Overwrite a file without destroying the previous contents

[tebasuna51]

I want modify some bytes in one big file, there are a way to do that?

There are:

$FO_APPEND (1) = Write mode (append to end of file)

$FO_OVERWRITE (2) = Write mode (erase previous contents)

But not a Read/Write mode to go to a position, modify some bytes and leave the rest intact.

The $FO_BINARY (16) = Force binary mode, don't help.

[Nine]

Maybe this :

#include <Constants.au3>

Local $hFile = FileOpen("Test.txt", $FO_APPEND)
FileSetPos($hFile, 5, $FILE_BEGIN)
FileWrite($hFile, "12345")
FileClose($hFile)

 

[Musashi]

Or maybe this :

#include <Constants.au3>

Global $bInsert = "0xFFFFFF"
Global $hFile = FileOpen("mslogo.jpg", BitOR($FO_APPEND, $FO_BINARY))
FileSetpos($hFile, 20, $FILE_BEGIN ) ; Pos = 20
FileWrite($hfile, $bInsert)
FileClose($hFile)

==> Info : A better way to work with binary data is to use a struct !

I would advise you to always create a backup of the file before you change it - safety first 

Edited Tuesday at 03:47 PM by Musashi

[TheSaint]

And you can also use other methods, especially if this is just a text file.

A very simple approach if you know your insertion point, and it is based on unique text, is to FileRead your file, then use StringSplit with the unique text, then add all three parts together and then FileWrite back to the file with that, overwriting it. The only tricky bit, is if you are not familiar with an array, which is what StringSplit creates.

But as suggested, back your file up first, just in case.

Have a play, and any further questions, keep asking here.

[tebasuna51]

Thanks all. Since my file isn't a text file, I need to open it as $FO_APPEND+$FO_BINARY.

If it opens as a text file by default, writing adds an unwanted extra chr(15) (?).

I use this to change the channel mask (4 bytes) of a wav file (maybe larger than 10 GB). Rewriting the entire file takes time and space.

[tebasuna51]

Code used:

$fh = FileOpen($wav_file, 17)   ; $FO_APPEND (1) + $FO_BINARY (16) = 17
FileSetPos($fh, $pos_mask, 0)   ; $FILE_BEGIN (0) = Beginning of the file.
FileWrite($fh, number($mask,1)) ; $NUMBER_32BIT (1) = 32bit integer (to be sure always 4 bytes)
FileClose($fh)

[TheSaint]

Goodo.

I remember having issues once upon a time (some years ago) with surround sound (multiple channel) wav files not being recognized as such, due to some header issue etc.