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Exploring iOS Core Bluetooth: Read Requests

Apple’s Core Bluetooth Progamming Guide does a good job of explaining how to use Core Bluetooth in an iOS app. And there are countless blog post tutorials that walk through scanning for peripherals, connecting, discovering services and characteristics, and reading/writing values. These resources were great for getting started, but eventually I ran into some questions that I couldn’t answer. Things like:

  • What happens when consecutive requests are made for multiple characteristics without waiting for a response each time?
  • Can read requests be made from multiple threads concurrently?

In this post I’ll discuss some experiments I ran to explore the -readValueForCharacteristic: method of the CBPeripheral class to try to get a better understanding of how it works.

Read more on Exploring iOS Core Bluetooth: Read Requests…

Also posted in iOS / OS X | Tagged , | Leave a comment

Test-Driven Infrastructure (TDI)

Atomic really has a passion for writing high quality code and for testing. While our internal server infrastructure has often been maintained in a semi-automated fashion, it has traditionally lagged far behind our development practices in terms of code quality, testing, and continuous integration.

Over the past year, however, Mike English and I have been working to revamp much of our server infrastructure using the Chef configuration management tool. Our goal has become to build a Test-Driven Infrastructure (TDI) in which we first write tests to model and validate the code that we later produce to configure and manage our servers and applications. Read more on Test-Driven Infrastructure (TDI)…

Also posted in Development Practices, DevOps & System Admin. | Leave a comment

Unwind Segues in iOS Storyboards

Introduced in 2012 with iOS 6, Unwind Segues give you a way to “unwind” the navigation stack and specify a destination to go back to. The first time you use them, they can be confusing. In fact there’s no other UI feature of iOS development that has caused more discussion in our office than Unwind Segues. In this post, I’ll help you understand the fundamentals.

Unwind exit buttonEver since I noticed the strange “Exit” outlet show up in Xcode, I’ve wondered what it did. Most people probably did what I did the first time they saw it. I Ctrl-draged from a button to the “Exit” outlet and nothing happened. This is the beginning of the confusion for new developers. How do I get the “Exit” outlet to do anything?

Read more on Unwind Segues in iOS Storyboards…

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Accessing a .NET Virtual Machine Application from a Host Machine

I’ve recently been working on a .NET web application. We are mainly a Mac-based development shop at Atomic, so I’m working on this application in a virtual machine. My weapons of choice are VMWare Fusion and Visual Studio 2012 (not much of a choice), but that is neither here nor there.

Our application uses the Helvetica Neue font family. This font comes pre-packaged with Mac, but isn’t included in Windows. So, in order to effectively design and preview our application, I needed to make it available from my host machine. Here is how I did that.

Read more on Accessing a .NET Virtual Machine Application from a Host Machine…

Also posted in .NET / WPF | Tagged , , , | Leave a comment

What You Should Already Know about JavaScript Scope

If you are a novice JavaScript programmer, or if you’ve been messing around with JQuery to pull off a few animations on your website, chances are you’re missing a few vital chunks of knowledge about JavaScript.

One of the most important concepts is how scope binds to “this”.

For this post, I’m going to assume you have a decent understanding of JavaScript’s basic syntax/objects and general terminology when discussing scope (block vs. function scope, this keyword, lexical vs. dynamic scoping). Read more on What You Should Already Know about JavaScript Scope…

Also posted in Web Apps | Tagged , | Leave a comment

Shellshock – CVEs, Patches, Updates, & Other Resources

First announced almost a month ago, Shellshock continues to endanger un-patched web servers and Linux devices. So what is it? How can you tell if you’re vulnerable? And how can it be addressed?

What Is Shellshock?

Shellshock is a vulnerability in the bash software program. Bash is a shell, installed to Linux and other operating systems in the Unix family. A shell is a software component that is deeply integrated into the operating system, which is what makes this vulnerability so insidious.

The Shellshock vulnerability is a bug in the parser. It was first introduced more than 20 years ago when a feature to allow exporting functions was added. The danger is that an attacker who could control the content of an environment variable could potentially execute arbitrary code on a vulnerable system. Remote code execution (RCE) vulnerabilities (also called “arbitrary code execution” vulnerabilities) are among the most dangerous. Paired with privilege escalation vulnerabilities or poor security practices (e.g. allowing web servers to run as privileged users), unaddressed arbitrary code execution vulnerabilities can lead to the complete takeover of vulnerable systems. Read more on Shellshock – CVEs, Patches, Updates, & Other Resources…

Also posted in DevOps & System Admin. | Tagged , | Leave a comment

Google Protocol Buffers Libraries for C

Google Protocol Buffers have become a very popular serialization mechanism for simple to complex/variant data structures. Due to the variation of message formats in a given protocol, the presence and/or lack of fields on an ad-hoc basis makes implementation in the C language somewhat of a challenge.

Three Protocol Buffers Libraries for C

Thanks to the open-source world, a couple of valid options exist, but choosing the right one depends on the complexity of the protocol and memory/performance requirements.

Google Protocol Buffers for C++

The obvious first choice may seem to go right to the source. Google’s Protocol Buffer Compiler supports the generation of a C++ protocol implementation from a .proto specification. Though you are developing in C, and your complier supports C++, you could use the C++ API directly from C, or at least create a C wrapper for the target protocol.

  • Pros: Formally supported by Google, bleeding edge
  • Cons: Well, it’s C++ and does lots of dynamic memory allocation

Read more on Google Protocol Buffers Libraries for C…

Also posted in C & C++ | Tagged , | 2 Comments

Using the Raspberry Pi as a Simple Current and Power Meter

On a recent project I needed to fairly accurately measure current and power consumption of a handful of parts in real time. We needed to measure current in the range of micro amps, so this was actually somewhat tricky.

I could have just used a bunch of super expensive current meters, but I managed to find a much more convenient and cheap solution using a Raspberry Pi. We were already using a Pi to allow for remote debugging, so I went looking for something I could hook up to that.

I found these ADCs:

They plug right into the GPIO headers on the Pi and even come with some python libraries for reading them.

Read more on Using the Raspberry Pi as a Simple Current and Power Meter…

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Serving Static Assets with DropWizard

Over the last few months, I’ve had the opportunity to do a lot of work with DropWizard, an awesome Java web framework for developing RESTful web services.

I was recently prototyping a small application, and I wanted to serve static files directly from DropWizard. While this isn’t what DropWizard is primarily designed to do, I didn’t want to go through the process of setting up another web server just to serve static assets for a prototype. While DropWizard has some out-of-the-box support for serving static assets, I found the documentation surrounding it to be incomplete and confusing. It actually took me a couple hours of debugging to figure out how to do exactly what I wanted, So I thought I would document what I learned here.

My end goal was to serve a small single page JavaScript app on the root of my DropWizard application, while having my DropWizard REST endpoints available at /api. The limited documentation around serving assets in DropWizard suggest that setting up an AssetsBundle for / and changing the application context path in your config file should be enough to achieve this, but that did not work for me. I had to do the following steps: Read more on Serving Static Assets with DropWizard…

Also posted in Java, Web Apps | Tagged | Leave a comment

Nicer C99 APIs with Designated Initializers

While working on a library for property-based testing in C, I discovered a trick that can be used to make significantly nicer library interfaces in C99: “designated initializers”. As of C99, struct literals can have their fields set by name, in any order. The C99 standard explicitly updated the behavior for how fields in struct literals are handled:

6.7.8 point 21:

“If there are fewer initializers in a brace-enclosed list than there are elements or members of an aggregate, or fewer characters in a string literal used to initialize an array of known size than there are elements in the array, the remainder of the aggregate shall be initialized implicitly the same as objects that have static storage duration.” (emphasis mine)

Since memory with static storage duration is already initialized to zero, this means that in C99 we can finally depend on stack-allocated structs’ fields being set to zero, rather than garbage data from previous stack frames. This is a huge improvement! If pointers to structs are used as arguments for function calls, it also gives C99 a portable way of using optional and keyword arguments. Read more on Nicer C99 APIs with Designated Initializers…

Also posted in C & C++ | Tagged , , , | 3 Comments