startup.md

Startup

Chrome is (mostly) shipped as a single executable that knows how to run as all the interesting sorts of processes we use.

Here's an overview of how that works.

1. First there's the platform-specific entry point: wWinMain() on Windows, main() on Linux. This lives in chrome/app/chrome_exe_main_*. On Mac and Windows, that function loads modules as described later, while on Linux it does very little, and all of them call into:
2. ChromeMain(), which is the place where cross-platform code that needs to run in all Chrome processes lives. It lives in chrome/app/chrome_main*. For example, here is where we call initializers for modules like logging and ICU. We then examine the internal --process-type switch and dispatch to:
3. A process-type-specific main function such as BrowserMain() (for the outer browser process) or RendererMain() (for a tab-specific renderer process).

Platform-specific entry points

Windows

On Windows we build the bulk of Chrome as a DLL. (XXX: why?) wWinMain() loads chrome.dll, does some other random stuff (XXX: why?) and calls ChromeMain() in the DLL.

Mac

Mac is also packaged as a framework and an executable, but they're linked together: main() calls ChromeMain() directly. There is also a second entry point, in chrome_main_app_mode_mac.mm, for app mode shortcuts: "On Mac, one can't make shortcuts with command-line arguments. Instead, we produce small app bundles which locate the Chromium framework and load it, passing the appropriate data." This executable also calls ChromeMain().

Linux

On Linux due to the sandbox we launch subprocesses by repeatedly forking from a helper process. This means that new subprocesses don't enter through main() again, but instead resume from clones in the middle of startup. The initial launch of the helper process still executes the normal startup path, so any initialization that happens in ChromeMain() will have been run for all subprocesses but they will all share the same initialization.