Function 45h Duplicate File Handle

mov bx, OldHandle ;handle to duplicate

mov ah, 45h ;Duplicate File Handle

int 21h

jc error_handler ;carry set means error

mov NewHandle, ax ;refers to same file as OldHandle

Duplicate File Handle (Function 45h) creates a new file handle that can be used to read from or write to the same file or device associated with the original handle.

Parameter

OldHandle

Identifies the handle to be duplicated.

Return Value

If the function is successful, the carry flag is clear and the AX register contains the new handle. Otherwise, the carry flag is set and the AX register contains an error value, which may be one of the following:

Value

Name

0004h	ERROR_TOO_MANY_OPEN_FILES
0006h	ERROR_INVALID_HANDLE

Comments

If this function is used to duplicate the handle of an open file, the file pointer for the new handle is set to the same position as the pointer for the old handle. Using either handle to read from or write to the file changes the file pointer for both handles.

Duplicate File Handle can also be used to keep a file open while its directory entry is changed. If a program creates a duplicate handle and then closes the original handle, the file's directory entry is updated, but the duplicate handle can still be used to read from or write to the file.