Find last occurrence of ASCII byte in file and truncate file from that point to end? - truncate

Question

In a Windows command line environment, I'd like to be able to search a binary file for the last (final) occurrence of hex 06 char ("Ack") and truncate the file from that char to the end of the file, meaning that the found char is also trimmed off. How can I do that? The files can be several hundred megabytes in size.
EDIT: To be fair, I did quite a lot of Googling for code ideas, but my search terms are not bringing me to some kind of way to tackle this. Something like "search binary file for ASCII char hex 06, find last occurrence of that char and truncate the file from that point on," is so vague as to be essentially useless. I'll keep looking!

Answer 1

If you start reading bytes from the end of the file you will find the last ACK (if there is one). Knowing its position, you can now truncate the file.
I'm not good at PowerShell, so there might be some cmdlet I don't know about, but this achieves what you want:
$filename = "C:\temp\FindAck.txt"
$file = Get-Item $filename
$len = $file.Length
$blockSize = 32768
$buffer = new-object byte[] $blockSize
$found = $false
$blockNum = [math]::floor($len / $blockSize)
$mode = [System.IO.FileMode]::Open
$access = [System.IO.FileAccess]::Read
$sharing = [IO.FileShare]::Read
$fs = New-Object IO.FileStream($filename, $mode, $access, $sharing)
$foundPos = -1
while (!$found -and $blockNum -ge 0) {
$fs.Position = $blockNum * $blockSize
$bytesRead = $fs.Read($buffer, 0, $blocksize)
if ($bytesRead -gt 0) {
for ($i = $bytesRead -1; $i -ge 0; $i--) {
if ($buffer[$i] -eq 6) {
$foundPos = $blockNum * $blockSize + $i
$found = $true
break
}
}
}
$blockNum--
}
$fs.Dispose()
if ($foundPos -ne -1) {
$mode = [System.IO.FileMode]::Open
$access = [System.IO.FileAccess]::Write
$sharing = [IO.FileShare]::Read
$fs = New-Object IO.FileStream($filename, $mode, $access, $sharing)
$fs.SetLength($foundPos)
$fs.Dispose()
}
Write-Host $foundPos
The idea of reading in 32KB blocks is to get a reasonable size chunk from the disk to process rather than reading one byte at a time.
References:
Creating Byte[] in PowerShell
file IO, is this a bug in Powershell?
How to truncate a file in c#?
Break