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

ID: Q128112

The information in this article applies to:

Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1
Microsoft Foundation Classes (MFC)
Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51

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

Keywords : kbdocfix
Version : winnt:4.0,4.1; :1.0,1.5,1.51
Platform : winnt
Issue type :