SendMessage() with shared memory?

[Sam Hobbs]

I think that the documentation is intentionaly misleading, but I am guessing about a lot of this. If the documentation is misleading, then I think it is unusual for the Windows documentation to be so misleading. The documentation often is intentionaly incomplete but it is seldom intentionally misleading. I think that the reason it is misleading is that Microsoft does not want to guarantee that GlobalAlloc will always work the way it works, but they should say that. It is quite common for the Windows documentation to say such things.

So now I am so curious about whether it will work that I am tempted to try it myself.

[mauricioprado00]

win32.hlp> under dinamic link library its a posible solution, it could be usefull this code:

// File: DLLSHMEM.C.
// The DLL entry-point function sets up shared memory using
// a named file-mapping object.

#include <windows.h>
#include <memory.h>

#define SHMEMSIZE 4096

static LPVOID lpvMem = NULL; // pointer to shared memory

BOOL DllEntryPoint(HINSTANCE hinstDLL, // DLL module handle
DWORD fdwReason, // reason called
LPVOID lpvReserved) // reserved
{
HANDLE hMapObject = NULL; // handle to file mapping

BOOL fInit, fIgnore;

switch (fdwReason)
{
// The DLL is loading due to process
// initialization or a call to LoadLibrary.

case DLL_PROCESS_ATTACH:

// Create a named file mapping object.

hMapObject = CreateFileMapping(
(HANDLE) 0xFFFFFFFF, // use paging file
NULL, // no security attributes
PAGE_READWRITE, // read/write access

0, // size: high 32-bits
SHMEMSIZE, // size: low 32-bits
"dllmemfilemap"); // name of map object
if (hMapObject == NULL)
return FALSE;

// The first process to attach initializes memory.

fInit = (GetLastError() != ERROR_ALREADY_EXISTS);

// Get a pointer to the file-mapped shared memory.

lpvMem = MapViewOfFile(

hMapObject, // object to map view of
FILE_MAP_WRITE, // read/write access
0, // high offset: map from
0, // low offset: beginning
0); // default: map entire file
if (lpvMem == NULL)
return FALSE;

// Initialize memory if this is the first process.

if (fInit)
memset(lpvMem, '\0', SHMEMSIZE);


break;

// The attached process creates a new thread.

case DLL_THREAD_ATTACH:
break;

// The thread of the attached process terminates.

case DLL_THREAD_DETACH:
break;

// The DLL is unloading from a process due to
// process termination or a call to FreeLibrary.

case DLL_PROCESS_DETACH:

// Unmap shared memory from the process's address space.


fIgnore = UnmapViewOfFile(lpvMem);

// Close the process's handle to the file-mapping object.

fIgnore = CloseHandle(hMapObject);

break;

default:
break;
}

return TRUE;
UNREFERENCED_PARAMETER(hinstDLL);
UNREFERENCED_PARAMETER(lpvReserved);
}

// SetSharedMem sets the contents of shared memory.

VOID SetSharedMem(LPTSTR lpszBuf)
{
LPTSTR lpszTmp;


// Get the address of the shared memory block.

lpszTmp = (LPTSTR) lpvMem;

// Copy the null-terminated string into shared memory.

while (*lpszBuf)
*lpszTmp++ = *lpszBuf++;
*lpszTmp = '\0';
}

// GetSharedMem gets the contents of shared memory.

VOID GetSharedMem(LPTSTR lpszBuf, DWORD cchSize)
{
LPTSTR lpszTmp;

// Get the address of the shared memory block.

lpszTmp = (LPTSTR) lpvMem;

// Copy from shared memory into the caller's buffer.


while (*lpszTmp && --cchSize)
*lpszBuf++ = *lpszTmp++;
*lpszBuf = '\0';
}


Note that the shared memory can be mapped to a different address in each process. For this reason, each process has its own instance of the lpvMem parameter, which is declared as a global variable so that it is available to all DLL functions. The example assumes that the DLL global data is not shared, so each process that loads the DLL has its own instance of lpvMem.
In this example, the shared memory is released when the last handle of the file-mapping object is closed. To create persistent shared memory, a DLL can create a detached process (see CreateProcess) when the DLL is first loaded. If this detached process uses the DLL and does not terminate, it has a handle of the file-mapping object that prevents the shared memory from being released.

[Sam Hobbs]

Welcome to the Codeguru forums, mauricioprado00. I am sure you will make many more contributions that will help very many.

Note that this thread is very old; more than 8 years old. So your contribution might help others in the future looking for relevant answers but just be aware that the person asking the question has probably either gotten a solution or given up trying.