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
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Build date: 11/19/2009