createfile to open comm port...

I am writing a program to receive data from a serial device. Works great on my win2k pro machine, but my win98se machine won't open com1.

Here's my code:

hCom = CreateFile("COM1:", GENERIC_READ, 0, Null, OPEN_EXISTING, 0, 0)

If hCom <= 0 Then Exit Sub
...

On a win2k pro machine, this returns the handle, and the rest of my code works perfectly.

On a win98se machine, hCom = -1, code exits.

I've tried several permutations of the lpFileName parameter:

"COM1", "COM1:", "\\.\COM1", "\\.\COM1:", ...

Is there a trick?

David <><

I tried your code and it wouldn't even compile at first because I
had a different declaration (lpSecurityAttributes) when I did get it
to run it would not open COM1 jast as you. I opened up the
Device Manager and found that it said I HAVE NO COM1. But wait
a minute, my modem is on COM1 and the Device Manager
confirmed this.

I went back to my code and switch the dwCreationDisposition
param from OPEN_EXISTING to OPEN_ALWAYS and it worked.
Below is my code.

Code:

---

'BAS
Option Explicit
Public Const OPEN_EXISTING = 3
Public Const OPEN_ALWAYS = 4

Declare Function WriteFile Lib "kernel32" (ByVal hFile As Long, lpBuffer As Any, ByVal nNumberOfBytesToWrite As Long, lpNumberOfBytesWritten As Long, ByVal lpOverlapped As Long) As Long
Declare Function CreateFile Lib "kernel32" Alias "CreateFileA" (ByVal lpFileName As String, ByVal dwDesiredAccess As Long, ByVal dwShareMode As Long, ByVal lpSecurityAttributes As Long, ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes&, ByVal hTemplateFile As Long) As Long
Declare Function CloseHandle Lib "kernel32" (ByVal hObject As Long) As Long
Declare Function FlushFileBuffers Lib "kernel32" (ByVal hFile As Long) As Long


'FORM

Option Explicit

Private Sub Form_Load()
    DialNumber "767-8900", "COM1"
End Sub



Private Sub DialNumber(sPhoneNumber, sCommPort As String)
   
    Dim sMsg As String
    Dim bModemCommand() As Byte, sCommand  As String
    Dim lOpenPort As Long
    Dim lRet As Long, lRetBytes As Long, i As Integer
   
    If MsgBox("Please pickup the phone and choose OK to dial " & sPhoneNumber, vbOKCancel, App.Title) = vbCancel Then
        Exit Sub
    End If
   
    lOpenPort = CreateFile(sCommPort, &HC0000000, 0, 0, OPEN_ALWAYS, 0, 0)
    If lOpenPort = -1 Then
        sMsg = "Unable to open communication port " & sCommPort
        GoTo Err_DialNumber
    End If
   
    sCommand = "ATDT" & sPhoneNumber & vbCrLf
    ReDim bModemCommand(Len(sCommand))
    For i = 0 To Len(sCommand) - 1
        bModemCommand(i) = Asc(Mid$(sCommand, i + 1, 1))
    Next
   
    lRet = WriteFile(lOpenPort, bModemCommand(0), Len(sCommand), lRetBytes, 0)
   
    If lRet = 0 Then
        sMsg = "Unable to dial number " & sPhoneNumber
        GoTo Err_DialNumber
    End If
   
    lRet = FlushFileBuffers(lOpenPort)
   
    MsgBox "Click OK when the phone finishes dialing.    "
    sCommand = "ATH0" & vbCrLf
    ReDim bModemCommand(Len(sCommand))

    For i = 0 To Len(sCommand) - 1
        bModemCommand(i) = Asc(Mid$(sCommand, i + 1, 1))
    Next
   
    lRet = WriteFile(lOpenPort, bModemCommand(0), Len(sCommand), lRetBytes, 0)
   
    lRet = FlushFileBuffers(lOpenPort)
   
    lRet = CloseHandle(lOpenPort)
   
    Exit Sub
   
Err_DialNumber:
    MsgBox sMsg & vbCr & vbCr & "Make sure no other devices are using Com port " & sCommPort
   
   
End Sub

---

Greg

I tried switching to open_always, to no avail...but wait!

You inspired me to do something I hadn't thought of yet...try mixing up the settings. Everything I found on MSDN said that lpSecurityAttributes had to be Null, not zero...and this is true on win2k. Passing zero won't work on win2k...but passing null apparently won't work on win9x! I changed the lpSecurityAttributes from zero to byval 0&, and I'm smokin'!!!

Basically I've gone to the following code to maintain compatibility between win2k and win9x:

'Declare statement for openfile
Public Declare Function CreateFile Lib "kernel32.dll" Alias "CreateFileA" _
(ByVal lpFileName As String, ByVal dwDesiredAccess As Long, _
ByVal dwShareMode As Long, lpSecurityAttributes As Any, _
ByVal dwCreationDisposition As Long, ByVal dwFlagsAndAttributes As Long, _
ByVal hTemplateFile As Long) As Long

'...in subroutine
strCom = "COM1"
'Open port in win2k
hCom = CreateFile(strCom, GENERIC_READ, 0, Null, OPEN_EXISTING, 0, 0
if hCom<=0 then
'Open Port in win9x
hCom = CreateFile(strCom, GENERIC_READ, 0, ByVal 0&, OPEN_EXISTING, 0, 0
if hCom <=0 then Exit Sub
end if

Hope this will be of benefit to anyone else who needs to maintain portability between win2k and win9x...