Hacking on Blink and Chromium

This blog documents my attempts to use and improve the Web platform

Searching the Chromium Codebase

After I started programming professionally, I was literally shocked when I asked my tech lead “where is X implemented?”, and his answer was “let’s grep for it”. My schoolwork consisted of writing a new piece of software, or completely understanding a piece of software, and then using or modifying it. At the same time, if I wanted to look up a piece of information, I would use Google instead of going to a library and trying to figure out what book might hold my answer.

Long story short, grep is a central piece to finding one’s way in a large codebase, just like Google is the only sensible way of finding specific bits of information in the large pool of human knowledge that we have on the Web.

Git Grep

git grep works for any repository, so it was go-to tool when I started hacking on the Chromium codebase. I still use it when I’m traveling and I don’t have Internet access. For most cases, Google’s code search (covered in the following section) is a better tool.

git grep is better than the plain vanilla grep because it searches over the entire repository. However, Chromium uses many repositories, so git grep does not cover the entire codebase.

For example, the search below does not cover the Blink repository.

cd ~/chromium/src
git grep -n -3 allowImage

Moving to Blink’s root directory will make git grep search the Blink repository. For this reason, I usually have at least one Terminal tab open in ~/chromium/src and one tab open in ~/chromium/src/third_party/WebKit.

cd ~/chromium/src/third_party/WebKit
git grep -n -3 allowImage

I found the command-line arguments above to be generally useful. -n shows line numbers, -3 provides 3 lines of context above and below the match. 3 is not a magic value, all digits work similarly.

Bash assigns special meaning to some characters. Remember to escape them or to quote the pattern when necessary.

cd ~/chromium/src/third_party/WebKit
git grep -n -3 "allowImage("
git grep -n -3 allowImage\(

If you find yourself using git grep a lot (presumably for other projects), consider upgrading to ack.

Code Search: Grep on Steroids

Google’s Code Search service was shut down in 2013, but it is still up and running for the Chromium code base, at cs.chromium.org.

The main advantage of Chromium Code Search is the search speed. For example, time git --no-pager grep -n -3 "allowImage(" took 1.9 seconds in the main Chromium repository and 2.7 seconds in the Blink repository, on a high-end mid-2012 Retina MacBook Pro. Chromium Code search answered the same query for the entire Chromium codebase in 0.2 seconds.

Here are the three features that I didn’t discover right away, but I used a lot once I figured them out.

  1. Hovering over a name turns it into a link to the definition for the respective variable, function, method or class. My Chromium setup opens a link in a new tab when I click on the middle mouse button, and I use this a lot when exploring Chromium code.

  2. Clicking over a name where it is defined opens up the XRefs (cross-references) pane, which is much better than a raw search for commonly used names.

  3. The file tree on the left can be replaced with an outline of the current file. Look for the Outline link above the tree.

If you’d like to use code search for other projects, you might be interested in the open-sourced Google Code Search back-end code. Unfortunately, this is not the full code for the Web application, but rather a starting point. Even if you don’t end up using the implementation there, the comments in the code are very insightful.

Active Exploration

This post covered passively exploring the Chromium codebase by searching. Sometimes, passive search is inefficient, and must be complemented by active exploration methods such as logging and getting stack traces (upcoming post).