To me, logging is a fancy way of saying printf-debugging. In a large project, like Chromium, it comes in very handy for learning about the code. Strategically-placed logging statements can help you prove that a function is called in response to an action, or figure out the values that some parameters take.

Chromium puts together many different projects, and they all have their logging subsystems. I will cover logging for the projects that I worked with. That being said, I'm a people pleaser, so I am likely to add a project, if you ask nicely.

Chromium

The Chromium source code follows Google's coding style, and the logging API resembles the API used in Google's internal codebases. In particular, most logging statements are compiled in both the release and the debugging builds, and can be activated via command-line flags.

To enable Chromium logging, run your build with the following arguments.

path/to/chromium --enable-logging=stderr --v=1

Logging statemtents look like using streams in the C++ standard library.

#include "base/logging.h"

 
VLOG(1) << "SomeFunction(" << arg << ")";

Using DVLOG instead of VLOG causes a logging statement not to be compiled into release builds. You shouldn't worry about this while you use logging for exploratoraty purposes. When you get in a position to commit a logging statement, your code reviewer can help you figure out the specific logging statement that you should use.

Blink

The Blink source code in /third_party/WebKit uses Apple-style logging. Most importantly, logging statements are only compiled in debugbuilds. Unfortunately, this means that if you need logging, you can't use release builds, which are faster to produce.

To enable Blink logging, run your (debug) build with the following arguments.

path/to/chromium --webcore-log-channels=Loading,ResourceLoading

The valid channels are defined in Logging.cpp in the WebKit source tree. The file has moved in the recent past, so the command below is a good way of finding it.

cd ~/chromium/src/third_party/WebKit

find . | grep Logging.cpp

Logging statements look like printf calls.

#include "platform/Logging.h"

 
LOG(Loading, "ResourceFetcher::requestResource %s", url.latin1().data());

V8

V8 can run on its own, so you should try to do your development that way whenever possible.

To enable V8 logging, run your build with the following arguments.

path/to/chromium --js-flags="--log_all --logfile=-"

Running a tool such as tick-processor requires that V8's logging output is neatly separated from Chromium's. To have V8 output its log to a file, Chromium's sandbox must be disabled. By default, V8 logs to v8.log.

path/to/chromium --no-sandbox --js-flags="--log_all"

I haven't hacked on the V8 source code yet, so I don't have an example of writing logging statements.

References