The loginwindow Process

Login occurs after system initialization with the appearance of the login window. The loginwindow process coordinates the login process and the individual user's session, calling on other system services as needed. Depending upon the user's login preferences, the login window may prompt the user for a valid login name and password, or may use cached values to log in the user automatically. When the user's login name and password have been authenticated, loginwindow proceeds to load the user environment.

When the user logs in, loginwindow does the following:

Loads the user's computing environment, including preferences, environment variables, device and file permissions, keychain access, and so on
Launches the Dock, Finder, and SystemUIServer
Launches the Setup Assistant if an installation is in progress
Automatically launches applications specified in the Login Items pane of Accounts preferences

When all of these applications are launched and running, the login procedure is complete.

The loginwindow application uses Launch Services to launch all applications, including the Finder, Dock, SystemUIServer, and user-specified applications. Most applications in the user session run as child processes of the Window Manager process. They are not owned by loginwindow.

Once the user session is running, loginwindow monitors the session and user applications in the following ways:

Manages logout, restart, and shutdown procedures
Manages the Force Quit window, which includes monitoring the currently active applications, responding to user requests to force quit applications, and relaunching the Finder
Displays alert dialogs when a notification is received from hidden applications (applications not visible in the user interface)
Writes any standard error (stderr) output to /Library/Logs/Console/user/console.log (where user is the short name), which is then used as input by the Console application
Saves a limited archive of logs in console.log.# files within the same folder, where # is a number from 0 through 9, which are all viewable in the Console application under /Library/Logs/Console/user

NOTE

If your computer is set to automatically log in, and you need to display the login window to log in as another user, press the Shift key when the blue screen appears during startup until the login window appears. If you were already pressing the Shift key to enter Safe mode, continue pressing it until the login window appears.