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, 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 (HANDLE)0xFFFFFFFF 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 (HANDLE)0xFFFFFFFF 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 Processes and Threads and Handles and Objects.

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.

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