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.