Non-blocking read of stdin?

Question

I need to have my form-based application check stdin periodically for input, but still perform other processing. Scripting.TextStream.Read() and the ReadFile() API are blocking,

Answer 1

I am afraid that I haven't managed to get this to work as of yet, however someone else might be able to have a go. The ideas was to use asynchronous I/O with the console std input (I assume the idea of your app is to allow people to write directly into the console window, and read the input as it comes).

I separated off all the API stuff into a module (MAsynchConsole):

Option Explicit

Private Const GENERIC_READ          As Long = &H80000000
Private Const GENERIC_WRITE         As Long = &H40000000
Private Const OPEN_EXISTING         As Long = 3&
Private Const FILE_FLAG_OVERLAPPED  As Long = &H40000000
Private Const FILE_SHARE_READ       As Long = &H1

Private Const FILE_FLAG_NO_BUFFERING As Long = &H20000000

Private Type OVERLAPPED
    Internal                    As Long
    InternalHigh                As Long
    OffsetOrPointer             As Long
    OffsetHigh                  As Long
    hEvent                      As Long
End Type

Private Type OVERLAPPED_ENTRY
    lpCompletionKey             As Long
    lpOverlapped                As Long ' pointer to OVERLAPPED
    Internal                    As Long
    dwNumberOfBytesTransferred  As Long
End Type

Private Declare Function AllocConsole Lib "kernel32" () As Long

Private Declare Function CancelIo Lib "Kernel32.dll" ( _
    ByVal hFile As Long _
) As Long

Private Declare Function CreateFile Lib "Kernel32.dll" Alias "CreateFileW" ( _
    ByVal lpFileName As Long, _
    ByVal dwDesiredAccess As Long, _
    ByVal dwShareModen As Long, _
    ByRef lpSecurityAttributes As Any, _
    ByVal dwCreationDisposition As Long, _
    ByVal dwFlagsAndAttributes As Long, _
    ByVal hTemplateFile As Long _
) As Long

Private Declare Function FreeConsole Lib "kernel32" () As Long

Private Declare Function GetStdHandle Lib "kernel32" ( _
    ByVal nStdHandle As Long _
) As Long


Private Declare Function ReadFile Lib "Kernel32.dll" ( _
    ByVal hFile As Long, _
    ByVal lpBuffer As Long, _
    ByVal nNumberOfBytesToRead As Long, _
    ByRef lpNumberOfBytesRead As Long, _
    ByRef lpOverlapped As OVERLAPPED _
) As Long

Private Declare Function ReadFileEx Lib "Kernel32.dll" ( _
    ByVal hFile As Long, _
    ByVal lpBuffer As Long, _
    ByVal nNumberOfBytesToRead As Long, _
    ByRef lpOverlapped As OVERLAPPED, _
    ByVal lpCompletionRoutine As Long _
) As Long

Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long

Private m_hStdIn                    As Long
Private m_uOverlapped               As OVERLAPPED
Private m_sUnicodeBuffer            As String

Private m_oReadCallback             As IReadCallback

Public Sub CloseConsole()

    CancelIo m_hStdIn
    Set m_oReadCallback = Nothing
    m_sUnicodeBuffer = vbNullString
    CloseHandle m_hStdIn

    FreeConsole

End Sub

Private Sub FileIOCompletionRoutine( _
    ByVal dwErrorCode As Long, _
    ByVal dwNumberOfBytesTransfered As Long, _
    ByRef uOverlapped As OVERLAPPED _
)

    On Error GoTo ErrorHandler

    m_oReadCallback.DataRead "FileIOCompletionRoutine"
    m_oReadCallback.DataRead "dwErrorCode = " & CStr(dwErrorCode)

    If dwErrorCode Then
        MsgBox "Error = " & CStr(dwErrorCode)
        CloseConsole
        Exit Sub
    End If

    m_oReadCallback.DataRead "dwNumberOfBytesTransfered = " & CStr(dwNumberOfBytesTransfered)

    m_oReadCallback.DataRead Left$(m_sUnicodeBuffer, dwNumberOfBytesTransfered)

Exit Sub

ErrorHandler:
    '
End Sub

Public Sub OpenConsoleForInput(ByRef the_oReadCallback As IReadCallback)

    Dim sFileName                   As String

    On Error GoTo ErrorHandler

    Set m_oReadCallback = the_oReadCallback

    AllocConsole

    'm_hStdIn = GetStdHandle(-10&)

    sFileName = "CONIN$"
    'm_hStdIn = CreateFile(StrPtr(sFileName), GENERIC_READ, FILE_SHARE_READ, ByVal 0&, OPEN_EXISTING,  0&, 0&)
    m_hStdIn = CreateFile(StrPtr(sFileName), GENERIC_READ Or GENERIC_WRITE, FILE_SHARE_READ, ByVal 0&, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, 0&)

    m_oReadCallback.DataRead "m_hStdIn = " & CStr(m_hStdIn)
    m_oReadCallback.DataRead "LastError = " & CStr(Err.LastDllError)

    m_sUnicodeBuffer = Space$(8192)

Exit Sub

ErrorHandler:
    Err.Raise Err.Number, Err.Source, Err.Description
End Sub

Public Sub Read()

    Dim nRet                            As Long
    Dim nBytesRead                      As Long

    On Error GoTo ErrorHandler

    m_oReadCallback.DataRead "About to call ReadFileExe"

    'm_uOverlapped.OffsetHigh = 0&
    'm_uOverlapped.OffsetOrPointer = 0&
    'nRet = ReadFile(m_hStdIn, StrPtr(m_sUnicodeBuffer), LenB(m_sUnicodeBuffer), nBytesRead, m_uOverlapped)
    nRet = ReadFileEx(m_hStdIn, StrPtr(m_sUnicodeBuffer), LenB(m_sUnicodeBuffer), m_uOverlapped, AddressOf FileIOCompletionRoutine)

    m_oReadCallback.DataRead "nRet = " & CStr(nRet)

    m_oReadCallback.DataRead "nBytesRead = " & CStr(nBytesRead)

    If nRet = 0 Then
        m_oReadCallback.DataRead "Err.LastDllError = " & CStr(Err.LastDllError)
    Else
        m_oReadCallback.DataRead StrConv(Left$(m_sUnicodeBuffer, nBytesRead), vbUnicode)
    End If

Exit Sub

ErrorHandler:
    Err.Raise Err.Number, Err.Source, Err.Description
End Sub

This relies on an interface (IReadCallback) to communicate with the main GUI.

Option Explicit

Public Sub DataRead(ByRef out_sData As String)
    '
End Sub

This is my sample form (FAsynchConsoleTest) - which uses a Timer (Timer) and RichTextBox (txtStdIn):

Option Explicit

Implements IReadCallback

Private Sub Form_Load()

    MAsynchConsole.OpenConsoleForInput Me

    Timer.Enabled = True

End Sub

Private Sub Form_Unload(Cancel As Integer)

    MAsynchConsole.CloseConsole

End Sub

Private Sub IReadCallback_DataRead(out_sData As String)

    txtStdIn.SelStart = Len(txtStdIn.Text)
    txtStdIn.SelText = vbNewLine & out_sData

End Sub

Private Sub mnuTimerOff_Click()

    Timer.Enabled = False

End Sub

Private Sub mnuTimerOn_Click()

    Timer.Enabled = True

End Sub

Private Sub Timer_Timer()

    MAsynchConsole.Read

End Sub

Unfortunately, whilst CreateFile() using FILE_FLAG_OVERLAPPED should create a file handle that can be used with async I/O, and the handle seems valid, ReadFileEx() returns 0, and GetLastError is 6, which is:

//
// MessageId: ERROR_INVALID_HANDLE
//
// MessageText:
//
// The handle is invalid.
//
#define ERROR_INVALID_HANDLE             6L

The console, interestingly, is frozen whilst this all happens.

Anyone else have any ideas? The docs seem to suggest that if you use CreateFile() with a console device name, the parameter is ignored.