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Because in AutoIt's helpfile there is very poor example for function FileSetEnd() I add here for the reference my better example demonstrating expanding/truncating size of file also with retaining of original data. Note: FileSetEnd() was added in AutoIt 3.3.13.11 (2014-07-27) #include <FileConstants.au3> #include <MsgBoxConstants.au3> Example() Func Example() Local Const $sFilePath = @TempDir & "\FileSetEnd.txt" ; create file 6 bytes in size FileDelete($sFilePath) FileWrite($sFilePath,'abc123') MsgBox($MB_SYSTEMMODAL, FileGetSize($sFilePath), FileRead($sFilePath)) ; 6 / abc123 ; expand size of file to 9 bytes but don't write any data to expanded area $hFileOpen = FileOpen($sFilePath, $FO_APPEND) FileSetPos($hFileOpen, 9, $FILE_BEGIN) FileSetEnd($hFileOpen) FileClose($hFileOpen) MsgBox($MB_SYSTEMMODAL, FileGetSize($sFilePath), FileRead($sFilePath)) ; 9 / abc123 ; write data to expanded area (more than size of expanded area) so size of file will auto expand to 12 bytes (6+6) $hFileOpen = FileOpen($sFilePath, $FO_APPEND) FileSetPos($hFileOpen, 6, $FILE_BEGIN) FileWrite($hFileOpen,'def456') FileClose($hFileOpen) MsgBox($MB_SYSTEMMODAL, FileGetSize($sFilePath), FileRead($sFilePath)) ; 12 / abc123def456 ; truncate size of file to 9 bytes $hFileOpen = FileOpen($sFilePath, $FO_APPEND) FileSetPos($hFileOpen, 9, $FILE_BEGIN) FileSetEnd($hFileOpen) FileClose($hFileOpen) MsgBox($MB_SYSTEMMODAL, FileGetSize($sFilePath), FileRead($sFilePath)) ; 9 / abc123def FileDelete($sFilePath) EndFunc ;==>Example Remarks: - current position for FileSetEnd() can be changed by FileSetPos() - for expanding/truncating of existing files with preserved content have to be used mode $FO_APPEND in FileOpen() EDIT: This my example in slightly changed version was added as example #2 to AutoIt's helpfile in version 3.3.13.14 (2014-08-02).