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The information in this article applies to:
SUMMARY
Under Windows NT, version 3.1, if you want a service to have access to the
application desktop, you must run the service in the LocalSystem account. A
service process running in the LocalSystem account (or a process started
from such a service) can display message boxes, windows, and dialog boxes.
Processes that are running in the LocalSystem account are not terminated by
the system during logoff. A number of changes were made to Windows NT,
version 3.5, that affect the way Windows NT interacts with these services.
In addition, Windows NT 3.51 has a richer set of desktop APIs.
NOTE: Running interactive services under the system account is a VERY
dangerous practice. This is especially true of the command processor and
batch files. A user who wants to control the system can just hit CTRL+C
to get an interactive system command prompt.
MORE INFORMATION
The following are new features of Windows NT, version 3.5, that affect
services:
- The account of the logged in user is the only account granted access to
the application desktop. The LocalSystem no longer has access.
Therefore, it is possible to get access to the desktop by impersonating
the user before making any USER or GDI calls.
- Console and GUI applications started from a service process during a
particular logon session are run on an invisible window station and
desktop that are unique to that session. The window station and desktop
are created automatically when the first application in the session
starts; they are destroyed when the last application exits. There is no
way to make these invisible desktops visible.
- If you want a service in the localsystem account to interact with the
logged-on user, specify the SERVICE_INTERACTIVE_PROCESS flag in the call
to CreateService(). For example:
schService = CreateService(
schSCManager,
serviceName,
serviceName,
SERVICE_ALL_ACCESS,
SERVICE_INTERACTIVE_PROCESS | SERVICE_WIN32_OWN_PROCESS,
SERVICE_DEMAND_START,
SERVICE_ERROR_NORMAL,
lpszBinaryPathName,
NULL,
NULL,
NULL,
NULL,
NULL );
If you specify an account other than localsystem when using
SERVICE_INTERACTIVE_PROCESS, you will get error INVALID_PARAMETER (87).
- If you use CreateProcess() to launch your process and you want your
service to log onto the users desktop, assign the lpdesktop parameter of
the STARTUPINFO struct with "WinSta0\\Default".
- Services that simply need a visible user notification can do this by
calling MessageBox() with the MB_SERVICE_NOTIFICATION flag. Using the
MB_DEFAULT_DESKTOP_ONLY flag works as well, but only if the user's
desktop is active. If the workstation is locked or a screen saver is
running, the call will fail.
NOTE: If you are writing code for an application that can be run as
either a service or an executable, you can't use MB_SERVICE_NOTIFICATION
as well as a non-NULL hwndOwner.
- Any output done to a window is not displayed or made available to the
application in any way. Attempts to read bits from the display results
in a failure.
- GUI services do not receive WM_QUERYENDSESSION/WM_ENDSESSION messages
at logoff and shutdown; instead, they receive CTRL_LOGOFF_EVENT
and CTRL_SHUTDOWN_EVENT events. These services are not terminated by the
system at logoff.
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