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Getting Started with Blink Debugging


While many of the tools and tips on this page can be used for it, this page focuses on debugging Blink outside the context of the Layout Tests. For more Layout Test specific instructions, see this page. For more general Chromium debugging info, see the respective pages for debugging on Windows, Mac, and Linux.


Getting Started

There are two main ways to get into blink: via debugging the chromium binary itself or content_shell. For most purposes of exclusive Blink debugging, the latter is the recommended option because it drastically reduces size and complexity. This means building content_shell, which should be as simple as making it the build target for your build method of choice. This should stick a content_shell binary in your out/Debug or out/Release directory, depending on your build type.

content_shell itself takes as an argument the HTML file you wish to run Blink on. Furthermore, one of the simplest types of debugging you might want to do is to see the basic page structure after a page load (this internal structure in Blink is called the Layout Tree, not to be confused with the DOM Tree or the Line Box Tree). You can do this with a simple command line option of --dump-render-tree. Thus, one of your simplest debugging tools, seeing the page structure after a page load, might look something like:
content_shell --dump-render-tree test.html

Starting the Debugger

Debugging on Linux is generally done with GDB, so we will assume that's what you are using here. Not surprisingly, you will almost always want to compile Blink in debug mode to get all the symbols and tools you will need.

Assuming you are using content_shell, you will probably want to run it in single process mode, unless, of course, you are dealing with threading issues. This will greatly simplify your setup and speed up your debugging process. Doing this is as simple as starting content_shell with the --single-process flag.

A common GDB command to get everything started is:
gdb --args content_shell --single-process test.html
If you can't use --single-process  you can use --renderer-startup-dialog instead. Simply run content_shell with the normal flags you pass it as well as --renderer-startup-dialog  The renderer will start in a paused state and allow you to attach a debugger before continuing.

General Useful Debugging Tools

Debugging functions

There are some key functions built into objects once you've reach a breakpoint inside Blink. For the examples here, we'll assume you're using GDB on Linux. These can be incredibly useful for showing the trees midway during execution to try and identify points when things change. You can use the GDB command print to display them. Here are some of Blink's debugging functions:
 FunctionObjects it's available on Description
 showTreeForThis()Nodes and LayoutObjectsOutputs the DOM tree, marking this with a *
 showLayoutTreeForThis()LayoutObjectsOutputs the Layout tree, marking this with a *
 showLineTreeForThis()LayoutObjects and InlineBoxesOutputs the Inline Box tree for the associated block flow, marking all matching inline boxes associated with this with a *
 showDebugData()DisplayItemListsOutputs the list of display items and associated debug data

Assuming a local variable child in scope that's a LayoutObject, the following will print the Layout Tree:

(gdb) print child->showLayerTreeForThis()

Blink GDB python library

When using a GDB build that supports python, there's a library of useful Blink functions and pretty printers that can make working with some of Blink's types easier and more convenient, such as pretty printers for LayoutUnit and LayoutSize classes. You can find it at third_party/webkit/Tools/gdb/; see LinuxDebugging for instructions.

Printing back trace

Make ASSERT print symbols

Run Chrome with --no-sandbox. You'll see Chromium's back trace for SEGV.

Use Chromium's StackTrace

#include <base/debug/stack_trace.h>
base::debug::StackTrace st;

and run Chrome with --no-sandbox command line option. Because base is not included in blink by default, this will only work if you do not use the component build (or, you update gyp/gn to include base in blink).

Make BACKTRACE() print symbols without Chromium code

Use component build mode

$ build/gyp_chromium -D"component=shared_library"

and then BACKTRACE(), ASSERT(), etc. will print (a few) symbols.