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

wqw's answer doesn't work for a form-based application, but the prototypes given there for Peek/ReadConsoleInput allow for one that does:

Private Declare Function AllocConsole Lib "kernel32" () As Long
Private Declare Function FreeConsole Lib "kernel32" () As Long
Private Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
Private Declare Function GetStdHandle Lib "kernel32" (ByVal nStdHandle As Long) As Long
Private Declare Function PeekConsoleInput Lib "kernel32" Alias "PeekConsoleInputA" (ByVal hConsoleInput As Long, lpBuffer As Any, ByVal nLength As Long, lpNumberOfEventsRead As Long) As Long
Private Declare Function ReadConsoleInput Lib "kernel32" Alias "ReadConsoleInputA" (ByVal hConsoleInput As Long, lpBuffer As Any, ByVal nLength As Long, lpNumberOfEventsRead As Long) As Long
Private Declare Function ReadFile Lib "kernel32" (ByVal hFile As Long, lpBuffer As Any, ByVal nNumberOfBytesToRead As Long, lpNumberOfBytesRead As Long, ByVal lpOverlapped As Long) As Long
Private Declare Function SetConsoleMode Lib "kernel32" (ByVal hConsoleInput As Long, dwMode As Long) As Long

Private Const STD_INPUT_HANDLE As Long = -10& ' GetStdHandle()

Private Const KEY_EVENT As Long = 1 ' PeekConsoleInput()

Private Const ENABLE_PROCESSED_INPUT As Long = &H1 ' SetConsoleMode()
Private Const ENABLE_ECHO_INPUT As Long = &H4

Dim hStdIn As Long

Private Sub Form_Load()

    AllocConsole

    hStdIn = GetStdHandle(STD_INPUT_HANDLE)
    SetConsoleMode hStdIn, ENABLE_PROCESSED_INPUT ' Or ENABLE_ECHO_INPUT ' uncomment to see the characters typed (for debugging)

    Timer1.Enabled = True

    Exit Sub

End Sub

Private Sub Form_QueryUnload(Cancel As Integer, UnloadMode As Integer)

    CloseHandle hStdIn
    FreeConsole

End Sub

Private Sub Timer1_Timer()

    Dim bytesRead As Long
    Dim buffer As String
    Dim baBuffer(0 To 512) As Byte
    Dim lEvents As Long

    PeekConsoleInput hStdIn, baBuffer(0), 1, lEvents
    If lEvents > 0 Then
        If baBuffer(0) = KEY_EVENT And baBuffer(4) <> 0 Then ' baBuffer(4) = INPUT_RECORD.bKeyDown
            buffer = Space$(1)
            Call ReadFile(hStdIn, ByVal buffer, Len(buffer), bytesRead, 0)

            ' buffer now contains one byte read from console
            ' Statements to process go here.

        Else
            Call ReadConsoleInput(hStdIn, baBuffer(0), 1, lEvents)
        End If
    End If
End Sub

PeekNamedPipe, GetConsoleMode and PeekConsoleInput will all return zero if your app isn't a true VB6 console app (though all that may be required is linking with the console subsystem, e.g., "C:\Program Files\Microsoft Visual Studio\vb98\LINK.EXE" /EDIT /SUBSYSTEM:CONSOLE MyApp.exe, I haven't tested it that far). They still work, however, at least Peek... does.

It is key that only one byte is read on each pass, as reading what is in baBuffer is problematic past the first record (INPUT_RECORD structure), but one byte at a time non-blocking is better than none at all. For me, Timer1 is set at 100 ms, but a better setting might be 55 ms, the events time slice.

Also key is that ReadConsoleInput is non-blocking if there is an event present on stdin, not just a key to be read. Using it when the recognized event isn't a key, effectively clears the event, allowing the application to proceed. It is possible to use this to read the bytes from the buffer without using ReadFile at all:

PeekConsoleInput hStdIn, baBuffer(0), 1, lEvents
If lEvents > 0 Then
    Call ReadConsoleInput(hStdIn, baBuffer(0), 1, lEvents)
    If baBuffer(0) = KEY_EVENT And baBuffer(4) <> 0 Then
        ' Chr(baBuffer(14)) now produces the character typed...

This hasn't been tested for reading true human input, except in the simplest debugging during construction, but it does work and should allow most VB6 form-based apps to effectively use a console. Thank you wqw!