Re: implementing DllCanUnloadNow questions

[Robert Shearman]

James Hawkins wrote:
> On Tue, 30 Nov 2004 15:34:50 -0600, Robert Shearman  wrote:
> > James Hawkins wrote:
> > > I would like to work on the DllCanUnloadNow janitorial task, but I was
> > > wondering if there are any patches or examples of dll's that correctly
> > > implement this so that I can do it right.
> >
> > I don't know of any examples so far, but Mike Hearn and I will be
> > tackling OLE soon, so that will involve cleaning up ole32. However, that
> > probably won't happen until we've done some groundwork on COM first, so
> > feel free to beat us to it and make a shining example of a DLL that
> > correctly implements DllCanUnloadNow.
> > > Is LockServer(TRUE/FALSE) the same as this example?
> > >
> > > static HRESULT WINAPI SFCF_LockServer(LPCLASSFACTORY iface,BOOL dolock) {
> > >      IClassFactoryImpl *This = (IClassFactoryImpl *)iface;
> > >      FIXME("(%p)->(%d),stub!\n",This,dolock);
> > >      return S_OK;
> > > }
> > >
> > > If they are the same, what is needed to complete this stub?  I would
> > > think that there would be a refCount variable that is incremented if
> > > dolock is true and decremented if dolock is false.  Also, should the
> > > LockServer functions be listed in the spec files of the dlls?
> >
> > No, the LockServer functions are part of IClassFactory vtables. Only
> > DllCanUnloadNow needs to be exported.
> > > After implementing LockServer, it should be an easy matter to do a
> > > check for refCount == 0 to see if the dll can be unloaded or not.  Am
> > > I heading in the right direction?
> >
> > To implement DllCanUnloadNow properly you need to do the following:
> > - Add a variable "LONG cLocks" and two function for manipulating it:
> >
> > void LockModule()
> > {
> >    InterlockedIncrement(&cLocks);
> > }
> >
> > void UnlockModule()
> > {
> >    InterlockedDecrement(&cLocks);
> > }
> > 2. Increment cLocks on construction of every heap object:
> > static HRESULT Example_Construct(...)
> > {
> > ...
> >    LockModule();
> >    return S_OK;
> > }
> > 3. Decrement cLocks on destruction of every heap object:
> > static ULONG Example_Release(...)
> > {
> >    ...
> >    res = InterlockedDecrement(&This->cRefs);
> >    if (!res)
> >    {
> >       /* Free object's resources, including memory, etc. */
> >       UnlockModule();
> >    }
> > }
> > 4. For non-heap based objects (commonly objects with no state, like
> > IClassFactory):
> > static ULONG Example_AddRef(...)
> > {
> >    LockModule();
> >    return 2; /* non-heap object */
> > }
> >
> > static ULONG Example_Release(...)
> > {
> >    UnlockModule();
> >    return 1;
> > }
> > 5. Implement IClassFactory_LockServer's as follows:
> > HRESULT WINAPI Example_LockServer(...)
> > {
> >    if (bLock)
> >       LockModule();
> >    else
> >       UnlockModule();
> >    return S_OK;
> > }
> > 6. Then implement DllCanUnloadNow:
> > HRESULT WINAPI DllCanUnloadNow()
> > {
> >    return cLocks > 0 : S_FALSE : S_OK;
> > }
> >
> > Rob
>
> Attached is my beginning attempts at adding DllCanUnloadNow to msi. 
> There are a couple questions I have though.
> > static HRESULT Example_Construct(...)
> > {
> > ...
> >    LockModule();
> >    return S_OK;
> > }
>
> Is _Construct the same as _AddRef()?  I'm thinking it's not.  If it
> isn't, I need to add a MSI_Construct to msi.  What are the parameters
> to the Construct function and what would usually go in the place of
> the "..."?

The _Construct function has a similar purpose to the C++ constructor, 
except that we usually allocate the memory in _Construct as well 
(whereas the C++ normally compiler does this). The parameters and return 
value vary according to the situation. If the object has a number of 
properties that are set up at creation time, then these should be passed 
to the constructor.
> > static ULONG Example_Release(...)
> > {
> >    ...
> >    res = InterlockedDecrement(&This->cRefs);
> >    if (!res)
> >    {
> >       /* Free object's resources, including memory, etc. */
> >       UnlockModule();
> >    }
> > }
> > static ULONG Example_Release(...)
> > {
> >    UnlockModule();
> >    return 1;
> > }
>
> Are these _Release's referring to the same function?  If not, what is
> the difference?  If so, which one should be implemented and what
> usually goes in the place of the "..."?

The difference is that the first one is a heap based object, so it does 
the ref-counting itself, whereas the second is a non-heap based object 
where it makes no sense to keep an internal ref-count so UnlockModule is 
always called (and LockModule is always called in AddRef).
> @@ -1567,12 +1570,32 @@
>   return S_OK;
> }
>
> +void LockModule()
> +{
> - InterlockedIncrement(&cLocks);
>
> +}
> -
>
> +void UnlockModule()
> +{
> - InterlockedDecrement(&cLocks);
>
> +}
> -
>
> +HRESULT WINAPI MSI_LockServer(LPCLASSFACTORY iface,BOOL bLock)
> +{
> - if (bLock)
> - LockModule();
> - else
> - UnlockModule();
> -
> - return S_OK;
>
> +}
> -

Not quite right. The _LockServer method should be the one included in 
the IClassFactory implementation's vtable and I don't see this in this 
patch.

Rob