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Introduction
Many developers who wrote their own Input Methods (IM) for the Software
Input Panel (SIP) on Pocket PC, or those who developed uninstallers for
custom Input Methods faced with the problem of unloading their DLL's in
order to update them or remove from the device. However it seems a bit hard
to find any information on how to achieve this without rebooting the
system. This short article aims to solve this problem.
Background
According to the official documentation the SIP should
release the Input Method COM object just after calling of
IInputMethod->Deselect() method, i.e. just after another Input
Method is selected. And I can confirm the SIP does release it. So it seems
that the easiest method would be to simply select another Input Method.
This does not work however.
As you know, the COM subsystem does not unload unused
DLL's immediately. Instead, it waits for the user code to call
CoFreeUnusedLibraries() or CoFreeUnusedLibrariesEx(). The
SIP does call the first function, but, unfortunately, this is not enough to
unload all unused input methods immediately. Inside this function the COM subsystem calls
DllCanUnloadNow() routine for all DLL's that do not seem to be used
anymore and puts all those DLL's that returned S_OK (DLL can be unloaded) into a
list of candidates for unloading. When a DLL from this list is needed to
the COM again (for example, the application requests to create a new COM object
that sits in this DLL) the DLL is removed from this candidate list. If some
DLL is still sitting in the candidate list when
either CoFreeUnusedLibraries(), or CoFreeUnusedLibrariesEx()
is called again, this DLL finally gets unloaded if and only if enough time
has passed since putting the DLL into the list.
When using CoFreeUnusedLibrariesEx() this time interval can be
specified as the first parameter, but for CoFreeUnusedLibraries() used by the SIP
this timeout is the system default value, which is 10 minutes! Quite a long time to ask the user to
wait for...
Solution
One of the possible ways to workaround this problem would be to call
CoFreeUnusedLibrariesEx() from the same process where the SIP lives
with zero or near to zero timeout parameter.
Fortunately, this is easily to achieve, because the SIP is implemented as a device
driver and resides where all device drivers reside--in the device.exe
process. So we can create a simple driver, which calls needed function(s), then load it
and enjoy the possibility to remove our DLL. Sure, the input method
should be deselected before this. There is also another trick.
The driver would be simpler, if we could call CoFreeUnusedLibrariesEx()
from the Init() entry point. This does not work, because the COM subsystem
needs the owner process ID to be equal to the current process ID (device.exe),
for which we need DLL's to be unloaded. However the owner process ID inside the Init()
call is equal to the ID of process which loaded our driver (i.e. our example program),
and therefore COM tries to unload COM DLL's for this example application.
To overcome this, we just need to create another thread and do our call from
the context of this thread. Owner process ID for this thread is equal to the ID
of device.exe, where the SIP and Input Methods live.
Using the code
The example can be compiled with eVC++ 4.0. There are two projects in the archieve:
a simple device driver, which when loaded calls CoFreeUnusedLibrariesEx()
as described above; and a simple executable that loads this driver.
The driver exposes a minimal set of required entry points (Init/Deinit,
Open/Close, IOControl), because we just need it to be
loaded into the device.exe, nothing more.
Notes
When it is needed to unload the Input Method DLL while an application is being uninstalled
from the device via the Remove Programs applet, it is possible to use a Custom Setup
DLL as a driver itself by providing needed entry points, i.e. there is no need in
a separate DLL.
I hope this article will be helpful to Input Method writers.
Do not hesitate to comment or ask any questions.
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