Ask HN: how do you read code?

I don't just read code for the hell of it, I'm usually trying to get something specific out of it, so usually I try to figure out where the core of what I'm reading is at (starting with grep), occasionally cutting away the surrounding code and perhaps irrelevant comments with my editor.

That said, I'm not downplaying reading code; it's an important skill. But you don't have to just read the code of the masters; you can jump in and add to and fix bugs in stuff from a wealth of OSS projects and there you'll get feedback from those folks too as you get involved.

I personally get a much better understanding of a block of code if I treat it as a living thing and hack around with it seeing what breaks it and what makes it go.

I need to read more code... but when I do get the time to read code I never start with "main" I start by searching for a feature I am interested in and then branching out to see how this feature fits in with the whole. I normally use N++ and open a huge number of files at once, its important to me to be able to switch between files quickly and I don't want the bloat of an IDE.

In my experience, writing a lot of your own code teaches you MUCH more about good design than looking at others'. Most of the design process is actually in the reasoning behind WHY a certain process was employed, which is not easily evident from code alone!

But since you asked:

(1) Run the code and understand what the software DOES. If a library, write a driver program which tries out most of the functions.

(2) Look at the high-level classes (or the major organizational structures, e.g. source files) and figure out the purpose of each (e.g. CLogger -> logs errors to a file or the display)

(3a) (the fun way) Come up with a cool, simple feature you'd like to add, or something simple you'd like to change. Do it. Repeat.

-OR-

(3b) (the boring way) Find the main() function and step through the code with a debugger, stepping over function calls which are self-explanatory. Step into ones you're unsure about.

In reality, this all depends on how well the code is written. Understanding some code just isn't worth the price of admission.

Usually with a cross-referencer, like Doxygen.

Github is very good for reading code since it can be done through the browser. If I am going to be using a particular library, I take a look at the commit history and read the code on the more interesting commits.

Also, hanging out on IRC rooms is very helpful. Many people ask for code reviews before committing to the libraries and you get to see how the library evolves and what are the design decisions behind the changes.

Profile the code. Once you understand where the time is spent, its probably the main loop of the program or atleast will give you a clue to the main function. Ofcourse you can ask someone or read the docs, but profiling it has the added advantage of giving you the map of implementation to the intended algorithm.

Further when you profile you can get a call graph nicely drawn out using many utils available online. This instantly gives you a big picture.

I also second stepping in, adding feature, fixing a bug etc.

Reading code for large systems is rarely fun. I usually start with a specific objective, a bug or feature, to teach myself the system. I root around until I arrive at a solution. The first bug is expensive, but they become cheaper as I learn. I have tried systematic top to bottom examination, but I find my mind quickly turns to glue.

Good tools are essential. If you are programming Java, use eclipse. I hated it for the first year, but now I tolerate it. Whatever your language, use its IDE. I am also partial to UltraEdit. Write some good tools. I wrote a version of unix find for Windows among other things.

If you want code that's fun to read try the Python Cookbook. http://code.activestate.com/recipes/langs/python/ is a good resource.

I don't find 'main' and friends to be very meaningful), but sometimes it's ok to start there, if you don't have anything in particular to find. And structure is usually an artifact of behavior, so I almost always start with trying to understand the behavior.

I like to first familiarize myself with the overall program flow. With an IDE I can usually do this with 'called from' or similar views; if I can run the code, throwing stack traces paints a nice picture of potentially-interesting control paths, taking note of interesting details along the way.

Other than that, there's usually something I'm looking for when I dive into a piece of code. So I find that function or something that looks like the right area and start poking around the places it goes and the places it comes from, see what parameters are moving around, what side effects are taking place, etc.

I usually try to build a mental model of what I would build if I were trying to solve the same problem. It's usually not spot-on, but just doing a little thinking ahead of time helps me navigate the program more easily, predict what is going to happen, and be aware of recognizable handholds.

I try to keep an eye out open for abstractions and patterns so I can keep a sightly more terse mental model and hopefully fit more of the program in my head. If it's a really big chunk of code I'll keep pencil and paper handy and map out the program's design as a I go.

Oh yeah -- keep docs handy. Every now and then they actually help.

My last job involved a lot of code review. I usually started with static analysis. Cyclomatic complexity is crude, but it seems like a reasonable heuristic for identifying important or interesting portions of the code.

I start by cleaning code a lot.

I've found that often it's better to make the code more legible (indenting, IDE, etc) than just trying to parsing it without.

I would say unless the code is already in great shape normally tidying it up a bit means comprehension happens much more quickly.

Main's usually pretty useless for nontrivial apps. Usually some sort of internal/library framework structures the system.

These usually work well together:

1. Some way to cross-reference source files. e.g. ctags.

2. Source-code level documentation. Doxygen, javadoc, whatever you have.

3. Stack traces of the running app. Set breakpoints and get stack traces. If your tools will let you do this, get periodic stack traces (e.g. once every 10th of a second) for a while. Look at what the code's doing.

I just went through two systems totaling 1.5 million lines of code.

First hint: don't even think of going through all the code. Instead, figure out what's important to you, and start putting together a demand-paged map of what you need to know.

Second hint: get a paper notebook and start writing things down soon. You'll save yourself a lot of re-researching facts that way.

You need this book, it's excellent: http://www.amazon.com/dp/0201799405 .

For a new code base, that I will be working on - First read documentation - On A3 paper, I draw what I understood, to me picture gives more clarity - Understand the files/directory/toolchain structure etc. - Use ctags and cscope - Navigate by functionality

For others, I skim through books/blogs/opensource code

I use emacs and its find-tag feature to jump to the definitions of functions I don't understand.

This is a really good question and one that I'm having trouble answering. I find that I don't read through code in a definable way, it's more organic. I think it really comes down to (as previously mentioned by someone) why I'm reading through it.

Example, at work sometimes I need to read through other peoples code for reviews / bug fixes / enhancements. In that case I'll read the bits that matter or that I'm not sure what they're doing. Other times I might read, for example, the source to jQuery or a site or something to find out how something being done. In that case I'll try and follow where it does things by jumping between function as the compiler / interpreter would.

Where I read code is, again, something that isn't hard and fast. I'll more often than not just open it in a text editor (notepad as I'm a Windows man) but sometimes I'll load it into an IDE if it's particularly long or I'm having trouble following it in notepad.

Good question though!

I wrote a brief article on the topic:

http://www.swombat.com/recognising-and-learning-good

I think that the best way is not just to read, but to try to recreate the code or part of the code using the original code as a guide. It's fascinating how many decisions you'll come to that were taken in the original code too.

The good thing about doing it this way is that you'll really understand why those decisions were made. Imho you learn the most this way.

If the code has any unit tests starting from there can be a great diving point.

Depends on the language, if I'm reading Java code, then I'll use eclipse (ctrl+h is much better than ctrl+right click, when searching for a methods definition, it's the only way i can follow code that uses interfaces a lot).

If you're reading C code, vim + cscope + grep work great.

I use GDB to read code.