11 September 2011

Reading CD-Text with VB.Net (1)

Note: most commercial CDs do not have any CD Text on them.

There are probably a few ways to read the CD Text. I’m going to look at using the windows api function DeviceIOControl function. The steps are (1) Get a device handle (2) Prepare a message (3) Send the message to the device (4) Decode the results.

To get a device handle, you call CreateFile sending the drive letter of the drive formatted as a device path. You also have to specify the minimum permissions you will be needing for the operations you will be performing. If you fail to ask for a handle with sufficient permissions then you will get errors later on when you are calling DeviceIOControl.

In .Net you can pick various types to represent handles. I’ve seen people use Integer – which isn’t good as Integer is always 32 bits whereas handles may be 64 bits. IntPtr is a better choice as it will match the size of the C HANDLE type it is representing. HandleRef is another good choice, this is used to prevent the garbage collector from destroying your handle before a call ends – which would be embarrassing. SafeHandle is the best choice to use as it ensures that the handle will be closed in certain unusual circumstances. SafeHandle is an abstract class, there are various concrete implementations. For a device handle we can use the SafeFileHandle.

The IOCTL_CDROM_READ_TOC_EX control code can be used to get the CD-Text back from the drive. Another alternative is to use a less specific control code such as  IOCTL_SCSI_PASS_THROUGH which lets you send any command to the drive, it’s a bit more involved – I’ll show how to use that later. When you are working from scratch, and trying to figure this stuff out for yourself, it helps to have a C++ project that you can use to dump out the values of the constants, size of structures, and offsets of structure fields. A bit of work in C++ can save a lot of time searching the internet with vague error messages like “A call to PInvoke function xyz has unbalanced the stack”.

If you want to set up a C++ project then the steps are:
1) Download and install the WDK. It’s a big download. The bit you need are the “Build Environments”, as selecting this will ensure that the header files are installed.
2) Start visual studio 2010 and a new C++ CLR Console application project called, say, StructSizes
3) Right click the project name in the solution explorer, click References, Add New Reference and add System.Windows.Forms.
4) Right click the project name in the solution explorer, click Properties, Drill down to Configuration Properties/VC++ Directories and then select Include Directories. Edit the string to include the path to the wdk header files. This will involve adding a “;” and then the path. Mine looks like this after the edit: $(VCInstallDir)include;$(VCInstallDir)atlmfc\include;$(WindowsSdkDir)include;$(FrameworkSDKDir)\include;C:\WinDDK\7600.16385.1\inc\api
5) As an example lets dump out the value of the IOCTL command and some info about a structure. This goes into the cpp file with the same name as the project, so StructSizes.cpp.

#include "stdafx.h"
#include <Windows.h>
#include <winioctl.h>
#include <ntddcdrm.h>
#include <ntddscsi.h>
#include <stddef.h> 
using namespace System;
using namespace System::Text;
using namespace System::Windows::Forms;
[STAThread()] // to make the clipboard happy.
int main(array<System::String ^> ^args)
    StringBuilder ^sb = gcnew System::Text::StringBuilder();
    sb->AppendLine(L"IOCTL_CDROM_READ_TOC_EX: " + (IOCTL_CDROM_READ_TOC_EX).ToString(L"x2"));
    sb->AppendLine(L"SCSI_PASS_THROUGH offsets + size:");    
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::Length)).ToString() + L" Length");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::ScsiStatus)).ToString() + L" ScsiStatus");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::PathId)).ToString() + L" PathId");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::TargetId)).ToString() + L" TargetId");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::Lun)).ToString() + L" Lun");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::CdbLength)).ToString() + L" CdbLength");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::SenseInfoLength)).ToString() + L" SenseInfoLength");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::DataIn)).ToString() + L" DataIn");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::DataTransferLength)).ToString() + L" DataTransferLength");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::TimeOutValue)).ToString() + L" TimeOutValue");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::DataBufferOffset)).ToString() + L" DataBufferOffset");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::SenseInfoOffset)).ToString() + L" SenseInfoOffset");
    sb->AppendLine((offsetof(SCSI_PASS_THROUGH, SCSI_PASS_THROUGH::Cdb[0])).ToString() + L" Cdb[0]");
    sb->AppendLine(L"**TOTAL SIZE: " + (sizeof(SCSI_PASS_THROUGH)).ToString());
    return 0;

6) It should build and run. It sets the clipboard with the text. You can’t dump out the offset of bit fields, like you get in the CDROM_READ_TOC_EX structure. You should dump out x86 and x64 versions so that you can check your .Net structures match on both.

Let’s say you’ve made your .Net structure, called ScsiPassThrough, and want to see the offset of the Length field and the size. You would use the SizeOf and OffsetOf methods of the Marshal class which is in the System.Runtime.InteropServices namespace. e.g.:

Console.WriteLine(Marshal.OffsetOf(GetType(ScsiPassThrough), "Length").ToString)

Ok, back to the task in hand. We want to send IOCTL_CDROM_READ_TOC_EX which has the value &H24054. The documentation says we have to send a structure CDROM_READ_TOC_EX to specify what exactly we want to get back from the TOC.

typedef struct _CDROM_READ_TOC_EX {
    UCHAR Format    : 4;
    UCHAR Reserved1 : 3; // future expansion
    UCHAR Msf       : 1;
    UCHAR SessionTrack;
    UCHAR Reserved2;     // future expansion
    UCHAR Reserved3;     // future expansion

This structure has a size of 4 bytes on x86 and x64. We need to set the Format field to CDROM_READ_TOC_EX_FORMAT_CDTEXT (value 5) to say we want to retrieve CD Text. All the others will be 0. The structure is made up of 4 bytes. SessionTrack, Reserved2 and Reserved3 get a byte each. The other byte is divided up into 3 fields – Format, Reserved1 and Msf. Format takes the lo 4 bits of the byte. Reserved takes the next 3 and Msf takes the hi bit. We can’t split fields up in .Net like that. We can either declare a structure that has 4 byte fields, and expose the three bit fields with properties, or, as we just want to set the one field, we could also just send an integer with the correct bit pattern - an integer has the same size and all the dll that we are calling cares about is that it gets 4 bytes with the correct bits set. I came up with this:

Friend Structure CDROM_READ_TOC_EX
    Public FormatReservedMsf As Byte
    Public SessionTrack As Byte
    Public Reserved2 As Byte
    Public Reserved3 As Byte
    ' properties will just read the public fields without contributing to the size of the structure.
    Public ReadOnly Property Format As ReadTocExFormat
            Return CType(FormatReservedMsf And &HF, ReadTocExFormat)
        End Get
    End Property
    Public ReadOnly Property Reserved1 As Integer
            Return (FormatReservedMsf >> 4) And &H7
        End Get
    End Property
    Public ReadOnly Property Msf As Boolean
            Return (FormatReservedMsf And &H80) = &H80
        End Get
    End Property
    Public Sub SetFormatReservedMsf(format As ReadTocExFormat, msf As Boolean)
        FormatReservedMsf = CByte(format Or If(msf, &H80, 0))
    End Sub
    Public ReadOnly Property Size As Integer
            Return 4
        End Get
    End Property
End Structure

The documentation says that when we send the message we should also send a buffer. On return the buffer will contain a CDROM_TOC_CD_TEXT_DATA structure, followed by an array of CDROM_TOC_CD_TEXT_DATA_BLOCK structures. So – how big should the buffer be to hold all this? The answer is that we don’t know. We can either create a huge buffer that will be big enough, or we can call once to read the CDROM_TOC_CD_TEXT_DATA.Length field and use that to size the buffer. The buffer can be an array of bytes – they are easy to marshal if one of the parameters states the size of the array. DeviceIOControl will look like this:

<DllImport("kernel32", SetLastError:=True)>
Public Shared Function DeviceIoControl(hVol As SafeFileHandle, controlCode As Integer,
   ByRef inbuffer As CDROM_READ_TOC_EX, inBufferSize As Integer,
   <MarshalAs(UnmanagedType.LPArray, SizeParamIndex:=6)> ByVal outBuffer() As Byte,
   outBufferSize As Integer,
   <Out()> ByRef bytesReturned As Integer, ovelapped As IntPtr) As Boolean
End Function

Note the structure goes ByRef – it’s a value type and we don’t want to send the whole structure to the function as it is expecting a pointer to the structure. Passing it ByRef tells the marshaller to send a pointer. The array, being a reference type, goes ByValue.

So the first time we call it we want to just get the Length field which is a WORD field, 2 bytes. Then we can call it again with a byte array that is this Length field + 2 as the value of the Length field is the size not including itself. The bytes are the wrong way round to read as a UShort with the BitConverter. I guess you could reverse the array. At this point my program gets the bytes and doesn’t do anything with them…

Imports System.IO
Imports System.Runtime.InteropServices
Public Class CdTextRetriever
    Private Const IOCTL_CDROM_READ_TOC_EX As UInteger = &H24054
    Public Shared Function GetCdText(driveInfo As DriveInfo) As Object
        Dim cdText As Object = Nothing ' don't know what the return will look like yet.
        Dim devicePath As String = String.Format("\\.\{0}:", driveInfo.Name(0))
        Const FILE_ATTRIBUTE_NORMAL As UInteger = &H80
        Const GENERIC_READ As UInteger = &H80000000UI
        Const GENERIC_WRITE As UInteger = &H40000000
        Const FILE_SHARE_READ As UInteger = 1
        Const FILE_SHARE_WRITE As UInteger = 2
        Const OPEN_EXISTING As UInteger = 3
        Using hDevice As SafeFileHandle = NativeMethods.CreateFile(devicePath,
                IntPtr.Zero, OPEN_EXISTING,
                FILE_ATTRIBUTE_NORMAL, IntPtr.Zero)
            If hDevice.IsInvalid OrElse hDevice.IsClosed Then Throw New Win32Exception
        End Using
        Return cdText
    End Function
    Private Shared Sub ReadCdText(hDevice As SafeFileHandle)
        Dim size As Integer = GetCdTextSize(hDevice)
        Dim bytes(size - 1) As Byte
        ReadCdText(hDevice, bytes)
        ' TODO: Decode the bytes!
    End Sub
    Private Shared Function GetCdTextSize(hDevice As SafeFileHandle) As Integer
        Dim bytes(1) As Byte ' 2 bytes to read the size of the buffer required
        ReadCdText(hDevice, bytes)
        Return (CType(bytes(0), Integer) << 8) Or bytes(1) + 2
    End Function
    Private Shared Sub ReadCdText(hDevice As SafeFileHandle, bytes() As Byte)
        Dim request As New CDROM_READ_TOC_EX
        request.SetFormatReservedMsf(ReadTocExFormat.CDROM_READ_TOC_EX_FORMAT_CDTEXT, False)
        Dim returned As Integer = 0
        Dim result As Boolean = DeviceIoControl(hDevice, IOCTL_CDROM_READ_TOC_EX, request,
                                                 request.Size, bytes, bytes.Length,
                                                 returned, IntPtr.Zero)
        If result = False Then Throw New Win32Exception
    End Sub
End Class

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