To share data, multiple processes can use memory-mapped files that the system paging file stores.

First Process

The first process creates the file mapping object by calling theCreateFileMapping function withINVALID_HANDLE_VALUE and a name for the object. By using thePAGE_READWRITE flag, the process has read/write permission to the memory through any file views that are created.

Then the process uses the file mapping object handle thatCreateFileMapping returns in a call toMapViewOfFile to create a view of the file in the process address space. TheMapViewOfFile function returns a pointer to the file view,pBuf. The process then uses theCopyMemory function to write a string to the view that can be accessed by other processes.

Prefixing the file mapping object names with "Global\" allows processes to communicate with each other even if they are in different terminal server sessions. This requires that the first process must have theSeCreateGlobalPrivilege privilege.

When the process no longer needs access to the file mapping object, it should call theCloseHandle function. When all handles are closed, the system can free the section of the paging file that the object uses.

#include <windows.h>#include <stdio.h>#include <conio.h>#include <tchar.h>#define BUF_SIZE 256TCHAR szName[]=TEXT("Global\\MyFileMappingObject");TCHAR szMsg[]=TEXT("Message from first process.");int _tmain(){   HANDLE hMapFile;   LPCTSTR pBuf;   hMapFile = CreateFileMapping(                 INVALID_HANDLE_VALUE,    // use paging file                 NULL,                    // default security                 PAGE_READWRITE,          // read/write access                 0,                       // maximum object size (high-order DWORD)                 BUF_SIZE,                // maximum object size (low-order DWORD)                 szName);                 // name of mapping object   if (hMapFile == NULL)   {      _tprintf(TEXT("Could not create file mapping object (%d).\n"),             GetLastError());      return 1;   }   pBuf = (LPTSTR) MapViewOfFile(hMapFile,   // handle to map object                        FILE_MAP_ALL_ACCESS, // read/write permission                        0,                        0,                        BUF_SIZE);   if (pBuf == NULL)   {      _tprintf(TEXT("Could not map view of file (%d).\n"),             GetLastError());       CloseHandle(hMapFile);      return 1;   }   CopyMemory((PVOID)pBuf, szMsg, (_tcslen(szMsg) * sizeof(TCHAR)));    _getch();   UnmapViewOfFile(pBuf);   CloseHandle(hMapFile);   return 0;}

Second Process

A second process can access the string written to the shared memory by the first process by calling theOpenFileMapping function specifying the same name for the mapping object as the first process. Then it can use theMapViewOfFile function to obtain a pointer to the file view,pBuf. The process can display this string as it would any other string. In this example, the message box displayed contains the message "Message from first process" that was written by the first process.

#include <windows.h>#include <stdio.h>#include <conio.h>#include <tchar.h>#pragma comment(lib, "user32.lib")#define BUF_SIZE 256TCHAR szName[]=TEXT("Global\\MyFileMappingObject");int _tmain(){   HANDLE hMapFile;   LPCTSTR pBuf;   hMapFile = OpenFileMapping(                   FILE_MAP_ALL_ACCESS,   // read/write access                   FALSE,                 // do not inherit the name                   szName);               // name of mapping object   if (hMapFile == NULL)   {      _tprintf(TEXT("Could not open file mapping object (%d).\n"),             GetLastError());      return 1;   }   pBuf = (LPTSTR) MapViewOfFile(hMapFile, // handle to map object               FILE_MAP_ALL_ACCESS,  // read/write permission               0,               0,               BUF_SIZE);   if (pBuf == NULL)   {      _tprintf(TEXT("Could not map view of file (%d).\n"),             GetLastError());      CloseHandle(hMapFile);      return 1;   }   MessageBox(NULL, pBuf, TEXT("Process2"), MB_OK);   UnmapViewOfFile(pBuf);   CloseHandle(hMapFile);   return 0;}

Related topics

Sharing Files and Memory