simonmar 2004/10/26 05:03:25 PDT
Modified files:
hdirect/comlib Com.lhs ComException.lhs ComPrim.idl
ComPrimSrc.c ComServ.lhs Makefile
hdirect/src MarshallCore.lhs MarshallType.lhs
Log:
Yet another branch of H/Direct.
On this branch, COM interface pointers are simply Ptrs rather than
ForeignPtrs. You must addRef/release interface pointers according to
the rules of COM, rather than relying on the finalisers to do the
business.
The ForeignPtr/finaliser story didn't work well for us, particularly
when implementing a COM server, for a couple of reasons:
- When a method you're implementing gets passed an interface ptr,
H/Direct arranges to AddRef() the interface and sets up a finalizer
to Release() it. One problem is that the Release() might not
happen for a long time. In a C++ implementation, this
AddRef()/Release() combo would never happen for an ordinary
method parameter.
- A worse problem is that the Release() finaliser might happen in a
different OS thread. The original OS thread that made the call-in
is probably no longer available - it's off outside Haskell land
doing other stuff.
Furthermore, interfaces as Ptrs is much more efficient. Marshalling
and unmarshalling are essentially no-ops, compared to creating a
ForeignPtr, an extra AddRef() & Release(), and the cost of running the
finalizer itself.
The down side is having to be careful about your addRefs/releases. I
handled one of the common cases, queryInterface, like this:
withQueryInterface :: IID (IUnknown a) -> IUnknown b
-> (IUnknown b -> IO c) -> IO c
with the obvious implementation using bracket.
I imagine we could implement some automatic checking to catch missing
addRefs & releases, perhaps with a special DEBUG version of comlib.
eg. go back to using ForeignPtrs, but have the finaliser just check
that you had called release.
Revision Changes Path
1.14 +9 -11 fptools/hdirect/comlib/Com.lhs
1.9 +17 -4 fptools/hdirect/comlib/ComException.lhs
1.9 +11 -15 fptools/hdirect/comlib/ComPrim.idl
1.5 +0 -29 fptools/hdirect/comlib/ComPrimSrc.c
1.8 +9 -11 fptools/hdirect/comlib/ComServ.lhs
1.18 +4 -0 fptools/hdirect/comlib/Makefile
1.58 +3 -2 fptools/hdirect/src/MarshallCore.lhs
1.136 +1 -1 fptools/hdirect/src/MarshallType.lhs
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