Q132079: HOWTO: How to Access CMemFile Directly When Using It with CArchi

Article: Q132079
Product(s): Microsoft C Compiler
Version(s): winnt:4.0,4.1; :1.0,2.0,2.1,2.2,4.0,4.1
Operating System(s): 
Keyword(s): kbFileIO kbMFC kbVC100 kbVC150 kbVC200 kbVC400 kbVC410 kbGrpDSMFCATL
Last Modified: 29-JUL-2001

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The information in this article applies to:

- Microsoft Visual C++, versions 4.0, 4.1 
- Microsoft Foundation Classes (MFC) 
- Microsoft Visual C++, versions 1.0, 2.0, 2.1, 2.2 
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SUMMARY
=======

When using CMemFile in conjunction with CArchive, problems may arise from
calling CArchive::GetFile and subsequently using the CFile pointer returned for
direct file access.

This is true for CFile and any class derived from CFile. Because CArchive buffers
I/O, you need to call CArchive::Flush() before using any of the members of
CFile.

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

When a CMemFile object is used with a CArchive object, the two objects share a
single buffer. Both objects retain separate pointers to this shared buffer. It
is possible for the two objects to become out of synch if the file is accessed
directly through a file pointer retrieved by calling CArchive::GetFile.

For example, say you open a CMemFile, and start writing to it using the
serialization functions. Then, suppose you call CMemFile::GetFile, and write to
the file using CFile::Write. The first thing that the CMemFile object does is
make sure it has enough space for the pending write. For this example, assume
that there is not enough space to satisfy the write. This means that the
CMemFile object must reallocate the buffer. This will most likely change the
location of the buffer. The problem occurs when you write to the CMemFile again
through the CArchive object. The CArchive object still has a pointer to the old
buffer, because the object has no way of knowing that the CMemFile reallocated
the shared buffer. Any attempt to write through this pointer does not write to
the appropriate file. Therefore, the data is missing from the file where it
should have been written.

Additional query words: kbinf 1.00 1.50 2.00 2.10 2.50 2.51 2.52 3.00 3.10 3.20 2.20 4.00 4.10

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Keywords          : kbFileIO kbMFC kbVC100 kbVC150 kbVC200 kbVC400 kbVC410 kbGrpDSMFCATL 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbMFC kbvc100 kbVC220 kbVC410 kbVC200 kbVC210
Version           : winnt:4.0,4.1; :1.0,2.0,2.1,2.2,4.0,4.1
Issue type        : kbhowto

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