#include <FileConstants.au3>
#include <MsgBoxConstants.au3>
#include <WinAPIFiles.au3>

Example()

Func Example()
        ; Create a constant variable in Local scope of the filepath that will be read/written to.
        Local Const $sFilePath = _WinAPI_GetTempFileName(@TempDir)

        ; Open the file for writing (overwrite the file) and store the handle to a variable.
        Local $hFileOpen = FileOpen($sFilePath, $FO_OVERWRITE)
        If $hFileOpen = -1 Then
                MsgBox($MB_SYSTEMMODAL, "", "An error occurred whilst writing the temporary file.")
                Return False
        EndIf

        ; Write data to the file using the handle returned by FileOpen.
        FileWriteLine($hFileOpen, "Line 1")
        FileWriteLine($hFileOpen, "Line 2")
        FileWriteLine($hFileOpen, "Line 3")

        ; Flush the file to disk.
        FileFlush($hFileOpen)

        ; Now, adjust the position to the beginning.
        FileSetPos($hFileOpen, 0, $FILE_BEGIN)

        ; Check file position and try to read contents for current position.
        MsgBox($MB_SYSTEMMODAL, "", "Position: " & FileGetPos($hFileOpen) & @CRLF & "Data: " & @CRLF & FileRead($hFileOpen))

        ; Write data to the file using the handle returned by FileOpen.
        FileWriteLine($hFileOpen, "Line 4")
        FileWriteLine($hFileOpen, "Line 5")
        FileWriteLine($hFileOpen, "Line 6")

        ; Now, adjust the position to the beginning.
        FileSetPos($hFileOpen, 0, $FILE_BEGIN)

        ; Check file position and try to read contents for current position.
        MsgBox($MB_SYSTEMMODAL, "", "Position: " & FileGetPos($hFileOpen) & @CRLF & "Data: " & @CRLF & FileRead($hFileOpen))

        ; Now, adjust the position to the beginning.
        FileSetPos($hFileOpen, 0, $FILE_BEGIN)

        ; Set the end of the file at the current position.
        FileSetEnd($hFileOpen)

        ; Check file position and try to read contents for current position. The contents have been destroyed.
        MsgBox($MB_SYSTEMMODAL, "", "Position: " & FileGetPos($hFileOpen) & @CRLF & "Data: " & @CRLF & FileRead($hFileOpen))

        ; Close the handle returned by FileOpen.
        FileClose($hFileOpen)

        ; Delete the temporary file.
        FileDelete($sFilePath)
EndFunc   ;==>Example

#include <FileConstants.au3>
#include <MsgBoxConstants.au3>
#include <WinAPIFiles.au3>

Example()

Func Example()
        ; Create a constant variable in Local scope of the filepath that will be read/written to.
        Local Const $sFilePath = _WinAPI_GetTempFileName(@TempDir)

        ; Open the file for writing (overwrite the file) and store the handle to a variable.
        Local $hFileOpen = FileOpen($sFilePath, $FO_OVERWRITE)
        If $hFileOpen = -1 Then
                MsgBox($MB_SYSTEMMODAL, "", "An error occurred whilst writing the temporary file.")
                Return False
        EndIf

        ; Write data to the file using the handle returned by FileOpen and set the end of the file.
        FileWrite($hFileOpen, "ABCDEF")
        FileSetEnd($hFileOpen)

        ; Display the file size and contents.
        MsgBox($MB_SYSTEMMODAL, "", "Size: " & FileGetSize($sFilePath) & @CRLF & "Data: " & @CRLF & FileRead($sFilePath))

        ; Expand the size of the file to 9 bytes and don't write any additional data.
        FileSetPos($hFileOpen, 9, $FILE_BEGIN)
        FileSetEnd($hFileOpen)

        ; Display the file size and contents.
        MsgBox($MB_SYSTEMMODAL, "", "Size: " & FileGetSize($sFilePath) & @CRLF & "Data: " & @CRLF & FileRead($sFilePath))

        ; Move the position after ABCDEF i.e. 6 bytes and then write additional data which will expand the file to 12 bytes.
        FileSetPos($hFileOpen, 6, $FILE_BEGIN)
        FileWrite($hFileOpen, "HIJKLM")
        FileSetEnd($hFileOpen) ; Set the end of the file.

        ; Display the file size and contents.
        MsgBox($MB_SYSTEMMODAL, "", "Size: " & FileGetSize($sFilePath) & @CRLF & "Data: " & @CRLF & FileRead($sFilePath))

        ; Truncate the file size to 9 bytes.
        FileSetPos($hFileOpen, 9, $FILE_BEGIN)
        FileSetEnd($hFileOpen) ; Set the end of the file.

        ; Display the file size and contents.
        MsgBox($MB_SYSTEMMODAL, "", "Size: " & FileGetSize($sFilePath) & @CRLF & "Data: " & @CRLF & FileRead($sFilePath))

        ; Close the handle returned by FileOpen.
        FileClose($hFileOpen)

        ; Delete the temporary file.
        FileDelete($sFilePath)
EndFunc   ;==>Example