Q128112: INFO: CFile::end (from CFile::Seek) Uses a Forward Offset

Article: Q128112
Product(s): Microsoft C Compiler
Version(s): winnt:4.0,4.1; :1.0,1.5,1.51
Operating System(s): 
Keyword(s): kbprogramming kbdocfix kbdocerr kbFileIO kbMFC kbVC100bug kbVC151bug kbVC152bug kbVC200
Last Modified: 07-MAY-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, 1.5, 1.51 
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SUMMARY
=======

When using CFile::Seek() with the CFile::end parameter to start seeking from the
end of a file, a negative offset is required to seek backward in the file. The
documentation for CFile::end states:

  CFile::end  Move the file pointer backward lOff bytes from the end
              of the file.

This incorrectly indicates that a positive offset value will seek backwards in
the file. Using 16-bit versions of MFC, CFile::Seek() performs the actual seek
using int 21h function 42h. The "MS-DOS Encyclopedia" states that when seeking
from the end of file, the offset:

  ... can be either a positive or negative 32-bit integer ... [which] can move
  the file pointer either forward or backward from the end of the file.

When using MFC under Windows NT, CFile::Seek() calls SetFilePointer.
Documentation for the offset parameter to SetFilePointer is as follows:

  ... specifies the number of bytes to move the file pointer. A positive value
  moves the pointer forward in the file and a negative value moves it backward.

For both 16 and 32-bit versions of MFC, the code that performs the actual seek
can be found in FILECORE.CPP.

NOTE: This problem was fixed in Microsoft Visual C++, 32-bit Edition, version
4.0. The Visual C++ 4.0 Books On-Line states the following:

  CFile::end Move the file pointer lOff bytes from the end of the file. Note
  that lOff must be negative to seek into the existing file; positive values
  will seek past the end of the file.

Additional query words: noupdate 2.5 2.50 2.51 3.0 3.00 1.00 1.50 1.51 2.00 2.10 4.00

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Keywords          : kbprogramming kbdocfix kbdocerr kbFileIO kbMFC kbVC100bug kbVC151bug kbVC152bug kbVC200bug kbVC210bug kbVC400fix kbGrpDSMFCATL 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbMFC kbvc150 kbvc100 kbVC410 kbVC151
Version           : winnt:4.0,4.1; :1.0,1.5,1.51

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