Directly Executing Chunks of Memory: Function Pointers In C

In the first part of this series, we covered the basics of pointers in C, and went on to more complex arrangements and pointer arithmetic in the second part. Both times, we focused solely on pointers representing data in memory.

But data isn’t the only thing residing in memory. All the program code is accessible through either the RAM or some other executable type of memory, giving each function a specific address inside that memory as entry point. Once again, pointers are simply memory addresses, and to fully utilize this similarity, C provides the concept of function pointers. Function pointers provide us with ways to make conditional code execution faster, implement callbacks to make code more modular, and even provide a foothold into the running machine code itself for reverse engineering or exploitation. So read on!

Function Pointers

In general, function pointers aren’t any more mysterious than data pointers: the main difference is that one references variables and the other references functions. If you recall from last time how arrays decay into pointers to their first element, a function equally decays into a pointer to the address of its entry point, with the () operator executing whatever is at that address. As a result, we can declare a function pointer variable fptr and assign a function func() to it: fptr = func;. Calling fptr(); will then resolve to the entry point of function func() and execute it.

Admittedly, the idea of turning a function into a variable may seem strange at first and might require some getting used to, but it gets easier with time and it can be a very useful idiom. The same is true for the function pointer syntax, which can be intimidating and confusing in the beginning. But let’s have a look at that ourselves.

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When 4 + 1 Equals 8: An Advanced Take On Pointers In C

In our first part on pointers, we covered the basics and common pitfalls of pointers in C. If we had to break it down into one sentence, the main principle of pointers is that they are simply data types storing a memory address, and as long as we make sure that we have enough memory allocated at that address, everything is going to be fine.

In this second part, we are going to continue with some more advanced pointer topics, including pointer arithmetic, pointers with another pointer as underlying data type, and the relationship between arrays and pointers. But first, there is one particular pointer we haven’t talked about yet.

The one proverbial exception to the rule that pointers are just memory addresses is the most (in)famous pointer of all: the NULL pointer. Commonly defined as preprocessor macro (void *) 0, we can assign NULL like any other pointer.

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The Basics and Pitfalls of Pointers in C

Pointers — you either love them, or you haven’t fully understood them yet. But before you storm off to the comment section now, pointers are indeed a polarizing subject and are both C’s biggest strength, and its major source of problems. With great power comes great responsibility. The internet and libraries are full of tutorials and books telling about pointers, and you can randomly pick pretty much any one of them and you’ll be good to go. However, while the basic principles of pointers are rather simple in theory, it can be challenging to fully wrap your head around their purpose and exploit their true potential.

So if you’ve always been a little fuzzy on pointers, read on for some real-world scenarios of where and how pointers are used. The first part starts with regular pointers, their basics and common pitfalls, and some general and microcontroller specific examples.

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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.

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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.

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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.

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