Sharing Files and Memory

File mapping can be used to share a file or memory between two or more processes. To share a file or memory, all of the processes must use the name or the handle of the same file mapping object.

To share a file, the first process creates or opens a file by using the CreateFile function. Next, it creates a file mapping object by using the CreateFileMapping function, specifying the file handle and a name for the file mapping object. The names of event, semaphore, mutex, waitable timer, job, and file mapping objects share the same name space. Therefore, the CreateFileMapping and OpenFileMapping functions fail if they specify a name that is in use by an object of another type.

To share memory that is not associated with a file, a process must use the CreateFileMapping function and specify INVALID_HANDLE_VALUE as the hFile parameter instead of an existing file handle. The corresponding file mapping object accesses memory backed by the system paging file. You must specify a size greater than zero when you specify an hFile of INVALID_HANDLE_VALUE in a call to CreateFileMapping.

The easiest way for other processes to obtain a handle of the file mapping object created by the first process is to use the OpenFileMapping function and specify the object's name. This is referred to as named shared memory. If the file mapping object does not have a name, the process must obtain a handle to it through inheritance or duplication. For more information on inheritance and duplication, see Inheritance.

Processes that share files or memory must create file views by using the MapViewOfFile or MapViewOfFileEx function. They must coordinate their access using semaphores, mutexes, events, or some other mutual exclusion technique. For more information, see Synchronization.

A shared file mapping object will not be destroyed until all processes that use it close their handles to it by using the CloseHandle function.

For information about file mapping object security, see File Mapping Security and Access Rights.

See Also

Creating Named Shared Memory

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.

When the process does not need 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>

#define BUF_SIZE 256
TCHAR szName[]=TEXT("Global\\MyFileMappingObject");
TCHAR szMsg[]=TEXT("Message from first process");

int main()
{
   HANDLE hMapFile;
   LPCTSTR pBuf;

   hMapFile = CreateFileMapping(
                 INVALID_HANDLE_VALUE,    // use paging file
                 NULL,                    // default security 
                 PAGE_READWRITE,          // read/write access
                 0,                       // max. object size 
                 BUF_SIZE,                // buffer size  
                 szName);                 // name of mapping object
 
   if (hMapFile == NULL) 
   { 
      printf("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) 
   { 
      printf("Could not map view of file (%d).\n", 
             GetLastError()); 
      return 2;
   }

   
   CopyMemory((PVOID)pBuf, szMsg, strlen(szMsg));
   _getch();

   UnmapViewOfFile(pBuf);

   CloseHandle(hMapFile);

   return 0;
}

Second Process

A second process can access the same data by calling the OpenFileMapping function with the same name as the first process. Then it can use the MapViewOfFile function to obtain a pointer to the file view.

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

#define BUF_SIZE 256
TCHAR szName[]=TEXT("Global\\MyFileMappingObject");

int main()
{
   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) 
   { 
      printf("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) 
   { 
      printf("Could not map view of file (%d).\n", 
             GetLastError()); 
      return 2;
   }

   MessageBox(NULL, pBuf, TEXT("Process2"), MB_OK);

   UnmapViewOfFile(pBuf);

   CloseHandle(hMapFile);
 
   return 0;
}