Debugging on macOS

If you experience crashes or issues in Electron that you believe are not caused by your JavaScript application, but instead by Electron itself, debugging can be a little bit tricky, especially for developers not used to native/C++ debugging. However, using lldb, and the Electron source code, you can enable step-through debugging with breakpoints inside Electron’s source code. You can also use XCode for debugging if you prefer a graphical interface.

Requirements

  • A debug build of Electron: The easiest way is usually building it yourself, using the tools and prerequisites listed in the build instructions for macOS. While you can attach to and debug Electron as you can download it directly, you will find that it is heavily optimized, making debugging substantially more difficult: The debugger will not be able to show you the content of all variables and the execution path can seem strange because of inlining, tail calls, and other compiler optimizations.

  • Xcode: In addition to Xcode, also install the Xcode command line tools. They include LLDB, the default debugger in Xcode on macOS. It supports debugging C, Objective-C and C++ on the desktop and iOS devices and simulator.

  • .lldbinit: Create or edit ~/.lldbinit to allow Chromium code to be properly source-mapped.

    1. # e.g: ['~/electron/src/tools/lldb']
    2. script sys.path[:0] = ['<...path/to/electron/src/tools/lldb>']
    3. script import lldbinit

Attaching to and Debugging Electron

To start a debugging session, open up Terminal and start lldb, passing a non-release build of Electron as a parameter.

  1. $ lldb ./out/Testing/Electron.app
  2. (lldb) target create "./out/Testing/Electron.app"
  3. Current executable set to './out/Testing/Electron.app' (x86_64).

Setting Breakpoints

LLDB is a powerful tool and supports multiple strategies for code inspection. For this basic introduction, let’s assume that you’re calling a command from JavaScript that isn’t behaving correctly - so you’d like to break on that command’s C++ counterpart inside the Electron source.

Relevant code files can be found in ./shell/.

Let’s assume that you want to debug app.setName(), which is defined in browser.cc as Browser::SetName(). Set the breakpoint using the breakpoint command, specifying file and line to break on:

  1. (lldb) breakpoint set --file browser.cc --line 117
  2. Breakpoint 1: where = Electron Framework`atom::Browser::SetName(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&) + 20 at browser.cc:118, address = 0x000000000015fdb4

Then, start Electron:

  1. (lldb) run

The app will immediately be paused, since Electron sets the app’s name on launch:

  1. (lldb) run
  2. Process 25244 launched: '/Users/fr/Code/electron/out/Testing/Electron.app/Contents/MacOS/Electron' (x86_64)
  3. Process 25244 stopped
  4. * thread #1: tid = 0x839a4c, 0x0000000100162db4 Electron Framework`atom::Browser::SetName(this=0x0000000108b14f20, name="Electron") + 20 at browser.cc:118, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
  5.     frame #0: 0x0000000100162db4 Electron Framework`atom::Browser::SetName(this=0x0000000108b14f20, name="Electron") + 20 at browser.cc:118
  6.    115  }
  7.    116
  8.    117  void Browser::SetName(const std::string& name) {
  9. -> 118    name_override_ = name;
  10.    119  }
  11.    120
  12.    121  int Browser::GetBadgeCount() {
  13. (lldb)

To show the arguments and local variables for the current frame, run frame variable (or fr v), which will show you that the app is currently setting the name to “Electron”.

  1. (lldb) frame variable
  2. (atom::Browser *) this = 0x0000000108b14f20
  3. (const string &) name = "Electron": {
  4.     [...]
  5. }

To do a source level single step in the currently selected thread, execute step (or s). This would take you into name_override_.empty(). To proceed and do a step over, run next (or n).

  1. (lldb) step
  2. Process 25244 stopped
  3. * thread #1: tid = 0x839a4c, 0x0000000100162dcc Electron Framework`atom::Browser::SetName(this=0x0000000108b14f20, name="Electron") + 44 at browser.cc:119, queue = 'com.apple.main-thread', stop reason = step in
  4.     frame #0: 0x0000000100162dcc Electron Framework`atom::Browser::SetName(this=0x0000000108b14f20, name="Electron") + 44 at browser.cc:119
  5.    116
  6.    117  void Browser::SetName(const std::string& name) {
  7.    118    name_override_ = name;
  8. -> 119  }
  9.    120
  10.    121  int Browser::GetBadgeCount() {
  11.    122    return badge_count_;

NOTE: If you don’t see source code when you think you should, you may not have added the ~/.lldbinit file above.

To finish debugging at this point, run process continue. You can also continue until a certain line is hit in this thread (thread until 100). This command will run the thread in the current frame till it reaches line 100 in this frame or stops if it leaves the current frame.

Now, if you open up Electron’s developer tools and call setName, you will once again hit the breakpoint.

Further Reading

LLDB is a powerful tool with a great documentation. To learn more about it, consider Apple’s debugging documentation, for instance the LLDB Command Structure Reference or the introduction to Using LLDB as a Standalone Debugger.

You can also check out LLDB’s fantastic manual and tutorial, which will explain more complex debugging scenarios.