HOWTO: How to Access CMemFile Directly When Using It with CArchive (132079)
The information in this article applies to:
- Microsoft Visual C++ 4.0
- Microsoft Visual C++ 4.1
- Microsoft Foundation Classes (MFC)
- Microsoft Visual C++ 1.0
- Microsoft Visual C++ 2.0
- Microsoft Visual C++ 2.1
- Microsoft Visual C++ 2.2
This article was previously published under Q132079 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.
| Modification Type: | Major | Last Reviewed: | 12/2/2003 |
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| Keywords: | kbFileIO kbhowto KB132079 |
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