DOC: CFile::end (from CFile::Seek) Uses a Forward Offset

Last reviewed: July 2, 1997
Article ID: Q128112

1.00 1.50 1.51 | 1.00 2.00 2.10

WINDOWS        | WINDOWS NT

kbprg kbdocerr kbdocfix

The information in this article applies to:

The Microsoft Foundation Classes (MFC), included with:

    - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51
    - Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1

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.

KBCategory: kbprg kbdocerr kbdocfix
KBSubCategory: MfcMisc
Additional reference words: 2.5 2.50 2.51 3.0 3.00 1.00 1.50 1.51 1.00 2.00
2.10 4.00
Keywords : kbdocerr kbdocfix kbprg
Technology : kbMfc
Version : 1.00 1.50 1.51 | 1.00 2.00 2.10
Platform : NT WINDOWS

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Last reviewed: July 2, 1997
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