A seek, read, or write operation that moves the file pointer for one of the two handles also moves the file pointer associated with the other.Given a handle for a currently open device or file, returns a new handle that refers to the same device or file at the same position.
Call with:
AH = 45H
BX = handle to be duplicated
Returns:
If function successful
Carry flag = clear
AX = new handle
If function unsuccessful
Carry flag = set
AX = error code
Notes:
A seek, read, or write operation that moves the file pointer for one of the two handles also moves the file pointer associated with the other.
This function can be used to efficiently update the directory for a file that has changed in length, without incurring the overhead of closing and then reopening the file. The handle for the file is simply duplicated with this function and the duplicate is closed, leaving the original handle open for further read/write operations.
[3.3] See also Int 21H Function 68H (Commit File).
Example:
Duplicate the handle stored in the variable fhandle, then close the duplicate. This ensures that all buffered data is physically written to disk and that the directory entry for the corresponding file is updated, but leaves the original handle open for subsequent file operations.
fhandle dw 0 ; file handle
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; get duplicate handle
mov ah,45h ; function number
mov bx,fhandle ; original file handle
int 21h ; transfer to MS-DOS
jc error ; jump if dup failed
; now close dup'd handle
mov bx,ax ; put handle into BX
mov ah,3eh ; function number
int 21h ; transfer to MS-DOS jc error
; jump if close failed
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