, and you will be able to fix it before your program goes live. The Haskell programming language is very safe to begin with, but a Haskell program verified by Liquid Haskell is practically Fort Knox!

## The Human Factor

Humans are imperfect, we make mistakes. However, we make up for it in our ability to respond to dynamic situations. A human would never fail to grab the 259th egg from a 12 egg carton and crack it into a bowl; the human wouldn’t even try. The human can see that there is only 12 eggs without having to be told to do so, and will respond accordingly. Machines do not make mistakes, they do exactly what you tell them to, exactly how you told them to do it. If you tell the machine to grab the 259th egg and crack it into a bowl, it will reach it’s hand down, grab whatever is in the space 258 egg lengths to the right of the first egg, and smash it on the edge of a mixing bowl. You can only hope that nothing valuable was in that spot.

Most people don’t necessarily have a strong intuition for what “undefined behavior” is, but mathematicians and programmers everywhere fight this battle every day.

# DMP Photo Booth 1.0

Well, the day has come and gone. DMP Photo Booth’s final test on June 21st went off without issue, and DMP Photo Booth has left Beta and is now considered “production ready”. The initial 1.0 release can be found on GitHub.

The significance of June 21st is the very reason DMP Photo Booth was created; the 21st is the day of my wedding. My wife wanted a photo booth for the reception. We looked into renting a photo booth, but it turns out that they run around \$1,000. I turned to open source. Some quick googling turned up some options, but they were all personal projects or out of date. Sure I could get somebody else’s project working, but what’s the fun in that? I decided that we didn’t need to rent one, or download one, I could build it!

In late 2013, I set to work in earnest. I had a couple of months of downtime in school, and since I’m not currently working it was the perfect time. I decided I had three main objectives for this project: get some arduino experience, get some GTK+ experience, and do this all as portably as possible. I had initially decided to mostly ignore GLib and focus on GTK, but slowly I grew to appreciate GLib for what it is: the standard library that C never had. First I used GModule to handle shared libraries in a portable manner. Next I decided to use GLib primitives to keep from having to deal with cross-platform type wonkiness. Next, having grown tired of dealing with return codes, I refactored the project to use GLib’s exception replacement: GError.

## Lessons Learned

It’s not all roses and puppies though. There are certainly things I’d do differently. DMP Photo Booth is developed in an Object Oriented style, passing opaque structs with “method” functions that operate on them. Each component of the program are organized into their own source file with file scoped globals scattered throughout. Said globals are protected by mutexes to create a semblance of thread safety. That said, threading issues have been a major thorn in my side. Long story short: I regret this design choice. While I still feel that this is the correct way to structure C code, and that if globals are required, this is the correct way to handle them; I feel that I should have made more of an effort to limit side effects. Recently, I’ve spent some time doing functional programming, and if I could do it again I’d try to write in a more functional style. Fortunately for me, this is something that a little refactoring could help with.

Additionally, one thing I thought would be a major help is something that began to be a major thorn in my side: NetBeans. As the size of the project grew, NetBeans got slower and slower. It seemed that I spent more time fiddling with IDE settings than actually coding. Even worse is that the IDE-generated makefile is so convoluted that it’s extremely difficult to modify by hand in a satisfying way. I’ve always coded with and IDE so I wouldn’t have even considered not using one, but then I spent some time with Haskell. One of Haskell’s “problems” is that it doesn’t have good IDE support. It doesn’t seem like any IDE really handles it well, so most people use Emacs. Personally, I haven’t really warmed up to Emacs, but GEdit has syntax highlighting for Haskell and a built-in terminal for GHCI. GEdit also has syntax highlighting for C. Next time, I will seriously consider using a lighter-weight text editor for a C project. All this said, I think NetBeans for Java remains the way to go.

## What’s Next

Like any program, version 1.0 is just one of many versions. There certainly remains a lot of work to do with DMP Photo Booth. Some major items you are likely to see whenever I get around to working on DMP Photo Booth some more:

#### Options Dialog

I think anybody who has seen it will agree: the options dialog in DMP Photo Booth is bad. It’s poorly organized, and kind of wonky. Personally, I modify settings using the `.rc` file, which is telling. This is certainly a high-priority improvement.

#### Functional Refactor

Like I said above, the code could use a pass to limit side effects. Funtions need to have their side effects limited, and globals need to be eliminated unless absolutely necessary. However, C is not a functional language. While one could argue that function pointers enable functional programming in C, this is a very pedantic argument. I won’t be going crazy with functional programming techniques. There will be no Monads, or for loops being turned into mappings of function pointers.

#### Optional Module API

An idea I’ve had on the back burner for a while is an optional module API. This would be used for very specific quality-of-life things. For instance, a module could provide a GTK widget to be shown in the options dialog. Any module that doesn’t want to implement any or all of the optional API can just ignore it. The module loading function will gracefully handle the `dlsym` failure, just treating it as it is: declining to implement the API. I have no plans to change the current existing API, so all you module developers can rest easy!

#### User Interface Module

It occurred to me that it might be good to have a UI module. This would provide the UI, and wouldn’t be tied to the trigger/printer/camera module start/stop system. This module would be loaded at startup and unloaded on shutdown. This would allow the Photo Booth to use different widget toolkits: QT, Curses, Cocoa, WinForms, or whatever else. Under this scheme, the current GTK+ interface would be abstracted into the reference UI Module.

# DMP Photo Booth: Release Candidate 1

It’s been a long time coming, but the day is almost here: the day DMP Photo Booth is officially released.

Following last week’s successful stress test, I’ve been doing some last-minute touch-up work. I’ve been preparing my laptop for the big day, and finding and squashing some bugs.

DMP Photo Booth RC1 using my fancy new dark GTK theme

Now things are coming along, and I feel the time has come to proceed to RC1. This means that barring any major new show stoppers, DMP Photo Booth will proceed to version 1.0 on June 22.

You can find the latest release here. On that page you’ll find the latest source for DMP Photo Booth and the reference modules, as well as a pre-compiled version for Debian/AMD64. Just extract the tarball into a folder and double-click the executable and you’re off! It comes pre-configured with sane defaults.

Moving forward, I plan to work on a “GTK Trigger Module”. This will just show a window with a button you can click to trigger the photo session. I understand that not everybody feels like constructing an Arduino thingamabober, and that this is surely the only thing preventing DMP Photo Booth from going viral on a global scale. Hopefully this is done by 1.0, but if not it will likely make it into a version 1.0.1, to be released shortly after 1.0.

# DMP Photo Booth: Crunch Time

Over the last few months, I’ve become distracted. As anybody who’s ever committed to one project can probably tell you: it stops being exciting. What was once your pride and joy becomes the daily grind. Things get stale. As was the case with me, I suspect that this happens for most people when development of new features ends and the focus shifts to bug fixes.

I became distracted. My mind began to wander to the next thing, which in my case ended up being Haskell. I began learning Haskell, and it was definitely educational. I learned a lot with Haskell, and I plan to stick with it so that when I list it on my resume, I don’t get destroyed on a whiteboard test. Then came The Great Matrix Affair of 2014; I got overwhelmed at school. I spent so much time studying and doing homework that I couldn’t muster up the motivation to program. Things fell by the wayside, as you can see in my blog post output for February. Luckily for me, that is done, and I have the next two months free to program!

## What Remains To Be Done?

It’s been a good 6 – 8 weeks since I’ve really focused on DMP Photo Booth, so the first order of business is the figure out what needs to get done. After doing some brainstorming I’ve settled on a list:

#### Finish The Trigger

I’ve mostly completed the trigger, but it doesn’t work. The button is soldered wrong, and while it was magically working for a while, it has since stopped. I need to fix the wiring issue, and then drill a hole in the box to put it through. After that and maybe a quick coat of paint it will be complete.

This particular task is due by Friday. I have a series of VIP demos coming up, the first of which is Saturday morning. A few of my cousins are coming in from out of town on Saturday to do wedding stuff, and I want to show it off then. While my cousin Allen is an engineer, and can appreciate a breadboard mockup of what will Totally Become A Real Thing, it certainly won’t be impressive. My cousin Laraine will likely be less amused, but I’m sure I’ll get a pat on the head for my “hard work”. Due to this, it’s important that the trigger be done before then.

The reference suite of modules was planned to be: a Trigger Module using my Arduino implementation, a Printer Module using CUPS, a Camera Module using LibGPhoto2, and a Lua module for each so that modules can be created using Lua. Of these, only the Lua Printer Module remains to be done. Since creating a Lua module is a trivial task (and not terribly important to be honest), this is a very low priority item.

However, the current Printer Module requires a physical printer. This might not always be ideal, since printers are big and heavy. What if you just want to bring a laptop and a camera and have a photo booth? My fiancée is having just this sort of situation; she wants to use the photo booth at her bachelorette party, but who wants to lug a huge printer to a hotel room? To solve this, I’ve promised her a Facebook Printer Module.

The idea is that when `dmp_pm_print()` is called, the photo strip will be uploaded to facebook. While I know this sort of thing can be done, I haven’t really researched it much. If it turns out that you have to pay facebook money to get this sort of access, I will find a hosting service that is free. Maybe Photobucket, or Dropbox, or whatever. The important thing is that the photo strips will end up in some central location on the internet so that anybody at the party can download the strip later. Ideally, this central location would be a facebook gallery so people can tag themselvs and be all Web 2.0.

My fiancée’s bachelorette party is in May, so this project isn’t a burning priority. However, this represents the most substantial addition of new functionality remaining to be done, so I plan to work it sooner rather than later. Code will be posted on my Github as it evolves, and like DMP Photo Booth will be available under the GPLv3.

#### Mac Support

To this point, all my development has been done in Linux. First using Ubuntu, and now using Debian. However, most people don’t use Linux. While Linux is the main target platform for DMP Photo Booth, I have been coding as portably as possible, so it should be little effort to port the application to Mac. Over the next few months, I’ll be making sure it compiles and runs correctly my old Macbook. My Macbook is vintage 2010, as such it is only running Snow Leopard. Therefore if anybody in the audience is a Mac user, and has a current version of OSX, feel free to give it a shot as well and let me know how it goes.

Ideally, my fiancée will bringing the Macbook to her bachelorette party and not my development laptop, therefore this project is due at the same time as the Facebook Printer Module, in May.

#### Bugs!

Finally, bugs. I need to identify them. I need to squash them. And I need unit tests.

After making a big show about being a good person and doing unit tests, I promptly lost the faith. Soon after implementing those first tests, I made a major change to how I handled errors in my code. Suddenly, all my tests were broken, and I was faced with the choice: rewrite them all, or delete them. After some thought I decided that my tests weren’t that great and that I’d probably change something again and break them all again. So I deleted them.

I’ve got to say, I miss those tests. There has been more than a few times where I’d refactored something and wasn’t sure if it still worked. Sure, it seems to work, but does it really? If I had unit tests in place, I could say with a greater degree of certainty that they do. Plus, “it sounds like a lot of work” is not a good reason not to do something, so it won’t be one for me.

On top of that, I’ll be identifying and squashing bugs the old fashioned way: by trying stuff. I’ve already found a few doozies, and I’m sure I’ll find more. As I find them I’m going to post them on the bug tracker for DMP Photo Booth on Github. I do this for a few reasons: 1) it will provide a public centralized repository of open issues so that I don’t lose or forget about them. 2) I would like others to post bugs here. If I post them here and show that I am fixing them, I feel it will establish confidence that it is looked at and acted upon.

This is due when DMP Photo Booth goes to version 1.0. That is planned to be on June 21, the day of my wedding. DMP Photo Booth will get a good solid night of work as the 80 or so people attending put it through its paces. Assuming all goes to plan with minimal issues, DMP Photo Booth will be declared to be out of Beta. That said, to be truly version 1.0, unit tests must be in place and “all bugs must be fixed”.

#### Packaging

Currently, DMP Photo Booth is available in source form only. No end-user ever had to compile Microsoft Office; I don’t feel they should have to compile DMP Photo Booth.

To that end, binary distributions will be available for Linux and Mac when DMP Photo Booth goes to version 1.0.

## Got My Work Cut Out For Me

It’s a long list to be sure, but I have 4 months to focus on it. However, I’ve decided that I should spend some time focusing on other things too, so that I don’t burn out. To that end, I plan to spend 1 day per week focusing on learning new technologies, and 1 day per week keeping my old skills sharp.

For new technologies, this means things like learning more Haskell, and other languages or frameworks. Whatever strikes my fancy. For old skills this means practicing with Lua and Java, and maybe even C++ if I’m feeling particularly masochistic that day. This will likely take the form of blog posts on topics relating to these, so stay tuned!

# DMP Photo Booth: First Commit

Well, it’s been a long time coming, but this weekend 45 days of work has culminated in my first milestone: DMP Photo Booth has received its first commit. If you go to this page, you’ll find the newly created Github repository for it. Feel free to go judge all the choices I made, I’ll wait…

…done? Good, now I’d like to use this post to talk about it.

## The Story So Far

So far, most of the work is done in the core. There are module stubs for each module, and they are loadable and “function”, but they don’t do much of anything. However, the UI for the core is, for the most part, done. If you build the core (Either open the projects in NetBeans or cd into its directory and enter `make build`), and run it (as of this writing, it must be run from the project root directory, since the supporting files are located here and referenced by relative paths) it will launch. At this point, you can launch the about window, the config window, or exit the program. From the config window you can load your modules (built stub modules are included for your convenience), and the status indicators on the main window will detect it.

All this is fine and good, but the real story here is the plumbing that has been laid.

## Said Plumbing

Not much works at the moment, but the groundwork has been laid for things to come. Going down the list:

#### Global Defines

In this header, you’ll find all the magic integer constant return values used throughout DMP Photo Booth and friends. I touched on this topic briefly in a previous post, but the cliff’s notes version is that I feel that using a bare integer instead of an enum reduces the burden on module developers, and that the benefits of an enum aren’t great enough to justify this burden.

As this header has no accompanying .c file, it can be `#include`d by module developers so they can make use of it if they like. I do this in my modules.

#### Module

One of the first things I worked on was module handling. This header holds all the functions required for the loading and unloading of modules, and the calling of module functions. At first I was implementing all of this with POSIX calls, but it occurred to me that I was defeating the purpose of my modular architecture by using non-portable calls. For this reason, I switched to GModule for module handling. I wrote a post about this if you want to hear more.

#### Module Callbacks

This header contains functions that will be added to modules as callbacks. Currently, this is the function that will be called when the user presses “the button”, and the function that will be called to push a message to the console queue.

#### Error Handling

My general approach to function design is that functions should not return void. All functions that would, instead return gint. This way, I have a mechanism to detect error conditions. In a language without Exception Handling, this is a nice thing to have. This header contains functions to extend this. Since sometimes you can’t return gint, I’ve created a facility to log error codes, similar to how the POSIX errno function works.

#### Console Queue

This header contains a thread safe queue to store messages to print to the console. Currently, this is implemented with a GQueue and mutex locking, but I made that choice before I knew about GAsyncQueue. I may refactor this at some point to use GAsyncQueue, but for now it works fine, and I see no immediate issue with my implementation.

The idea behind the console queue is that there should be a generic way to store messages. Currently the console is a GtkTextBuffer, but if I build around this, it limits future functionality. Suppose I wanted to support a command line only option? It’d be silly to require GTK for that, wouldn’t it?

That said, the default implementation uses a function placed in the GtkMainLoop to pop all messages out of the queue and place them into the GtkTextBuffer.

#### Module Status Watchdog

This header file is in charge of keeping those fancy module status indicators up to date. There are two main moving parts here: a thread that polls the module status, and a function run by the GtkMainLoop.

The Thread polls the modules every 3 seconds and pushes any status changes to a GAsyncQueue. Meanwhile the GtkMainLoop checks the GAsyncQueue for updates, and sets the indicator status accordingly. Why doesn’t the GtkMainLoop just check the module status you ask? I wanted to make sure the GtkMainLoop does as little work as possible. Currently the calls to check module status are pretty trivial, but there is no guarantee that they will stay that way. Since nobody likes an unresponsive interface, I kept them separate.

#### User Interface

This rather hefty header contains two things: a function to build and launch the UI, and all the callbacks called by said UI. All the callbacks are named according to their glade widget name to maintain some semblance of readability.

## The Next Step

Obviously, looking through the User Interface source, there are signals to be hooked up. But aside from that, the core is coming along. The one big portion of the core remaining to be done is the photo booth strip logic. The camera module will take X pictures and download them. At this point it is up to the core to assemble them into on image file to be printed.

For now, I will continue to hook up signals and refine the existing portions of the core. While I do this, I will research image handling libraries. I’d like to keep things within the GLib family, but failing this, I will try to keep it limited to libraries shipped with Ubuntu (The default platform). Windows compatibility will come, but version 1.0 will work on Ubuntu.

Also looming on the horizon are the modules. I have a general idea of how I will deal with these, but nothing is set in stone. The first module that I will likely handle is the trigger module. I plan to implement the trigger module using Arduino. This is a topic that interest me, but I’ve never had a good project idea. Whenever I get tired of dealing with GTK, it should be a nice change of pace.