Unionize Your Variables – An Introduction to Advanced Data Types in C

Programming C without variables is like, well, programming C without variables. They are so essential to the language that it doesn’t even require an analogy here. We can declare and use them as wildly as we please, but it often makes sense to have a little bit more structure, and combine data that belongs together in a common collection. Arrays are a good start to bundle data of the same type, especially when there is no specific meaning of the array’s index other than the value’s position, but as soon as you want a more meaningful association of each value, arrays will become limiting. And they’re useless if you want to combine different data types together. Luckily, C provides us with proper alternatives out of the box.

This write-up will introduce structures and unions in C, how to declare and use them, and how unions can be (ab)used as an alternative approach for pointer and bitwise operations.

Continue reading “Unionize Your Variables – An Introduction to Advanced Data Types in C”

Hack your C++ with LLVM

Have you ever wanted to analyze or mutate some C or C++ code? You can do some simple pattern matching with regular expressions, but there’s always some special case or another that will break your logic. To do it right, you need to develop an entire parser, perhaps using a formal grammar and a tool like Yacc. That’s a big job, though, just to change all the floats to doubles.

[Adrian Sampson] wrote a blog entry to make you go from “mostly uninterested in compilers to excited to use LLVM to do great work.” LLVM – the Low Level Virtual Machine compiler infrastructure — provides tools for a lot of languages, including CLANG for C and C++. [Adrian] points out a few key differences between LLVM and other compilers and tools you might use for a similar purpose:

  • LLVM uses a consistent intermediate representation that is human-readable
  • It is extremely modular
  • It is both highly hackable and an industrial-strength, well-supported compiler

He points out that compiler tools aren’t just for compiling. You can use them to analyze source code, build simulators, and inject code for security or testing, among other things (speaking of security testing, check out the use of LLVM to analyze binaries for security issues in the video after the break). The high hackability of LLVM is due to its modular nature. By default, a front end chews up the C or C++ code into the intermediate representation. Then multiple passes can modify the representation before handing it off for the next pass. The final pass does actual code generation for the target processor.

Continue reading “Hack your C++ with LLVM”

C bit field structures for microcontroller multitasking

So you’re getting better at programming microcontrollers and now you want to do several things at once? You know better than that, microcontrollers are only capable of processing one thing at a time. But if you’re clever with your coding you can achieve something that behaves as if several things are going on at once. The most common way to do this is to set a flag using an interrupt, then use the main loop to check for that flag. [S1axter] posted a tutorial on this topic where he uses bit field structures to help simplify time sensitive events.

We think [S1axter] did a fantastic job of explaining this moderately difficult topic clearly and quickly. In the video after the break he begins by explaining what a bit field is and how it is defined. Basically you’re using a C structure to track a flag using just one bit of storage. This way the flag is either set or not. We suggest you pay careful attention to how he declares the structures as volatile, so you don’t have unexpected behavior when you try it yourself.

Continue reading “C bit field structures for microcontroller multitasking”