Debugging Native Code¶
Table of contents¶
Debugging Native Code in Android Studio.¶
If you want to work on the C++ code that powers GeckoView, you will need to be able to perform native debugging inside Android Studio. This article will guide you through how to do that.
If you need to get set up with GeckoView for the first time, follow the Quick Start Guide.
Perform a debug build of Gecko.¶
mozconfigfile and add the following lines. These will ensure that the build includes debug checks and symbols.
ac_add_options --enable-debug ac_add_options --with-android-ndk="<path>/.mozbuild/android-ndk-r17b"
Ensure that the following lines are commented out in your
./mach configurewill not allow artifact builds to be enabled when generating a debug build.
# ac_add_options --enable-artifact-builds
To be absolutely sure that Android Studio will pick up your debug symbols, the first time you perform a debug build it is best to clobber your
MOZ_OBJDIR. Subsequent builds should not need this step.
Build as usual. Because this is a debug build, and because you have clobbered your
MOZ_OBJDIR, this will take a long time. Subsequent builds will be incremental and take less time, so go make yourself a cup of your favourite beverage.
Set up lldb to find your symbols¶
~/.lldbinit file (or create one if one does not already
exist) and add the following lines.
The first line tells LLDB to enable inline breakpoints - Android Studio will need this if you want to use visual breakpoints.
The remaining lines tell LLDB where to go to find the symbols for debugging.
settings set target.inline-breakpoint-strategy always settings append target.exec-search-paths <PATH>/objdir-android-opt/toolkit/library/build settings append target.exec-search-paths <PATH>/objdir-android-opt/mozglue/build settings append target.exec-search-paths <PATH>/objdir-android-opt/security
Set up Android Studio to perform native debugging.¶
Edit the configuration that you want to debug by clicking
Run -> Edit Configurations...and selecting the correct configuration from the options on the left hand side of the resulting window.
Debug typeselect box. Dual will allow debugging of both native and Java code in the same session. It is possible to use
Native, but it will only allow for debugging native code, and it’s frequently necessary to break in the Java code that configures Gecko and child processes in order to attach debuggers at the correct times.
Symbol Directories, add a new path pointing to
<PATH>/objdir-android-opt/toolkit/library/build, the same path that you entered into your
OKto close the window.
Debug Native code in Android Studio¶
The first time you are running a debug session for your app, it’s best to start from a completely clean build. Click
Build -> Rebuild Projectto clean and rebuild. You can also choose to remove any existing builds from your emulator to be completely sure, but this may not be necessary.
If using Android Studio visual breakpoints, set your breakpoints in your native code.
Run the app in debug mode as usual.
When debugging Fennec or geckoview_example, you will almost immediately hit a breakpoint in
ElfLoader.cpp. This is expected. If you are not using Android Studio visual breakpoints, you can set your breakpoints here using the lldb console that is available now this breakpoint has been hit. To set a breakpoint, select the app tab (if running Dual, there will also be an
<app> javatab) from the debug window, and then select the
lldbconsole tab. Type the following into the console:
b <file>.cpp:<line number>
Once your breakpoints have been set, click the continue execution button to move beyond the
ElfLoaderbreakpoint and your newly set native breakpoints should be hit. Debug as usual.
Attaching debuggers to content and other child processes¶
Internally, GeckoView has a multi-process architecture. The main Gecko process lives in the main Android process, but content rendering and some other functions live in child processes. This balances load, ensures certain critical security properties, and allows GeckoView to recover if content processes become unresponsive or crash. However, it’s generally delicate to debug child processes because they come and go.
The general approach is to make the Java code in the child process that you want to debug wait for a Java debugger at startup, and then to connect such a Java debugger manually from the Android Studio UI.
Bug 1522318 added environment variables that makes GeckoView wait for Java debuggers to attach, making this debug process more developer-friendly. See Configuring GeckoView for Automation for instructions on how to set environment variables that configure GeckoView’s runtime environment.
Making processes wait for a Java debugger¶
The following environment variable makes the main (Gecko) process wait for a Java debugger to connect:
This is a superset of Android Studio’s built-in debugging support so it’s not particularly useful (unless you want to attach a different jdwp debugger).
The following environment variable makes every child process wait for a Java debugger to connect:
MOZ_DEBUG_CHILD_WAIT_FOR_JAVA_DEBUGGER=suffix in the environment
to make child processes with an Android process name ending with
suffix wait for a Java debugger to connect. For example, the
following environment variable makes every child content process wait
for a Java debugger to connect:
An easy way to set this is with
./mach run --setenv MOZ_DEBUG_CHILD_WAIT_FOR_JAVA_DEBUGGER=:tab
Attaching a Java debugger to a waiting child process¶
This is standard: follow the Android Studio instructions.
You must attach a Java debugger, so you almost certainly want to attach
Dual debugger and you definitely can’t attach only a
Determining the correct process to attach to is a little tricky because
the mapping from process ID (pid) to process name is not always clear.
Gecko content child processes are suffixed
:tab at this time.
If you attach
Dual debuggers to both the main process and a content
child process, you will have four (4!) debug tabs to manage in Android
Studio, which is awkward. Android Studio doesn’t appear to configure
attached debuggers in the same way that it configures debuggers
connecting to launched Run Configurations, so you may need to manually
configure search paths – i.e., you may need to invoke the contents of
lldbinit file in the appropriate
lldb console by hand,
using an invocation like
command source /absolute/path/to/topobjdir/lldbinit.
Android Studio also doesn’t appear to support targeting breakpoints from
the UI (say, from clicking in a gutter) to specific debug tabs, so you
may also need to set breakpoints in the appropriate
lldb console by
Managing more debug tabs may require different approaches.
Using Android Studio on Windows¶
You can now use artifact builds
mode on MozillaBuild environment even if you are
not using WSL. If you want to debug GeckoView using Android Studio on
Windows, you have to set an additional environment variable via the
Control Panel to run the gradle script. The
mach command sets these
variables automatically, but Android Studio cannot.
If you install MozillaBuild tools to
installation path), you have to set the
variable to recognize MozillaBuild installation path.
To set environment variable on Windows 10, open the
Windows System, then select
System and Security -
Advanced system settings -
Environment Variables ....
To set the
MOZILLABUILD variable, click
User variables for, then
Variable name: is
Variable value: is
You also have to append some tool paths to the
To append the variables to PATH, double click
User Variables for, then click
New. And append the following