how to get the pointer variable address from a dll

hi all,

I have a DLL which has a output  parameter  LPVOID, A application
calls this DLL function, the Dll responsibility is to  assign a valid
pointer to the lpBuffer,

Till here it is OK,  but once it returns to the Application this
lpBuffer becomes NULL.

Prototype,
__declspec(dllexport) BOOL TRSTMonGetStatus(LPVOID lpBuffer)
Please let me know what to do, I am stuck in this.

0
6/18/2007 3:13:27 PM
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try this

 __declspec(dllexport) BOOL TRSTMonGetStatus(LPVOID &lpBuffer)

AliR.



<karunyalakshmi@gmail.com> wrote in message 
news:1182179607.989434.318540@x35g2000prf.googlegroups.com...
>
> hi all,
>
> I have a DLL which has a output  parameter  LPVOID, A application
> calls this DLL function, the Dll responsibility is to  assign a valid
> pointer to the lpBuffer,
>
> Till here it is OK,  but once it returns to the Application this
> lpBuffer becomes NULL.
>
> Prototype,
> __declspec(dllexport) BOOL TRSTMonGetStatus(LPVOID lpBuffer)
> Please let me know what to do, I am stuck in this.
> 


0
AliR3470 (3235)
6/18/2007 3:17:17 PM
I'm not sure what's happening to you without seeing more of the code, but 
typically in these kinds of cases it would be the responsibility of the 
calling program to cast the pointer on the return from the function. 
Sometimes there is an additional argument or flag that lets you know what 
kind of structure to expect in pointer.

Tom

<karunyalakshmi@gmail.com> wrote in message 
news:1182179607.989434.318540@x35g2000prf.googlegroups.com...
>
> hi all,
>
> I have a DLL which has a output  parameter  LPVOID, A application
> calls this DLL function, the Dll responsibility is to  assign a valid
> pointer to the lpBuffer,
>
> Till here it is OK,  but once it returns to the Application this
> lpBuffer becomes NULL.
>
> Prototype,
> __declspec(dllexport) BOOL TRSTMonGetStatus(LPVOID lpBuffer)
> Please let me know what to do, I am stuck in this.
> 

0
tom.nospam (3240)
6/18/2007 3:21:48 PM
You are confusing a pointer with a poitner-to-a-pointer.

The LPVOID parameter is NOT an "output" parameter; it is a POINTER to a buffer which will
be filled.  Therefore, if the pointer lpBuffer is set inside the function, only the
parameter value is set, e.g.,

LPVOID buff = NULL;
TRSTMonGetStatus(buff);

will correctly leave 'buff; as NULL when you return.  That's because NOTHING you can do
inside the function can change that variable.

that is,
	lpBuffer = new BYTE[size];

changes the local copy of lpBuffer, but cannot possibly change the value of 'buff' because
you passed the pointer in, NOT the pointer-to-buff, but the contents-of-buff.

Had you done

TRSTMonGetStatus(LPVOID & lpBuffer)

then you are passing a reference-to-a-pointer, so if in the DLL if you do

	lpBuffer = new BYTE[size];

then it stores the value in 'buff' in the input parameter.
			joe
On Mon, 18 Jun 2007 08:13:27 -0700, "karunyalakshmi@gmail.com" <karunyalakshmi@gmail.com>
wrote:

>
>hi all,
>
>I have a DLL which has a output  parameter  LPVOID, A application
>calls this DLL function, the Dll responsibility is to  assign a valid
>pointer to the lpBuffer,
>
>Till here it is OK,  but once it returns to the Application this
>lpBuffer becomes NULL.
>
>Prototype,
>__declspec(dllexport) BOOL TRSTMonGetStatus(LPVOID lpBuffer)
>Please let me know what to do, I am stuck in this.
Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm
0
newcomer (15974)
6/18/2007 3:42:03 PM
On Mon, 18 Jun 2007 08:13:27 -0700, "karunyalakshmi@gmail.com"
<karunyalakshmi@gmail.com> wrote:

>I have a DLL which has a output  parameter  LPVOID, A application
>calls this DLL function, the Dll responsibility is to  assign a valid
>pointer to the lpBuffer,
....
>Prototype,
>__declspec(dllexport) BOOL TRSTMonGetStatus(LPVOID lpBuffer)
>Please let me know what to do, I am stuck in this.

As others have said, the problem is that you may need a new level of
indirection (pointer to a pointer).

AliR or Joe seems to have suggested using reference (&), but if your
DLL has a C-interface, I would suggest using a real
pointer-to-pointer, i.e. void ** (or LPVOID *), as COM does e.g. for
QueryInterface:

 __declspec(dllexport) BOOL TRSTMonGetStatus(LPVOID * ppBuffer);

In your function implementation:

  // Check valid parameters
  if ( ppBuffer == NULL )
    return FALSE; // Error: bad parameter

  // Clear output parameter (*ppBuffer is LPVOID)
  *ppBuffer = NULL;

  // Allocate/process what you need here...
  // Use *ppBuffer, e.g.:

  BYTE * pBytes = new BYTE[1000];
  // ...or whatever...

  // This value is returned to caller
  *ppBuffer = (LPVOID)pBytes;


Note that you should also provide in your DLL a function to *release*
the buffer:

__declspec(dllexport) void TRSTMonReleaseBuffer(LPVOID * ppBuffer)
{
  if ( ppBuffer == NULL )
    return;

  BYTE * pBytes = (BYTE*) (*ppBuffer);
  if ( pBytes == NULL )
    return;

  // Release the allocated array
  delete [] pBytes;

  // Clear caller pointer
  *ppBuffer = NULL;
}


You can use your code like so:

  // The caller
  LPVOID pBuffer = NULL;
  if ( TRSTMonGetStatus( &pBuffer ) ... )

  // ... Work ...

  // Release
  TRSTMonReleaseBuffer( &pBuffer );


(BTW: I've not tested the code with the compiler...)

MrAsm
0
mrasm (715)
6/18/2007 10:05:46 PM
Reply:

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