Principles for C programming March 15, 2017 on Drew DeVault's blog

In the words of Doug Gwyn, “Unix was not designed to stop you from doing stupid things, because that would also stop you from doing clever things”. C is a very powerful tool, but it is to be used with care and discipline. Learning this discipline is well worth the effort, because C is one of the best programming languages ever made. A disciplined C programmer will…

Prefer maintainability. Do not be clever where cleverness is not required. Instead, seek out the simplest and most understandable solution that meets the requirements. Most concerns, including performance, are secondary to maintainability. You should have a performance budget for your code, and you should be comfortable spending it.

As you become more proficient with the language and learn about more features you can take advantage of, you should also be learning when not to use them. It’s more important that a novice could understand your code than it is to use some interesting way of solving the problem. Ideally, a novice will understand your code and learn something from it. Write code as if the person maintaining it was you, circa last year.

Avoid magic. Do not use macros1. Do not use a typedef to hide a pointer or avoid writing “struct”. Avoid writing complex abstractions. Keep your build system simple and transparent. Don’t use stupid hacky crap just because it’s a cool way of solving the problem. The underlying behavior of your code should be apparent even without context.

One of C’s greatest advantages is its transparency and simplicity. This should be embraced, not subverted. But in the fine C tradition of giving yourself enough rope to hang yourself with, you can use it for magical purposes. You must not do this. Be a muggle.

Recognize and avoid dangerous patterns. Do not use fixed size buffers with variable sized data - always calculate how much space you’ll need and allocate it. Read the man pages for functions you use and handle their failure modes. Immediately convert unsafe user input into sanitized C structures. If you later have to present this data to the user, keep it in C structures until the last possible moment. Learn of and use extra care around sensitive functions like strcat.

Writing C is sometimes like handling a gun. Guns are important tools, but accidents with them can be very bad. You treat guns with care: you don’t point them at anything you love, you exercise good trigger discipline, and you treat it like it’s always loaded. And like guns are useful for making holes in things, C is useful for writing kernels with.

Take care organizing the code. Never put code into a header. Never use the inline keyword. Put separate concerns in separate files. Use static functions liberally to organize your logic. Use a coding style that gives everything enough breathing room to be easy on the eyes. Use single letter variable names when their purpose is self-evident and descriptive names when it’s not, and avoid neither.

I like to organize my code into directories that implement some group of functions, and give each function its own file. This file will often contain lots of static functions, but they all serve to organize the behavior this file is responsible for implementing. Write up a header to give others access to this module. And use the Linux kernel coding style, god dammit.

Use only standard features. Do not assume the platform is Linux. Do not assume the compiler is gcc. Do not assume the libc is glibc. Do not assume the architecture is x86. Do not assume the coreutils are GNU. Do not define _GNU_SOURCE.

If you must use platform-specific features, describe an interface for it, then write platform-specific support code separately. Under no circumstances should you ever use gcc extensions or glibc extensions. GNU is a blight on this Earth, do not let it infect your code.

Use a disciplined workflow. Have a disciplined approach to version control, too. Write thoughtful commit messages - briefly explain the change in the first line, and add justification for it in the extended commit message. Work in feature branches with clearly defined goals, and do not include changes that don’t serve that goal. Do not be afraid to rebase and edit your branch’s history so that it presents your changes clearly.

When you have to return to your code later, you will be thankful for the detailed commit message you wrote. Others who interact with your code will be thankful for this as well. When you see some stupid code, it’s nice to know what the bastard was thinking at the time, especially when the bastard in question was you.

Do strict testing and reviews. Identify the different possible code paths that your changes may take. Test each of them for the correct behavior. Give it incorrect input. Give it inputs that could “never happen”. Pay special attention to error-prone patterns. Look for places to simplify the code and make the processes clearer.

Next, give your changes to another human to review. This human should apply the same process and sign off on your changes. Review with discipline as well, taking all of the same steps. Review like it’ll be your ass on the line if there’s a problem with this code.

Learn from mistakes. First, fix the bug. Then, fix the real bug: your process allowed this mistake to happen. Bring your code reviewer into the discussion - this is their fault, too. Critically examine the process of writing, reviewing, and deploying this code, and seek out the root cause.

The solution might be simple, like adding strcat to the list of functions that should trigger your “review this code carefully” reflex. It might be employing static analysis so a computer can detect this problem for you. Perhaps the code needs to be refactored so it’s simpler and easier to spot errors in. Failing to reflect on how to avoid future fuck-ups would be the real fuck-up here.

It’s important to remember that rules are made to be broken. There may be cases where things that are discouraged should be used, and things that are encouraged disregarded. You should strive to make such cases the exception, not the norm, and carefully justify them when they happen.

C is the shit. I love it, and I hope more people can learn to see it the way I do. Good luck!

  1. Defining constants with them is fine, though ↩︎

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