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Fixing GEdit’s Unconfigurable Terminal

Lately I’ve been distro hopping. Between projects and lacking inspiration, I’ve been in search of a new DE and a new text editor. I’ve come to realize that I don’t have the patience to deal with XMonad and friends, and of the mainstream DE’s, I like Gnome 3 the best.

However, that still leaves the question of the text editor. I’ve been using GEdit to do Haskell, and it’s been working for the most part. However, while GEdit supports many languages by way of GtkSourceView, it doesn’t support them all. What if I decide to learn some fringe langauge?

Of course a google search for “GEdit fringe-lang syntax highlighting” turns up “Use EMACS” or “Use VIM”. I’ve some experience with VIM from my past life, and I’ve tried to get into EMACS in my new life. Neither of these are terribly appealing. Which leaves me with GEdit for the time being.

So today, determined to do something productive, I fired up GEdit for the first time since reloading Ubuntu GNOME and was greeted with this:

not_ok

Yeah… Vaguely remembering dealing with this issue before, I went into the module preferences and tried to configure the build-in terminal.

cant_config

Irritating… After some googling, and message-board hopping, I found the answer. So that I don’t have to figure this out again next year, I’d like to preserve the solution here.

The Fixing

The fix is pretty simple. First, launch dconf Editor:

dconf_editor

If this isn’t installed, it should be in your distro’s repositories. On Ubuntu:

sudo apt-get install dconf-editor

Browse to org -> gnome -> gedit -> plugins -> terminal.

dconf-ed

There are plenty of interesting configuration options in here, things that would normally be done in the gnome-terminal profile config dialog. However, I just want my terminal to use the built-in theme. To do this, we need to ensure the use-theme-colors option is selected. After that’s done, close the dconf editor and re-open GEdit. It should now look like this:

much_better

I now have a nice, well-behaved embedded terminal. Much better!

Now For Something Completely Different

For as long as I’ve been trying (successfully or not) to program, I’ve been using C like languages. When I was a kid, I struggled in vain to learn C++. As an adult, I learned Java. After that, I used Java as a spring-board into the wonderful world of C Like Languages: C, C++, Perl, Lua. I wrote hello world in dozens of others as well. I found myself proudly proclaiming that “I’m confident I could pick up any C Like Language!”

Then one day I thought “what about the rest of them?” Sure, maybe I can speak Latin. Maybe I can pick up any Latin based language with relative ease. But what if I need to move to China? I speak C, but what if C falls out of favor for something else? I decided it was time to try something else.

But What?

C and it’s cousins broadly represent the Procedural and Object Oriented paradigms. We’ve all been there and done that. Procedures and Subroutines may or may not take arguments, do something, and may or may not return a result. The global or local state may or may not change. Loops happen. I don’t think it is a stretch to say that these are the two most mainstream paradigms. For the purposes of this blog post, I’m going to lump the Procedural and Imperative paradigms together. I understand that they are not the same thing, but roughly speaking, the Procedural paradigm is an evolution of the Imperative paradigm.

This leaves us with Functional Programming. Unlike the functions of an Object Oriented or Procedural language, the functions of a Functional language closely resemble those in math. In math, f(x+2), where x = 2, will always return 4. Similarly, a function in a Functional language will always return the same result given the same input. Where a function in a Procedural or Object Oriented language describes the steps to perform some task (usually, this involves some sort of loop construct), a function in a functional language just describes what the result of some function is. (usually involving recursion) f(x+f(x-1)) adds x to the result of f(x-1), which recursively adds x-1 to f(x'-1) and so on until the end of time.

So, what programming language to choose? Many languages support functional programming to an extent. Python, Lua, and even C# if you squint hard enough. However, these languages are multi-paradigm. As such, it will be easy to fall back into my C Like ways. What about Lisp?

Lisp is a family of languages: Common Lisp, Scheme, Clojure, Emacs Lisp. Sure, I could learn one, and theoretically be able to transition with ease, but this isn’t a level of fragmentation that I’m comfortable with. In addition, Lisps are multi-paradigm, so I’m more likely to not keep the faith. Which leaves me with…

Haskell

Haskell is a “pure” Functional programming language. While any useful program must have the side effect of reading from or writing to some external source, Haskell places that part of the program neatly in a corner. Let’s talk about some of the neat features of Haskell:

Lazy Evaluation

Expressions in Haskell are evaluated lazily. What this means is that a value isn’t computed until it’s needed. Let’s take a look at an example:

embiggen :: Int -> [Int] embiggen x = x:embiggen (x + 1)

This function takes an integer, and creates a list out of it. (Lists in Haskell behave much the same way as a normal linked list: O(1) insertion, O(n) traversal) The passed-in integer is pushed on to the front of the list resulting from embiggen (x + 1). You may have noticed that this function will go on forever. While maybe not ideal, this is ok in Haskell because of Lazy evaluation. The infinityeth element of this list will not be evaluated until it’s needed!

show (take 5 (embiggen 5)) [5,6,7,8,9] show (embiggen 5) !! 17 22 show (embiggen 5) [OMG INFINITE RECURSION!!!!!]

In the first example, we call the library function take, which returns a list containing the first n elements of the passed in list. In the second example, we call the library function !! (all operators are functions), which is the list indexing operator, which returns the nth element of the list. In a language with strict evaluation, the list would need to be completely evaluated before these things could happen. In Haskell, it doesn’t! Only in the third example, where we attempt to call show on the entire list, does infinite recursion occur.

Tail Call Optimization

This is one of those terms that gets thrown around a lot, but what does it actually mean? The short answer is that it prevents a recursive function call from consuming a new stack frame. In a language without this feature, if foo() calls itself, the new call will consume a new stack frame. This will cause a stack overflow if allowed to go on too long. In Haskell, this isn’t a problem because of Tail Call Optimization.

Type System

Haskell’s type system is quite different from the usual type systems. Sure there are Ints, Chars, Floats, Bools, and the like, but there’s more to it than that. Haskell is very strongly typed. There is no casting in Haskell, if a function takes an Int, there’s no getting around giving it an Int. However, the whole type system operates in a manner similar to generics in languages like C++ or Java. Take the following examples:

putInList :: a -> [a] putInList thing = [thing] addStuff :: (Num a) => a -> a -> a addStuff lhs rhs = lhs + rhs

The first function takes some arbitrary type, and returns a singleton list containing the passed-in argument. Much like generics, the argument shouldn’t depend on any type-specific behavior.

The second function takes two arguments of the same type that behaves like a number (Int, Float, Double, and friends) adds them, and returns the result. the addStuff function accomplishes this by specifying that arguments of type a should be members of the Num Typeclass. Despite the word “class”, Typeclasses aren’t the same as classes in Object Oriented languages. You CAN think of them as being the same as Java’s interfaces. When you create a type, you can make it a member of any number of Typeclasses. You must then implement the functions specified by the Typeclass, just like when a class in Java implements some interface, it must define the methods of that interface.

This is just the tip of the iceberg, but I’m sure you’re beginning to see how you can make very general functions in a very type-safe way.

Partial Function Application

A feature of functional programming is higher order functions. This means that functions can take functions as arguments, and functions can return functions. While nice, this isn’t exactly a new concept. Even C supports this to an extent with function pointers. What is new is partial application of functions. Recall the addStuff function above. It takes two arguments of type a and returns a result of type a. Now let’s look at an example:

doNumFunc :: (Num a) => (a -> a) -> a -> a doNumFunc f a = f a addThree :: (Num a) => a -> a addThree a = addStuff 3 a

The doNumFunc function takes a function that takes a type a and returns a type a (This is what (a -> a) means), and a second type a, and returns a type a. doNumFunc calls the passed in function with the second passed in argument. The addThree function takes a type a and returns a type a. addThree takes an argument, and calls the addStuff function we defined earlier with its argument and 3. How does this all pan out?

addThree 3 6 doNumFunc addThree 3 6

Seems pretty straightforward, right? Though, this isn’t very re-usable. What if I want to add 4? Do I need to define a function addFour? No, I can partially apply addStuff. If you call a function in Haskell with less arguments than it takes, it will return a function that takes the remaining arguments and returns a result! Observe:

doNumFunc (addStuff 3) 3 6

Now things are getting cool. By calling (addStuff 3), we’ve created a function that takes a type a, adds 3 to it, and returns the result! You can’t do that in C!

Getting Started

Excited yet? You know you are, don’t try to act like you’re not. But how does one get started? Like any language, you need two things to begin: a compiler/interpreter and some reading material.

Environment

First up, you should go download the Haskell Platform. This package contains your compiler/interpreter and all the standard libraries. Haskell can be compiled, or interpreted. Or, you could use ghci, the interactive interpreter, if you just want to doop around and try stuff.

If you’re running a Linux distro, haskell-platform is likely in the repositories. In Debian or Ubuntu, it’s a simple:

sudo apt-get install haskell-platform

… and you’re set! Unfortunately, there doesn’t seem to be a great IDE for Haskell. NetBeans definitely has nothing to offer in this regard. Luckily for us, Haskell is simple enough to not really need an IDE. GEdit, the default text editor that ships with Gnome, has built-in syntax highlighting for Haskell. Just enable the built-in terminal in GEdit to test stuff and you should be good to go. I like to run ghci in the embedded terminal to test functions as I write them. Plus, as you code, you can periodically attempt to load the script in ghci to make sure everything is formatted correctly and you haven’t messed up your syntax/types.

Literature

One of the biggest barriers to learning a new language is money. Nobody wants to put down cold hard cash on learning something new when what they have is working just fine. Luckily for us, you can learn you a Haskell for free! Learn You A Haskell For Great Good is a beginner’s guide to learning Haskell aimed at developers coming from C Like Languages. The best part is that the whole book is available to read online for free! You can check it out for the low-low price of zero dollars. If you like it, maybe you buy a copy for your bookshelf. Or maybe you just spread the word.

Whatever you do, you should have a good base of knowledge in Haskell. At that point, you can just consult Hoogle to learn more.

I Installed Something Called “Debian Unstable”

So, after weeks of procrastinating, the day finally came; it was time to upgrade Ubuntu. As many of you likely know, Ubuntu has a 6 month release cycle. New versions come out in November and April. The release of Saucy Salamander marked the first time I’ve had to deal with a Linux distro upgrade since I was running Fedora 8 back in 2008 (Not counting a brief encounter with Debian Squeeze just prior to using Ubuntu). As I recall, my attempt to upgrade to Fedora 9 was a disaster. Nothing worked, and it was a huge amount of effort. It was so bad that I decided to cut my losses and just go back to Windows Vista.

Needless to say, I wasn’t terribly excited about upgrading to Saucy. Finally, about a week ago I decided to stop being lazy and just do it. While it wasn’t quite the disaster that Fedora 9 was, I wouldn’t call the upgrade “smooth”. The first thing that I noticed was the fact that I could no longer lock the display. Since my cat likes to perform unauthorized refactoring of my code if I leave the display unlocked, this would not do. I did some googling, and it turns out that Gnome removed gnome-screensaver in Gnome 3.8. Gnome-screensaver controlled, among other things, locking the screen. All of the functionality was rolled into GDM. Ubuntu uses LightDM, so in order to protect my precious codebase I have to either switch it out for GDM, or use a Gnome shell plugin. First, I tried to install GDM, but every time I logged in I would get a popup saying that GDM crashed. I switched back to LightDM and installed the plugin. Everything seemed to be going fine, but things were just a bit more wonky. Every so often, when I’d go to unlock, the screen would freeze. I could just hope it was working and type my password and press enter to unlock it, but I like things to work right.

Not a huge deal though, I thought. I decided that I’d just grin and bear it. However, things continued to come apart. I went about re-compiling DMP Photo Booth and its modules to make sure everything was working correctly with the updated software versions. For the most part it was, but my working splash screen was broken. When shown, the window would pop, but the image on the window would not show. It seemed my call to while (gtk_events_pending()) gtk_main_iteration(); was returning early. In the course of my investigation I decided to open the Glade UI file to make sure everything was right. The only problem? The version of Glade shipped with Saucy has a major bug that causes it to crash when you open a file with a dialog in it. You can read the bug report here.

For me, this was the straw that broke the camel’s back. It was time to try a new distro.

Let’s Meet Our Contestants!

Ubuntu GNOME

I’ve been running Ubuntu for a while now, and have been mostly satisfied with it. I do have some concerns about their direction, but I’m not quite ready to break out the torches and pitch forks. However, I much prefer Gnome 3 to Unity, so I figured it was time to switch to a Gnome-centric distro. Luckily, there is a Ubuntu distro that focuses on Gnome: Ubuntu GNOME. My concern with this is that they seem to have manpower issues. I don’t feel like getting attached, just to have the rug pulled out from under me, so I won’t be using Ubuntu GNOME.

Fedora 20

I feel that it is fair to say that Fedora is to Red Hat as Ubuntu is to Debian. Fedora is an old, mainstream Linux distro that has the financial backing of a large company behind it. It is likely to be around for years to come. Better yet; Fedora is a Gnome distro. Fedora 20 ships with Gnome 3.10, the current latest and greatest.

Back in 2008, I tried to run Ubuntu. Back then, it didn’t “just work”. Fedora did. Maybe it was time to don my Fedora and come home to my first Linux distro. I downloaded the live DVD for Fedora 20, and booted it up. Everything seemed great; Gnome 3.10’s fancy new UI elements were incredibly profound. Mozart and Da Vinci would surely be reduced to tears at the sight of their magnificence. I was sold. I started the installer and got to work. I set my language, hostname, and then went to configure my partitions. …aaaaaaand no hard drives detected. Crud. After some googling around, this seems to be a known issue. The Googler told me that I could disable SELinux and it would work, but no luck. I was told that I could use the non-live image and it would work, but no luck. Well, so much for that idea. I filed my Fedora installation media in the round file and decided what to do next.

Debian Sid

It seems that the cool kids are running Debian these days. I’ve used Debian before, and had good experiences with it (uptime on my Debian Squeeze home server shows 102 days). The one sticking point is how old the software is. That is, old in the stable release; Debian Unstable has up-to-date software. The cool kids assure me that Sid is still more stable than Ubuntu or Fedora, so I decided to give it a shot.

The Installation

Installing Sid is slightly more tricky than Ubuntu or Fedora. Here’s the installation blurb on the Debian Wiki:

Use the stable installer to install a minimal stable system and then change your /etc/apt/sources.list file to testing and do an update and a dist-upgrade, and then again change your /etc/apt/sources.list file to unstable and again do an update and a dist-upgrade. ... If this seems too complicated you should probably not be using unstable.

With those words of encouragement, I set off to work. I downloaded the Debian 7 net install media, and installed. I followed the wizard, setting up the usual things. For partitioning, I formatted my /boot and / partitions, and preserved my /home partition. I spoke about this before in a previous post, but the short answer is that this keeps you from having to back up your data and settings. You should probably still do that stuff in case you do something stupid, but if all goes well you won’t need to.

When the time came to select additional software, I deselected everything. I finished the install and rebooted. The system booted up to the command line, and I logged in and su‘d to root. Now that my Wheezy install was complete, it was time to upgrade to Jessie. This is accomplished by editing /etc/apt/sources.list and changing every instance of the word wheezy to testing. While I was at it, I added contrib and non-free so I could get things like my wifi driver and flash. Next order of business was to install apt-listbugs and apt-listchanges. These two packages change apt-get to warn you of bugs in software, so you don’t blindly install some software that will light your computer on fire. After that:

apt-get update apt-get dist-upgrade

…then I ate lunch. This process will upgrade my system to testing, and it takes a while. After it’s done, I repeated the steps above, replacing all instances of testing with unstable in my sources.list. Additionally I had to delete the lines:

deb http://URL/ testing/updates main deb-src http://URL/ testing/updates main deb http://URL/debian/ testing-updates main deb-src http://URL/debian/ testing-updates main

…these don’t exist in Unstable.

While the apt-get dist-upgrade was running, it was time to watch some TV.

Finally, when apt-get dist-upgrade completed, I had a Debian Sid system. One problem: it was command line only.

A Few More Things

First things first, I needed to set up sudo:

adduser [username] sudo init 6

After the reboot, my user is set up to use sudo.

I had to install some software. First up is Gnome:

sudo apt-get install gnome

This is starts a 1.3 GB download, so I watched some more TV. When that finished, I needed to install my wifi driver so that I could disconnect my temporary cat-5 cable:

sudo apt-get install firmware-iwlwifi

Next up is the Debian laptop applications. This package installs the software that would be installed by selecting the laptop task in tasksel:

sudo apt-get install task-laptop

I rebooted into Gnome. I logged in and connected to my wifi. Since I preserved my /home partition, all my settings are still set up from Ubuntu, so there is very little asthetic configuration to be done.

The gnome package in Debian installs some other things besides Gnome. Among those things is LibreOffice, so I don’t have to worry about that. However, there are a few usability packages to install:

sudo apt-get install flashplugin-nonfree sudo apt-get install synaptic sudo apt-get install pkg-config

At this point I had a basic system set up. Now it is time to make sure DMP Photo Booth still works. Since I preserved my /home, NetBeans is still installed. However, there is no JDK installed. This was an easy fix:

sudo apt-get install openjdk-7-jdk

Now it is time to install the dependencies for DMP Photo Booth:

sudo apt-get install libmagickwand-dev sudo apt-get install libglib2.0 sudo apt-get install libgtk-3-dev sudo apt-get install cups libcups2-dev

Some of the development tools I need still aren’t installed. GCC is installed, but for some reason gdb isn’t. Also, to do work on the trigger, I’ll need avr-gcc:

sudo apt-get install gdb arduino sudo adduser [username] dialout sudo init 6

Finally, I need to install Glade to modify DMP Photo Booth’s UI:

sudo apt-get instal glade

And that’s it!

Impressions

It took me a good half of a day to get it all working, but so far so good. Iceweasel is still a thing, but mozilla.org thinks it’s the latest version of firefox, and my addons still work so I’m not going to worry about it. Plus, weasels rule and foxes drool.

Glade is working now, but DMP Photo Booth’s working screen is still broken. However, I’m beginning to think it never was really working right in the first place.

All in all, it’s been a successful load. 1 week in, and I still don’t miss Ubuntu. Hopefully Sid is good to me, and I’ve found my salvation from getting a new Distro version every 6 months.

DMP Photo Booth: Underwater

You’ve heard it before: “Premature optimization is the root of all Evil.” Capital Evil. So you go on about your day, arranging the ones and zeros in pretty christmas tree shapes and suddenly the day arrives: your program is slow as molasses. What are you going to do now?

Last monday was that day for me, and I’ve been underwater ever since. “Why is this happening to me?!” I thought. While not prematurely optimizing, I thought I did things right. I have no nested for loops. I’m not using an array when I need a list. Threads aren’t modifying the UI willy-nilly. Why has God forsaken me?

The Symptoms

I first noticed it while working on the printer module. After the program is open for some length of time, my whole computer begins to lag. Not just a little bit either; things completely fall apart. In the space of about 5 minutes, the computer becomes unusably slow. Killing the Photo Booth process doesn’t help; only physically shutting the computer off helps. Of course, the computer is so slow that I can’t use the shutdown option; I have to press The Button.

At this point, I feel some context is in order. I had been trying to figure out how to make my printer print on photo paper. Apparently printing is one of the areas Linux still hasn’t caught up to windows on, so this was proving to be difficult. After printing a few strips, I realized that my low-res photo strips weren’t going to cut it, so I bumped the resolution from 100 pixels wide to 1000. It was then that I noticed things were off.

Ten years of troubleshooting experience kicked in: “what changed?” I thought. The obvious answer was the image size. Clearly my photo strip assembly algorithm was operating at O(n^n^n) or something. What can be done?

Doing It Wrong

I took a look at my assemble strips function. After poking around for a while, I zeroed in on something that had been bugging me for a while. I had been using a function MagickResetImagePage combined with MagickCoalesceImages to composite images over each other. I had decided to use these functions before I knew this operation was called “compositing”, and I had found them in a tutorial on making animated .gif files in MagickWand. At the time, I was never really happy with this implementation, so I went back to the API docs to see if there was a function with “composite” in its name. There was.

MagickCompositeImage is a lot more intuitive to use than MagickResetImagePage. It doesn’t have that Magickal formatting string that MagickResetImagePage uses, it just takes coordinates. Perhaps this was the solution to my problem. I refactored, and recompiled.

Still broke.

Measure, Don’t Guess

That old gem: I’m sure you’ve heard it too. I decided that maybe this was my best course of action. I decided it was time to learn how to use this Valgrind thing all the Cool Kids are talking about these days. For those of you not in the know, Valgrind is a utility that will tell you various things about your program. The most important/most well-known thing that it can do for you is identify memory leaks. Thinking that prehaps I have a memory leak, I installed Valgrind and got to work.

It turns out that GTK has more than a few memory leaks. Allegedly this is due to the fact that it doesn’t cleanup on exit, relying on the OS to free the memory on program termination. While the general consensus is that this is fine, it doesn’t help us. The folks at Gnome are aware of this, and there is even a Wiki page on ways to mitigate this. The cliff’s notes version of that page being: “Just search for ‘definitely lost'”.

Armed with this piece of wisdom, I set off. I ran the Photo Booth in Valgrind, and examined the results. Valgrind actually turned up some memory leaks, which I corrected. Maybe now we’re set!

Nope.

Breaking Out The Profiler

This is what they usually want you to do when they tell you to Measure. Unfortunately for me, NetBeans’ built-in profiler is only for Java. After some google searching, I found gprof. Gprof is a pretty bare-bones profiler. It does what it says and not much else, which is fine. I hooked my program into the profiler and got to work. The results? Nothing. My two GTK idle functions ran some 7 million times, returning basically immediately each time as expected. Every other function performed as expected.

What now?

Trying The Process Monitor

Having run through Valgrind and GProf, coming out empty-handed, I was at a loss. I got into development because I wanted to fix my own broken code instead of mitigate somebody else’s, and fix it I will. Luckily I have 10 years of sysadmin experience to fall back on. I dusted off my process monitor and got to work.

I fired up DMP Photo Booth, and watched it in the process monitor. I pushed the button. I pushed it again. And again. memory use rose and fell predictably as the strip was assembled, but CPU usage stayed relatively low. Then boom!

I tried again, this time doing literally nothing. Still my computer sputtered and died. I killed the process, but again it was too late.

But wait, isn’t the OS supposed to clean up after me when my process ends? Something fishy is going on.

Have I Mentioned That Threads Are Hard?

Having eliminated all other possibilities, I was forced to consider that I was having a threading issue. “But I was so careful!” I thought. Shortly thereafter I noticed it: I was getting random pthread mutex errors on my console. Clearly I had a threading issue on my hand. Was I spawning extra threads? Was something not releasing its lock? Was I being victimized by gremlins? I set a break point on line one of main() and fired up my debugger. It was time to see just what was being done when nothing was being done.

So, I stepped through my program. Whenever I got to a g_thread_new call, I made sure the thread function was solid. Finally, I got to my g_idle_add calls. I had two of them, one to monitor the status indicators, and one to retrieve photo strip thumbnails. Both of these functions pop from a result from a GAsyncQueue. These Queues are fed by worker threads. I thought back to my profiler output and remembered how often these are called. Looking a few lines down I saw a call to g_timeout_add_seconds. This function is basically adds an idle function, but is only called at most X seconds. Maybe replacing the g_idle_add calls with g_timeout_add_seconds was my answer. I refactored and reran.

Nope.

Well, crud. “Are these functions even my problem?” I thought. I commented them out, recompiled and reran.

Fixed.

“So, what’s the difference?” I wondered. All three of these functions rely on the same basic behavior: pop from a GAsyncQueue some result placed there by a worker thread. I looked at the three threads: the thread that was working properly calls g_async_queue_ref/unref, and the two that don’t work do not take a reference, instead accessing the static global variable in their module. I refactored all thread functions that access a GAsyncQueue to take a reference and work on their local copy only. I recompiled, reran, and went to bed. 46,100 seconds later, everything was humming along just fine.

Wait, So I Just Had To Increment A Reference Count?

It certainly seemed odd. That’s like your car not starting if the headlights are out. Sure, they’re important, but the car should still start right?

Looking through the source of glib didn’t help. So far as I can tell, all that does is increment the reference count, and return a pointer. I turned to the documentation, which says “… Whenever another thread is creating a new reference of (that is, pointer to) the queue, it has to increase the reference count (using g_async_queue_ref()). Also, before removing this reference, the reference count has to be decreased (using g_async_queue_unref()). …” While not definitive, this certainly seems to indicate that taking a reference is important.

Frankly, I’m not happy about this answer. This is just the sort of magic solution that I hate; it’s fixed, but I’m not sure why. For the time being, I won’t dwell on it. Moving forward, I’ll be sure that my threads take a reference of a GAsyncQueue before calling methods on it. At some point when all of this is said and done, perhaps I’ll investigate this mysterious reference count.

I have taken away from this a new appreciation of just how brittle threads are. Sure, they are powerful, but shooting yourself in the foot with a 50 cal hurts a lot more than with a 9 mm. I’ll have to be more careful.

It was also a good introduction to GProf and Valgrind. Expect blog posts on the usage of each of these tools soon!

DMP Photo Booth: Throw It All Out

Another week goes by, another major refactor. This time on the chopping block: return codes. As you may know if you’ve been following development, I’ve been using integer return values to indicate success and failure of functions. This was working for a while, but as we all know, the worst problems don’t rear their ugly heads until some time has gone by. After working around them, I’ve realized this strategy has 3 main issues:

  • It’s really easy to just not check the return value, and not notice when some function returns DMP_PB_OMG_WERE_ALL_GONNA_DIE
  • It can be problematic if you have to return an integer as an actual result
  • You have to settle on some “failure” return value for functions that return values, and this can be a problem if you can’t guarantee some value will never be valid

Too bad I didn’t pick a modern language with exception handling…

GError To The Rescue

Luckily for me, GLib has the answer once again. GLib has a feature called GError, that is its answer to exception handling. I’d tell you all about how to use GError, but there is no need. GLib’s documentation is on par with Java’s documentation, and the GError documentation page is a shining example of how good documentation can make the difference between good and great.

Seriously, if anybody from Gnome is reading this: thank you for writing some decent documentation. This is a huge pet peeve of mine. No, your function called do_stuff(some_struct * zanzibar) is not self documenting, because if you don’t write a comment telling me if it has side effects, or if I remain responsible for zanzibar, then I have to look at your code to be sure. But I digress…

The basic idea behind GError is that any function that can throw has a GError ** as its final argument. If you’d like to call a function that can throw, you need to pass in an unallocated pointer to the function:

void some_func() { GError * error = NULL; dangerous_func(&error); ...

At this point, dangerous_func will be called, and return. Afterwards, if your GError pointer is no longer NULL, then an exception was thrown:

if (error != NULL) { //what now? }

This test is equivalent to a catch {} block. At this point, you traditionally have 3 options: Handle the exception, re-throw the exception, or wrap the exception in some other exception type and throw.

Personally, I’m a huge fan of Java’s checked exceptions. Sure, it can be annoying having to catch 15 different exceptions because some library author thought they needed that many on one method, but it sure beats having to magically know if some function throws, as C++ handles things. I feel that GError strikes a good balance. If a function has a GError ** argument, then it throws. At this point, you can check the docs to see if the author felt fit to say what GErrors they set, or you can just read the error and see what it is. You don’t have to catch some specific Exception class as in traditional exception handling, and you can determine exception type without resorting to instanceof.

Now, let’s examine our options…

Handle It

if (error != NULL) { printf("%s %d: %s\n", g_quark_to_string(error->domain), error->code, error->message); g_error_free(error); }

A GError has 3 public fields: domain, code, and message. Domain is a GQuark that is [NAMESPACE]_[MODULE]_ERROR. Think of this as your exception base class: i.e. BaseBeanFactoryBeanException. Code is an integer that represents a specific error. Think of this as your derived exception class: i.e. FluffyPinkBeanFactoryBeanException Finally, we have message. This is a human readable message for the error, and is equivalent to a Java call to Exception.getMessage().

GError has a few methods to ease working with these, but any C programmer worth their salt should be able to “make do” with these 3 fields.

Re-throw It

if (error != NULL) { g_propagate_error(this_funcs_gerror, error); return; }

Yes, it is that simple. Of course, in this case, the function signature would be:

void some_func(GError ** this_funcs_gerror)

…but aside from that, there’s not much going on here. The GError will be copied into the passed-in GError **. At this point you can (must) return; the error is now the caller’s problem!

Wrap It

if (error != NULL) { g_set_error(this_funcs_gerror, error->domain, error->code, "I know better, so I'm wrapping this!\n"); g_error_free(error); return; }

Pretty self-explanatory. As with re-throwing the exception, this requires a modified function signature. Also, this is how you’d throw a new exception as well, minus the call to g_error_free.

The Empty Catch Block

We’re all guilty of this, don’t try to act like you’re not. It’s just so much easier to do this:

try { fancyObj.someDangerousMethod(); } catch (Exception ex) {}

Don’t worry, you’re among friends. GLib has you covered on this front as well. If you pass NULL to a function that throws, it will not attempt to set an error, and you can just go on with your life:

void some_func() { dangerous_func(NULL); //catch (Exception ex) {} }

Feel free to omit that comment, I promise it’ll still work. As we all know, sometimes you just don’t care about some exception. In these cases, you aren’t forced to care, you can just pass NULL and get on with your life.

Bringing The Portability With GModule

As I’ve been writing DMP Photo Booth, I’ve been taking great pains to improve portability. I’ve got a fancy module-based architecture designed to segregate the non-portable sections of the project from the Core. The only problem? A bunch of ugly POSIX calls: dlopen(), dlsym(), and dlclose(). Kind of defeats the purpose of using modules for portability if I don’t load said modules in a portable way, doesn’t it? I thought so too…

Enter GModule

GModule is part of the GLib family of libraries. GModule provides a portable way to handle working with shared libraries. It works on any POSIX compliant platform, as well as Windows, and HP-UX via its shl_load() mechanism. You can read more about GModule in the GLib Reference Manual. While I’m sure there is some edge case that GLib doesn’t cover, this is far more portable than I’d initially envisioned DMP Photo Booth being. (Yay, HP-UX support!)

Another consideration in all of this is the adding of dependencies. However, since I’m already using GTK3 for my GUI, I already have a GLib dependency, so there is no added burden to using GModule.

The How

GModule is actually quite similar to using POSIX dlfcn.h functions. Some semantics are different, but GModule has functions that are roughly equivalent to the POSIX functions.

GModule * g_module_open(const gchar *file_name, GModuleFlags flags);

G_module_open() is the replacement for dlopen() in POSIX. The GModule pointer that it returns is the replacement for the void pointer returned by dlopen(). GModuleFlags is an integer flag that can be boolean or’d in. Your options for this are G_MODULE_BIND_LAZY and G_MODULE_BIND_LOCAL which are equivalent to RTLD_LAZY and RTLD_LOCAL.

gboolean g_module_symbol(GModule *module, const gchar *symbol_name, gpointer *symbol);

This is your replacement for dlsym(), and functions mostly in the same way. module is the gmodule pointer to extract symbols from, symbol_name is the symbol to get, and symbol is the function pointer to populate. This function returns true if successful, and false if not. This function is commonly called like this:

if (!g_module_symbol(dmp_pb_camera_module, "dmp_cm_capture", (gpointer *) &dmp_cm_capture)) { /* error handling here */ }

This idiom can be found throughout the GLib documentation. Did you see the craziness that is argument number 3? Dmp_cm_capture is a function pointer, as you may remember, but GObject tends to make things a little tricky, and will throw thousands of warnings if you don’t cast your function pointer to a gpointer *. The definition of gpointer is:

typedef void* gpointer;

That means that a gpointer * is actually void **. Therefore, you are expected to pass in the address of your pointer using an ampersand, hence (gpointer *) &dmp_cm_capture.

gboolean g_module_close(GModule *module);

As a nice change of pace from the last function, this one is quite straight forward. You pass in your GModule pointer, and it closes it, just like dlclose(), then returns true (or false, if an error occurs). Nothing particularly noteworthy here.

gboolean g_module_supported();

While this function does not line up with a POSIX function, I felt it was important to mention this one. This function returns true if you are on a platform that GModule supports. If you’re on Linux/UNIX/OSX, or Windows, or HP-UX, this should return true. If you’re hacking your Atari-2600, it will most likely return false. Stick this in front of calls to GModule functions and save yourself a headache.

Moving Forward

My design philosophy for DMP Photo Booth is that the Core should compile on any (typical) platform with no changes. Using GLib, this seems to be within reach. With GModules, GTK3 User Interfaces, I’ve kept the faith so far. Looking through the GLib documentation, it has GThreads, so when I inevitably get mired in threaded programming, I should be golden. GLib also has support for Pipes and file IO via GIOChannel, but the documentation claims only partial support in Windows.

Here’s hoping all goes well!

How to Create a Gnome 3 Launcher

Everybody loves Gnome 3, right?

I thought so.

Unfortunately, sometimes you have to install a program that’s not available in your distro’s repository. When this happens, the installer probably won’t create a launcher.

So, what to do?

Luckily for us, we can create one manually. First:

sudo vi /usr/share/applications/[appname].desktop

This will create a new text file. In this file, enter:

[Desktop Entry]
Name=[application name]
Comment=[comment text]
Exec=[executable file]
Path=[directory containing executable file
Terminal=false
Icon=[path to icon file]
Type=Application
Categories=[category of this application]

Save and close the file. Your application should show up in the activities menu.

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