If the handle passed in CX already refers to an open file, that file is closed first.Given two handles, makes the second handle refer to the same device or file at the same location as the first handle. The second handle is then said to be redirected.
Call with:
AH = 46H
BX = handle for file or device
CX = handle to be redirected
Returns:
If function successful
Carry flag = clear
If function unsuccessful
Carry flag = set
AX = error code
Notes:
If the handle passed in CX already refers to an open file, that file is closed first.
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 is commonly used to redirect the standard input and output handles to another file or device before a child process is executed with Int 21H Function 4BH.
Example:
Redirect the standard output to the list device, so that all output directed to the console will appear on the printer instead. Later, restore the original meaning of the standard output handle.
stdin equ 0
stdout equ 1
stderr equ 2
stdaux equ 3
stdprn equ 4
dhandle dw 0 ; duplicate handle
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.
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; get dup of stdout
mov ah,45h ; function number
mov bx,stdout ; standard output handle
int 21h ; transfer to MS-DOS
jc error ; jump if dup failed
mov dhandle,ax ; save dup'd handle
;
; redirect standard output
; to standard list device
mov ah,46h ; function number
mov bx,stdprn ; standard list handle
mov cx,stdout ; standard output handle
int 21h ; transfer to MS-DOS
jc error ; jump if redirect failed
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.
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; restore standard output
; to original meaning
mov ah,46h ; function number
mov bx,dhandle ; saved duplicate handle
mov cx,stdout ; standard output handle
int 21h ; transfer to MS-DOS
jc error ; jump if redirect failed
; close duplicate handle
; because no longer needed
mov ah,3eh ; function number
mov bx,dhandle ; saved duplicate handle
int 21h ; transfer to MS-DOS
jc error ; jump if close failed
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