Re: Using a structure as shared memory

From:
"Scott McPhillips [MVP]" <org-dot-mvps-at-scottmcp>
Newsgroups:
microsoft.public.vc.language
Date:
Mon, 31 Dec 2007 12:14:51 -0500
Message-ID:
<uzW49C9SIHA.2268@TK2MSFTNGP02.phx.gbl>
"Sean Farrow" <sean.farrow@seanfarrow.co.uk> wrote in message
news:%23toTj58SIHA.5400@TK2MSFTNGP04.phx.gbl...

Hi:
Firstly, I apologise if thi is the wrong group, but I'm hoping someone can
help:
I am currently working on a project that needs to use a structure as a
shared memory block between processes. The structure is defined as
follows:
struct SATreeviewInfo {

IAccessible* TreeviewAccessible;

VARIANT VarObject;

VARIANT VarObjectState;

HWND TreeviewWindow;

TV_ITEM CurrentItem;

LPWSTR Text[1024];

};

I have then declared a variable in a namespace (the structure is declared
inside this namespace also:

extern SATreeviewInfo TreeviewInfo;

The code for mapping this structure in to the process is as follows:

//Create the file mapping and event objects for treeviews.

HTreeviewMSAAMapFile =CreateFileMapping( INVALID_HANDLE_VALUE, NULL,
PAGE_READWRITE, 0, sizeof(SATreeviewInfo), TEXT("TreviewMapFile"));

The code tomap the structure is:

TreeviewInfo =(SATreeviewInfo)MapViewOfFile(HTreeviewMSAAMapFile, NULL, 0,
0, 0);

The error I obtain when trying to compile is:

1>c:\work\serotek\libserotek\treeview.cpp(26) : error C2440: 'type cast' :
cannot convert from 'LPVOID' to 'Treeview::SATreeviewInfo'


MapViewOfFile returns a pointer. Quite simply, you cannot cast a pointer
into an object. Among other problems, they are not the same size!

You can cast the returned pointer into a pointer to the object.

You should also recognize that putting a pointer into shared memory is
useless. Your IAccessible* cannot be used in another process because each
process has an independent memory space. So an address in one process is
invalid (and dangerous!) in another process.

--
Scott McPhillips [VC++ MVP]

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