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
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 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.
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
I found the command-line arguments above to be generally useful.
-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.
If you find yourself using
git grep a lot (presumably for other projects),
consider upgrading to ack.
Code Search: Grep on Steroids
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.
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.
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.
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.
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).