Q142377: MMFILE - Class for Memory-Mapped Files

Article: Q142377
Product(s): Microsoft C Compiler
Version(s): winnt:2.0,2.1,2.2,4.0
Operating System(s): 
Keyword(s): kbcode kbfile kbsample kbFileIO kbMFC kbVC200 kbVC400 kbGrpDSMFCATL
Last Modified: 06-MAY-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 2.2, 4.0 
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SUMMARY
=======

Memory-mapped files allow a file on disk to be associated with an address space.
Once this mapping is done the data in the file can be accessed as if the file
was in memory. Memory-mapped files provide a method on Win32 for sharing blocks
of memory between processes.

The CMemoryMappedFile class provides a C++ wrapper class for creating a
memory-mapped file and using it. The MMFILE.TXT file included with code provides
more information about how to use the class.

The following files are available for download from the Microsoft Download
Center:

Mmfile.exe

For additional information about how to download Microsoft Support files, click
the article number below to view the article in the Microsoft Knowledge Base:

  Q119591 How to Obtain Microsoft Support Files from Online Services

Microsoft used the most current virus detection software available on the date of
posting to scan this file for viruses. Once posted, the file is housed on secure
servers that prevent any unauthorized changes to the file.


MORE INFORMATION
================

CMemoryMappedFile is derived from CMemFile. It wraps the file mapping APIs to
manage the file mapping. A memory-mapped file does not necessarily have to be
associated with a disk file.

To create a memory-mapped file without an associated disk file, call
CMemoryMappedFile::Open(). The first argument has to be supplied and it can be
any string. This memory-mapped file can be accessed by other processes by using
this string when they open the file. The other arguments to Open specify the
size and protection flags for this file. The default parameters creates a
memory-mapped file with a size of 4096 bytes and does not map to a disk file.

Mapping a file on disk into memory involves opening the disk file with the
correct access. Then call CMemoryMappedFile::Open() and pass in the handle to
the file. The other parameters specify protection flags for the mapping, size of
the mapping, and the part of the file to map. The default parameters try to map
the complete file into memory.

The code below shows the two ways of creating the memory-mapped file.

REFERENCES
==========

"Advanced Windows" - Jeffrey Richter Win32 SDK Docs

Sample Code
-----------

     // Code to open a memory-mapped file
     // 
     CMemoryMappedFile SharedFile1;

     // Opens a memory-mapped file without creating a file on disk.
     // This or other processes can access the file by using the
     // name. Default size is 4096 bytes.
     SharedFile1.Open("Test");
     ...
     SharedFile1.Close();

     //Open a file on disk and map it into memory
     CFile file("DiskFile", CFile::modeReadWrite |
                CFile::modeCreate | CFile::shareDenyNone);

     CMemoryMappedFile SharedFile2;
     SharedFile2.Open("MapFile", (HANDLE) file.m_hFile);
     ...
     SharedFile2.Close();
     file.Close();

Additional query words: 4.00 2.00 2.10 2.20 memory mapped

======================================================================
Keywords          : kbcode kbfile kbsample kbFileIO kbMFC kbVC200 kbVC400 kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:2.0,2.1,2.2,4.0

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