Using Shared Memory in a Dynamic-Link Library

The following example demonstrates how the DLL entry-point function can use a file-mapping object to set up memory that can be shared by processes that load the DLL. The shared DLL memory persists only as long as the DLL is loaded. Applications can use the SetSharedMem and GetSharedMem functions to access the shared memory.

DLL that Implements the Shared Memory

The example uses file mapping to map a block of named shared memory into the virtual address space of each process that loads the DLL. To do this, the entry-point function must:

1. Call the CreateFileMapping function to get a handle to a file-mapping object. The first process that loads the DLL creates the file-mapping object. Subsequent processes open a handle to the existing object. For more information, see Creating a File-Mapping Object.
2. Call the MapViewOfFile function to map a view into the virtual address space. This enables the process to access the shared memory. For more information, see Creating a File View.

Note that while you can specify default security attributes by passing in a NULL value for the lpAttributes parameter of CreateFileMapping, you may choose to use a SECURITY_ATTRIBUTES structure to provide additional security.

C++

// The DLL code

#include <windows.h> 
#include <memory.h> 
 
#define SHMEMSIZE 4096 
 
static LPVOID lpvMem = NULL;      // pointer to shared memory
static HANDLE hMapObject = NULL;  // handle to file mapping

// The DLL entry-point function sets up shared memory using a 
// named file-mapping object. 
 
BOOL WINAPI DllMain(HINSTANCE hinstDLL,  // DLL module handle
    DWORD fdwReason,              // reason called 
    LPVOID lpvReserved)           // reserved 
{ 
    BOOL fInit, fIgnore; 
 
    switch (fdwReason) 
    { 
        // DLL load due to process initialization or LoadLibrary
 
          case DLL_PROCESS_ATTACH: 
 
            // Create a named file mapping object
 
            hMapObject = CreateFileMapping( 
                INVALID_HANDLE_VALUE,   // use paging file
                NULL,                   // default security attributes
                PAGE_READWRITE,         // read/write access
                0,                      // size: high 32-bits
                SHMEMSIZE,              // size: low 32-bits
                TEXT("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; 
 
        // DLL unload due to process termination or 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); 
} 

// The export mechanism used here is the __declspec(export)
// method supported by Microsoft Visual Studio, but any
// other export method supported by your development
// environment may be substituted.

#ifdef __cplusplus    // If used by C++ code, 
extern "C" {          // we need to export the C interface
#endif
 
// SetSharedMem sets the contents of the shared memory 
 
__declspec(dllexport) VOID __cdecl SetSharedMem(LPWSTR lpszBuf) 
{ 
    LPWSTR lpszTmp; 
    DWORD dwCount=1;
 
    // Get the address of the shared memory block
 
    lpszTmp = (LPWSTR) lpvMem; 
 
    // Copy the null-terminated string into shared memory
 
    while (*lpszBuf && dwCount<SHMEMSIZE) 
    {
        *lpszTmp++ = *lpszBuf++; 
        dwCount++;
    }
    *lpszTmp = '\0'; 
} 
 
// GetSharedMem gets the contents of the shared memory
 
__declspec(dllexport) VOID __cdecl GetSharedMem(LPWSTR lpszBuf, DWORD cchSize) 
{ 
    LPWSTR lpszTmp; 
 
    // Get the address of the shared memory block
 
    lpszTmp = (LPWSTR) lpvMem; 
 
    // Copy from shared memory into the caller's buffer
 
    while (*lpszTmp && --cchSize) 
        *lpszBuf++ = *lpszTmp++; 
    *lpszBuf = '\0'; 
}
#ifdef __cplusplus
}
#endif

Shared memory can be mapped to a different address in each process. For this reason, each process has its own instance of lpvMem, 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.

Note that the shared memory is released when the last handle to the file-mapping object is closed. To create persistent shared memory, you would need to ensure that some process always has an open handle to the file-mapping object.

Processes that Use the Shared Memory

The following processes use the shared memory provided by the DLL defined above. The first process calls SetSharedMem to write a string while the second process calls GetSharedMem to retrieve this string.

This process uses the SetSharedMem function implemented by the DLL to write the string "This is a test string" to the shared memory. It also starts a child process that will read the string from the shared memory.

C++

// Parent process

#include <windows.h>
#include <tchar.h>
#include <stdio.h>

extern "C" VOID __cdecl SetSharedMem(LPWSTR lpszBuf);

HANDLE CreateChildProcess(LPTSTR szCmdline) 
{ 
   PROCESS_INFORMATION piProcInfo; 
   STARTUPINFO siStartInfo;
   BOOL bFuncRetn = FALSE; 
 
// Set up members of the PROCESS_INFORMATION structure. 
 
   ZeroMemory( &piProcInfo, sizeof(PROCESS_INFORMATION) );
 
// Set up members of the STARTUPINFO structure. 
 
   ZeroMemory( &siStartInfo, sizeof(STARTUPINFO) );
   siStartInfo.cb = sizeof(STARTUPINFO); 
 
// Create the child process. 
    
   bFuncRetn = CreateProcess(NULL, 
      szCmdline,     // command line 
      NULL,          // process security attributes 
      NULL,          // primary thread security attributes 
      TRUE,          // handles are inherited 
      0,             // creation flags 
      NULL,          // use parent's environment 
      NULL,          // use parent's current directory 
      &siStartInfo,  // STARTUPINFO pointer 
      &piProcInfo);  // receives PROCESS_INFORMATION 
   
   if (bFuncRetn == 0) 
   {
      printf("CreateProcess failed (%)\n", GetLastError());
      return INVALID_HANDLE_VALUE;
   }
   else 
   {
      CloseHandle(piProcInfo.hThread);
      return piProcInfo.hProcess;
   }
}

int _tmain(int argc, TCHAR *argv[])
{
   HANDLE hProcess;

   if (argc == 1) 
   {
      printf("Please specify an input file");
      ExitProcess(0);
   }

   // Call the DLL function
   printf("\nProcess is writing to shared memory...\n\n");
   SetSharedMem(L"This is a test string");

   // Start the child process that will read the memory
   hProcess = CreateChildProcess(argv[1]);

   // Ensure this process is around until the child process terminates
   if (INVALID_HANDLE_VALUE != hProcess) 
   {
      WaitForSingleObject(hProcess, INFINITE);
      CloseHandle(hProcess);
   }
   return 0;
}

This process uses the GetSharedMem function implemented by the DLL to read a string from the shared memory. It is started by the parent process above.

C++

// Child process

#include <windows.h>
#include <tchar.h>
#include <stdio.h>

extern "C" VOID __cdecl GetSharedMem(LPWSTR lpszBuf, DWORD cchSize);

int _tmain( void )
{
    WCHAR cBuf[MAX_PATH];

    GetSharedMem(cBuf, MAX_PATH);
 
    printf("Child process read from shared memory: %S\n", cBuf);
    
    return 0;
}