Weird WriteUint8 behavior

General
1

Hello

I want to parse a BinaryStream file and change bytes that way:

var Track As UInt8

D81Image.BytePosition = 400128

Do
  MainWindow.Console.Value = MainWindow.Console.Value  + "Read before: " + D81Image.BytePosition.ToString + EndOfLine
  Track = D81Image.ReadUInt8
  MainWindow.Console.Value = MainWindow.Console.Value  + "Read after: " + D81Image.BytePosition.ToString + EndOfLine

  If Track = 0 Then
    MainWindow.Console.Value = MainWindow.Console.Value  + "Write before: " + D81Image.BytePosition.ToString + EndOfLine
    D81Image.WriteUInt8( &hFF )
    MainWindow.Console.Value = MainWindow.Console.Value  + "Write after: " + D81Image.BytePosition.ToString + EndOfLine

    Exit

  Else
    // Jump over to next sector
    D81Image.BytePosition = D81Image.BytePosition + 255
  End If

Loop Until D81Image.BytePosition >= 409600

The output to the console is this:

Read before: 400128
Read after: 400129
Read before: 400384
Read after: 400385
Write before: 400385
Write after: 404481

In second loop iteration the code Track is 0 and heads to “D81Image.WriteUInt8( &hFF )”. The weird thing: Before writing the BytePosition is at 400385 (correct) but after writing a single byte the BytePosition is set to BytePosition 404481 instead of expected 400386. Either there is a bug or I am missing something trivial.

Any ideas about this?

2

I see no bug here, you’ve done a good job of adding effective logging.

Does it behave this way when you step through the debugger?

3

Glad to hear it’s not my bug…

Yes, that’s the output running in debug mode, the output is sent to a textarea item. I compiled it as well but same result.

I never worked with the debugger, have to find out how to step through it.

Xojo for Mac, V2025, R3.1

4

I tried this, but doesn’t show the same problem:

var f as FolderItem = GetFolderItem("test.jpg")
Var b As BinaryStream = BinaryStream.Open(f, true)

b.Position = 123456
Var p1 As Integer = b.Position
b.Write "Hello World"
Var p2 As Integer = b.Position
System.DebugLog str(p2-p1)+" bytes"

Var p3 As Integer = b.Position
b.WriteUInt8 123
Var p4 As Integer = b.Position
System.DebugLog Str(p4-p3)+" bytes"

Var p5 As Integer = b.Position
b.WriteUInt16 123
Var p6 As Integer = b.Position
System.DebugLog Str(p6-p5)+" bytes"

Var p7 As Integer = b.Position
b.Writeint8 123
Var p8 As Integer = b.Position
System.DebugLog Str(p8-p7)+" bytes"

Break

b.Close

Works fine:

     : 11 bytes
     : 1 bytes
     : 2 bytes
     : 1 bytes