Firefox OS启动过程分析－b2g进程启动

来源：互联网发布：uncomtrade数据库编辑：程序博客网时间：2024/06/05 08:11

b2g启动时，运行”/system/b2g/b2g”，入口在”gecko/b2g/app/B2GLoader.cpp”中，如下：

intmain(int argc, const char* argv[]){  /**   * Reserve file descriptors before loading static data.   */  FdArray reservedFds;  ReserveFileDescriptors(reservedFds);  /*   * Before fork(), libxul and static data of Gecko are loaded for   * sharing.   */  bool ok = LoadStaticData(argc, argv);  if (!ok) {    return 255;  }  return RunProcesses(argc, argv, reservedFds);

代码执行流程如下所示：

main()     (b2g/app/B2GLoader.cpp)||-ReserveFileDescriptors||-LoadStaticData||-RunProcesses       |       |-fork       |       |-XRE_ProcLoaderServiceRun      (child(nuwa) process)       |       |-b2g_main               (parent(b2g) process) (b2g/app/nsBrowserApp.cpp)                |                |-android::ProcessState::self()->startThreadPool();                |                |-(void)setsid();                |                |-do_main                         |                         |-mozilla::StartBootAnimation();                         |                         |-XRE_main                               |                               |-XREMain::XRE_main                                      |                                      |-new ScopedAppData(aAppData);                                      |                                      |-XREMain::XRE_mainInit                                      |                                      |-XREMain::XRE_mainStartup                                      |                                      |-mScopedXPCOM = MakeUnique<ScopedXPCOMStartup>();                                      |                                      |-mScopedXPCOM->Initialize();          (toolkit/xre/nsAppRunner.cpp)                                      |           |                                      |           |-NS_InitXPCOM2                       (xpcom/build/XPCOMInit.cpp)                                      |                  |                                      |                  |-sMessageLoop = new MessageLoopForUI(MessageLoop::TYPE_MOZILLA_UI);                                      |                  |                                      |                  |-ioThread = MakeUnique<BrowserProcessSubThread>(BrowserProcessSubThread::IO);                                      |                  |-ioThread->StartWithOptions                                      |                  |                                      |                  |-nsThreadManager::get()->Init();                                      |                  |                                      |                  |-nsTimerImpl::Startup();                                      |                  |                                      |                  |-nsComponentManagerImpl::gComponentManager = new nsComponentManagerImpl();                                      |                  |                                      |                  |-nsCycleCollector_init                                      |                  |                                      |                  |-nsCycleCollector_startup                                      |                  |                                      |                  |-JS_Init                                      |                  |                                      |                  |-nsComponentManagerImpl::gComponentManager->Init();(xpcom/components/nsComponentManager.cpp)                                      |                  |     |                                      |                  |     |-nsComponentManagerImpl::InitializeStaticModules()                                      |                  |     |                                      |                  |     |-RegisterModule(...)                                      |                  |     |                                      |                  |     |-greOmnijar =    mozilla::Omnijar::GetReader(mozilla::Omnijar::GRE);                                      |                  |     |                                      |                  |     |-cl->location.Init(greOmnijar, "chrome.manifest");                                      |                  |     |                                      |                  |     |-nsComponentManagerImpl::RereadChromeManifests                                      |                  |               |                                      |                  |               |-nsComponentManagerImpl::RegisterManifest                                      |                  |                       |                                      |                  |                       |-DoRegisterManifest                                      |                  |                                |                                      |                  |                                |-ParseManifest                                      |                  |                                      |                                      |                  |                                      |-...                                      |                  |                                      |                                      |                  |                                      |-nsComponentManagerImpl::ManifestContract                                       |                  |                                      |                  |-XPTInterfaceInfoManager::GetSingleton();                                      |                  |                                      |                  |-nsDirectoryService::gService->RegisterCategoryProviders();                                      |                  |                                      |                  |-SharedThreadPool::InitStatics();                                      |                  |                                      |                  |-AbstractThread::InitStatics();                                      |                  |                                      |                  |-mozilla::scache::StartupCache::GetSingleton();                                      |                  |                                      |                  |-mozilla::AvailableMemoryTracker::Activate();                                      |                  |                                      |                  |-NS_CreateServicesFromCategory(...)                                      |                  |                                      |                  |-mozilla::HangMonitor::Startup();                                      |                  |                                      |                  |-mozilla::BackgroundHangMonitor::Startup();                                      |                  |                                      |                  |-sMainHangMonitor = new mozilla::BackgroundHangMonitor                                      |                                                        |-XREMain::XRE_mainRun()                                               |                                               |-mozilla::ipc::ProcLoaderClientGeckoInit();                                               |                                               |-mScopedXPCOM->SetWindowCreator(mNativeApp);                                               |                                               |-startupNotifier->Observe(nullptr, APPSTARTUP_TOPIC, nullptr);                                               |                                               |-mDirProvider.DoStartup();                                               |                                               |-cmdLine->Init(...)                                               |                                               |-obsService->NotifyObservers(cmdLine, "command-line-startup", nullptr);                                               |                                               |-appStartup->CreateHiddenWindow();                                               |                                               |-obsService->NotifyObservers(nullptr, "final-ui-startup", nullptr);                                               |                                               |-cmdLine->Run();     (toolkit/components/commandlines/nsCommandLine.cpp)                                               |          |                                               |          |- nsCommandLine::EnumerateValidators                                               |          |                                               |          |-nsCommandLine::EnumerateHandlers                                               |                  |                                               |                  |-EnumRun                                               |                        |                                               |                        |-nsICommandLineHandler->Handle   (first page will be loaded in here)                                                       |                                                                                 |-mNativeApp->Enable();                                               |                                               |-appStartup->Run();     (toolkit/components/startup/nsAppStartup.cpp)                                                         |                                                         |-mAppShell->Run();         (widget/gonk/nsAppShell.cpp->widget/nsBaseAppShell.cpp)                                                                  |                                                                  |-MessageLoop::current()->Run();  // run forever~~~~~

b2g进程启动中，会做以下几件事：
1. 在加载库文件之前保留5个文件描述符（从STDERR_FILENO+1开始）

  FdArray reservedFds;  ReserveFileDescriptors(reservedFds);

2.加载进程间能共享的各种数据（主要加载libxul等各种共享库，从Omnijar文件中加载各种模块的xpt信息），这样可以节约些内存。

bool ok = LoadStaticData(argc, argv);

3.开始fork进程。父亲就是b2g进程，儿子就是nuwa进程（后续再讲）。其中b2g进程具有系统root权限，负责处理系统的io，显示，操作底层硬件等。
在b2g进程中运行后，需要初始化xpcom，即调用：

  rv = NS_InitXPCOM2(&mServiceManager,gDirServiceProvider->GetAppDir(), gDirServiceProvider);

最后，b2g进程会进入一个message loop中，等待处理各种请求：

appStartup->Run();

Firefox OS中进程可以分为三类：b2g进程，nuwa进程，以及content进程。其中content进程都是由nuwa进程生成的（当然在nuwa进程未ready，但又需要fork进程的时候，有可能会由b2g进程直接生成）。

nuwa进程是如何创建新进程？b2g是如何加载第一个程序(system)的？后续文章中进行讲解。

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