out parameter in COM method
JJ
I programmed CORBA before. In CORBA, if a method has out string parameter,
server allocates memory and client is responsible to release the memory. Does
COM work the same way? Can someone help?
JJ
Igor Tandetnik
news:73BB8AF5-29FE-4E5A...@microsoft.com
> I wonder how COM allocates and releases out parameter memory.
The callee allocates, and the caller later releases when it no longer
needs the data.
> I programmed CORBA before. In CORBA, if a method has out string
> parameter, server allocates memory and client is responsible to
> release the memory. Does COM work the same way?
Yes, COM works the same way.
--
With best wishes,
Igor Tandetnik
With sufficient thrust, pigs fly just fine. However, this is not
necessarily a good idea. It is hard to be sure where they are going to
land, and it could be dangerous sitting under them as they fly
overhead. -- RFC 1925
JJ
this topic? I tried to search the answer myself, but I could not find it.
JJ
Alexander Nickolov
standard beginner's COM book is:
"Inside COM" by Dale Rogerson, MS Press
--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnic...@mvps.org
MVP VC FAQ: http://www.mvps.org/vcfaq
=====================================
"JJ" <J...@discussions.microsoft.com> wrote in message
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JJ
JJ
Snow
want to use another parameter for marshalling size, ie [out,
size_is(dwSize)] and I only want an "out" direction. Is this possible?
HRESULT GetStruct ( [out] TestStructType** ppStruct)
ppStruct has dynamic size
Thanks in advance
"Igor Tandetnik" <itand...@mvps.org> wrote in message
news:OiUyt%23HeGH...@TK2MSFTNGP03.phx.gbl...
Igor Tandetnik
news:es$2fqWfG...@TK2MSFTNGP05.phx.gbl
> I have a struct with dynamic size, how do I marshall this struct?
You don't. Standard marshalling does not support variable length
structures. Make it a fixed struct with a pointer to dynamically
allocated variable size array.
> I
> do not want to use another parameter for marshalling size, ie [out,
> size_is(dwSize)] and I only want an "out" direction. Is this possible?
A [size_is] member can be part of the structure:
struct MyData {
DWORD dwSize;
[size_is(dwSize)] BYTE* pData;
Alexander Nickolov
pair of methods and some code you write for the P/S DLL:
[local]
HRESULT GetStruct([out] TestStructType** ppStruct);
[call_as(GetStruct)]]
HRESULT GetStruct_remote([out] long* psz, [out, size_is(*psz)] BYTE**
ppStruct);
You need to manually code the conversion routines for
your interface and compile and link them in the P/S DLL
(note this is in C, not C++):
#define COBJINTERFACES
#include "MIDL_header.h"
HRESULT STDMETHODCALLTYPE IFace_GetStruct_Proxy(LPVOID pThis, struct
TestStructType** ppStruct)
{
long sz;
return IFace_GetStruct_remote_Proxy(pThis, &sz, (BYTE**)ppStruct);
}
HRESULT STDMETHODCALLTYPE IFace_GetStruct_Stub(LPVOID pThis, long* psz,
BYTE** ppStruct)
{
HRESULT hr = IFace_GetStruct(pThis, (struct TestStructType**)ppStruct);
if (SUCCEEDED(hr)) {
*psz = ?; // From the struct data
} else {
*psz = 0;
}
return hr;
}
Note I typed this based on the MIDL documentation after correcting
glaring errors, but I haven't actually compiled it.
Also note, I assumed the struct doesn't have embedded pointers.
If it does, you need to serailize it at the stub and then recreate it
at the proxy making this a true excersize in marshaling :)...
I leave the excersize in passing the struct as an [in] parameter
to you. It shouldn't be very different, but you'd be passing the
data in the opposite direction.
You'd probably quickly realize that writing a pair of methods
and the associated code each time you want to transmit a struct
quickly gets old. MIDL has more advanced mechanisms, but
I haven't used them myself, so can't provide you with snippets.
Look at the following MIDL attributes: transmit_as, represent_as,
wire_marshal, user_marshal. I'd suggest [wire_marshal] based
on the MIDL description.
--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnic...@mvps.org
MVP VC FAQ: http://www.mvps.org/vcfaq
=====================================
"Snow" <filame...@hotmail.com> wrote in message
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Snow
However its still a good piece of information :)
"Alexander Nickolov" <agnic...@mvps.org> wrote in message
news:ee3iHQdf...@TK2MSFTNGP02.phx.gbl...
Snow
Igor Tandetnik
> I tried your method but getting access violation. Not sure whether
> I'm doing it the right way or not.
>
> typedef struct _tagTestStruct2Type
> {
> DWORD dwSize;
> [size_is(dwSize)] BYTE* pData;
> } TestStruct2Type;
>
> typedef struct _tagMoreTestStructType
> {
> DWORD dwData;
> LONG lData;
> INT iData;
> } MoreTestStructType;
>
> [helpstring("method GetStruct2")] HRESULT GetStruct2([out]
> TestStruct2Type** ppStruct);
>
> STDMETHODIMP CTestStruct::GetStruct2(TestStruct2Type** ppStruct)
> {
> ULONG ulSize = sizeof(TestStruct2Type) +
> sizeof(MoreTestStructType);
> *ppStruct = (TestStruct2Type*)CoTaskMemAlloc(ulSize);
No, that's not the way you do it. You have to use CoTaskMemAlloc to
allocate memory for TestStruct2Type structure alone, then a separate
CoTaskMemAlloc call to allocate a buffer and store it in
TestStruct2Type::pData. Set TestStruct2Type::dwSize to the size of this
latter buffer.
Alexander Nickolov
is a bit higher.
--
=====================================
Alexander Nickolov
Microsoft MVP [VC], MCSD
email: agnic...@mvps.org
MVP VC FAQ: http://www.mvps.org/vcfaq
=====================================
"Snow" <filame...@hotmail.com> wrote in message
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