Function 45h (69)        Duplicate a File Handle (DUP)

Creates a new file handle by duplicating the existing handle of an
open file or device. The new handle shares the existing file pointer.

On entry:      AH         45h
BX         File handle

Returns:       AX         Duplicate file handle, or
Error code, if CF is set

Error Codes:   4          No handles available
6          Invalid handle

Call Function 59h for extended error code information
(DOS 3.0 and above).

                                                                          

Function 45h (DUP) returns a new file handle that refers to the same
file or device as the old file handle specified in BX.

Notes:         Moving the file pointer of either handle by a Read,
Write, or Lseek causes the other handle to be
changed also. However, closing one handle does not
close the other.

By duplicating a handle and closing one of the
handles, you can flush the file to disk and update
the directory; since the other handle will still be
open, you won't need to re-open the file, a time-
consuming process.

See also: 46h
See also: 59h

45h (69) Duplicate a File Handle (DUP)