Creating Named Shared Memory
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 the CreateFileMapping function with INVALID_HANDLE_VALUE and a name for the object. By using the PAGE_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 that
CreateFileMapping returns in a call to
MapViewOfFile to create a view of the file in the
process address space. The MapViewOfFile function
returns a pointer to the file view, pBuf. The process then uses the
CopyMemory 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 the SeCreateGlobalPrivilege privilege.
When the process no longer needs access to the file mapping object, it should call the CloseHandle 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 256 TCHAR 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 the
OpenFileMapping function specifying the same name for
the mapping object as the first process. Then it can use the
MapViewOfFile 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 256 TCHAR 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; }
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