First see get the code for checkout and build instructions.
You can use Visual Studio's built-in debugger or WinDBG to debug Chromium. You don't need to use the IDE to build in order to use the debugger: most developers use Ninja to build, and then open the IDE for debugging as necessary. To start debugging an executable from the command line:
This assumes you have Visual Studio installed and have devenv.exe on your path. Note that the path to the binary must use backslashes and must include the ".exe" suffix or Visual Studio will open and do nothing.
Goma (the internal Google distributed build) will not produce symbols so most debugging won't work (see discussion thread). You will want to do local builds when using a debugger.
It's a good idea to use a different profile for your debugging. If you are debugging Google Chrome branded builds, or use a Chromium build as your primary browser, the profiles can collide so you can't run both at once, and your stable browser might see profile versions from the future (Google Chrome and Chromium use different profile directories by default so won't collide). Use the command-line option:
Using the IDE, go to the Debugging tab of the properties of the chrome project, and set the Command Arguments.
Enable Chrome debug logging to a file by passing --enable-logging --v=1 command-line flags at startup. Debug builds place the chrome_debug.log file in the out\Debug directory. Release builds place the file in the top level of the user data Chromium app directory, which is OS-version-dependent. For more information, see logging and user data directory details.
If you are debugging official Google Chrome release builds, use the symbol server:
In Visual Studio, this goes in Tools > Options under Debugging > Symbols. It will be faster if you set up a local cache in a empty directory on your computer.
Chromium can be challenging to debug because of its multi-process architecture. When you select Run in the debugger, only the main browser process will be debugged. The code that actually renders web pages (the Renderer) and the plugins will be in separate processes that's not (yet!) being debugged. The ProcessExplorer tool has a process tree view where you can see how these processes are related. You can also get the process IDs associated with each tab from the Chrome Task Manager (right-click on an empty area of the window title bar to open).
The easiest way to debug issues is to run Chromium in single-process mode. This will allow you to see the entire state of the program without extra work (although it will still have many threads). To use single-process mode, add the command-line flag
This approach isn't perfect because some problems won't manifest themselves in this mode and some features don't work and worker threads are still spawned into new processes.
You can attach to the running child processes with the debugger. Select Tools > Attach to Process and click the chrome.exe process you want to attach to. Before attaching, make sure you have selected only Native code when attaching to the process This is done by clicking Select... in the Attach to Process window and choosing Native. You can now debug the two processes as if they were one. When you are debugging multiple processes, open the Debug > Windows > Processes window to switch between them.
Sometimes you are debugging something that only happens on startup, and want to see the child process as soon as it starts. Use:
You have to disable the sandbox or the dialog box will be prohibited from showing. When the dialog appears, visit Tools > Attach to Process and attach to the process showing the Renderer startup dialog. Now you're debugging in the renderer and can continue execution by pressing OK in the dialog.
Startup dialogs also exist for other child process types: --gpu-startup-dialog, --ppapi-startup-dialog, --plugin-startup-dialog (for NPAPI).
You can also try the vs-chromium plug-in to attach to the right processes.
The following flags cause child processes to wait for 60 seconds in a busy loop for a debugger to attach to the process. Once either condition is true, it continues on; no exception is thrown.
--wait-for-debugger-children[=filter]The filter, if provided, will fire only if it matches the --type parameter to the process. Values include renderer, plugin (for NPAPI), ppapi, gpu-process, and utility.
When using this option, it may be helpful to limit the number of renderer processes spawned, using:
The debugger can be configured to automatically not step into functions based on regular expression:
V8 supports many command-line flags that are useful for debugging. V8 command-line flags can be set via the Chromium command-line flag --js-flags; for instance:
chrome.exe --js-flags="--trace_exception --heap_stats"
Note that some V8 command-line flags exist only in the debug build of V8. For a list of all V8 flags try:
GPU Acceleration of rendering can be more easily debugged with tools. See:
Sometimes it's useful to debug installation and execution on a machine other than your primary build box. To run the installer on said other machine, first build the mini_installer target on your main build machine (e.g., ninja -C out\Debug mini_installer). Next, on the debug machine:
Consider reading the documentation at the top of copy-installer.bat to see how you can run it. It tries to be smart and copy the right things, but you may need to be explicit (e.g., "copy-installer.bat out Debug"). It is safe to re-run the script to copy only modified files (after a rebuild, for example).
The Windows heap manager has a really useful debug flag, where it can be asked to capture and store a stack trace with every allocation. The tool to scrape these stack traces out of processes is UMDH, which comes with WinDbg.
UMDH is great. It will capture a snapshot of the heap state as many times as you like, and it'll do it fairly quickly. You then run it again against either a single snapshot, or a pair of snapshots, at which time it'll symbolize the stack traces and aggregate usage up to unique stack traces.
Turning on the user stack trace database for chrome.exe with gflags.exe makes it run unbearably slowly; however, turning on the user stack trace database on for the browser alone is just fine.
While it's possible to turn on the user stack database with the "!gflag" debugging extension, it's too late to do this by the time the initial debugger breakpoint hits. The only reasonable way to do this is to
If you then ever suffer a browser memory leak, you can snarf a dump of the process with