Shared Memory

In the Win32 API, shared memory is implemented by file mapping. All memory allocated by the other allocation methods (the GlobalAlloc, LocalAlloc, HeapAlloc, or VirtualAlloc function) is accessible only to the calling process. Memory allocated by a DLL, however, is in the address space of the process that called the DLL and is not accessible to other processes using the same DLL.

Named file mapping provides an easy way to create a block of shared memory. A process can specify a name when it uses the CreateFileMapping function to create a file-mapping object. Other processes can specify the same name to either the CreateFileMapping or OpenFileMapping function to obtain a handle of the mapping object. The names of event objects, semaphore objects, mutex objects, and file-mapping objects share the same name space. If a specified name matches the name of an existing object of a different type, an error occurs. When creating named objects, try to use unique names and check the function return values for duplicate name errors.

Each process specifies its handle of the file-mapping object in the MapViewOfFile function to map a view of the file into its own address space. The views of all processes for a single file-mapping object are mapped into the same sharable pages of physical storage. However, the virtual addresses of the mapped views can vary from one process to another, unless the MapViewOfFileEx function is used to map the view at a specified address. Although sharable, the pages of physical storage used for a mapped file view are not global; they are not accessible to processes that have not mapped a view of the file.

A file-mapping object is associated with a disk file that the system uses when the mapped view is swapped out of physical memory and onto the disk. This disk swap file can be the system's paging file, or it can be some other file that was specified when the file-mapping object was created. In that case, the memory is initialized along with the contents of the file. Mapping a specified file in the file system is useful for processes that need to share the data in an existing file or that want to use the file to save data generated by the sharing processes. If you map a specified file, you should open it for exclusive access and keep the handle open until you are finished with the shared memory. Keeping it open will prevent other processes from opening another handle on the file to use ReadFile or WriteFile or from creating additional mapping objects for the same file, any of which actions can lead to unpredictable results.

Any pages committed by mapping a view of a file are released when the last process with a view of the mapping object either terminates or unmaps its view by calling the UnmapViewOfFile function. At this time, the specified file (if any) associated with the mapping object is updated. A specified file can also be forced to update by calling the FlushViewOfFile function.

For more information about file mapping, see File Mapping. For an example of shared memory in a DLL, see Dynamic-Link Libraries.

If multiple processes have write access to shared memory, access to the memory should be synchronized. For more information about interprocess synchronization, see Synchronization.

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