Using Win32 functions in Visual FoxPro Image Gallery
Code examples:
How to change display settings: screen resolution, screen refresh rate
Capturing keyboard activity of another application with the Raw Input API (VFP9)
Converting Unicode data from the Clipboard to a character string using a given code page
Changing file attributes
Copying picture of the active form to the Clipboard using Enhanced Metafile API functions
Enumerating data formats currently available on the clipboard
How to print FoxPro form
Obtaining current Internet Explorer browser version and UserAgent
Custom GDI+ class
Enumerating raw input devices attached to the system (keyboard, mouse, human interface device)
GDI+: copying to the Clipboard (a) image of active FoxPro window/form, (b) image file
Creating a clipping region from the path selected into the device context of a form
Storing content of the Clipboard to a bitmap file
FindText -- the hopeless and useless Common Dialog
How to create transparent areas inside a form -- punching holes in the form
Enumerating Processes -- WinNT
Using EnumPrinters function to enumerate locally installed printers
A procedure for setting file times
Retrieving the name of the primary domain controller (PDC) and join status information
Using FrameRgn for displaying system colors
Placing a button on the VFP form as a new child window
Using the DeleteFile
Using the Semaphore object to allow only one instance of VFP application running
Storing screen shot of a form to bitmap file
Testing serial ports

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Before you begin:
Another example: Enumerating the ports that are available for printing on a specified server
DO decl

LOCAL nIndex, cPort
FOR nIndex=1 TO 8
    cPort = "COM" + TRANSFORM(nIndex)
    ? "Testing port " + m.cPort + ":", TestPort(m.cPort)
ENDFOR
* end of main

FUNCTION TestPort(cPort)
#DEFINE FILE_SHARE_READ   1
#DEFINE FILE_SHARE_WRITE  2
#DEFINE OPEN_EXISTING     3
#DEFINE GENERIC_READ      0x80000000
#DEFINE GENERIC_WRITE     0x40000000
#DEFINE FILE_FLAG_OVERLAPPED 0x40000000
#DEFINE INVALID_HANDLE_VALUE -1
#DEFINE FILE_ATTRIBUTE_NORMAL 128

    LOCAL hPort, lnErr

*    hPort = CreateFile(cPort, GENERIC_READ, 0,0,;
        OPEN_EXISTING, FILE_FLAG_OVERLAPPED, 0)

    hPort = CreateFile(cPort,;
        BITOR(GENERIC_READ,GENERIC_WRITE),;
        0,0, OPEN_EXISTING,;
        BITOR(FILE_FLAG_OVERLAPPED,FILE_ATTRIBUTE_NORMAL), 0)

    IF hPort = INVALID_HANDLE_VALUE
        lnErr = GetLastError()
        RETURN "Error " + TRANSFORM(lnErr) +;
            ". " + GetErrorMessage(lnErr)
    ELSE
        = CloseHandle(hPort)
        RETURN "Ok"
    ENDIF

PROCEDURE decl
    DECLARE INTEGER CreateFile IN kernel32;
        STRING lpFileName, INTEGER dwAccess, INTEGER dwShareMode,;
        INTEGER lpSecurityAttr, INTEGER dwCreationDisp,;
        INTEGER dwFlagsAndAttr, INTEGER hTemplateFile

    DECLARE INTEGER CloseHandle IN kernel32 INTEGER hObject
    DECLARE INTEGER GetLastError IN kernel32

    DECLARE INTEGER FormatMessage IN kernel32;
        INTEGER dwFlags, INTEGER lpSource, INTEGER dwMessageId,;
        INTEGER dwLanguageId, INTEGER @lpBuffer,;
        INTEGER nSize, INTEGER Arguments

    DECLARE RtlMoveMemory IN kernel32 As CopyMemory;
        STRING @Destination, INTEGER Source, INTEGER nLength

FUNCTION GetErrorMessage(lnErr)
#DEFINE FORMAT_MESSAGE_ALLOCATE_BUFFER 256
#DEFINE FORMAT_MESSAGE_FROM_SYSTEM     4096
#DEFINE FORMAT_MESSAGE_IGNORE_INSERTS  512

    LOCAL dwFlags, lpBuffer, lnLength, lpResult
    dwFlags = FORMAT_MESSAGE_ALLOCATE_BUFFER +;
        FORMAT_MESSAGE_FROM_SYSTEM + FORMAT_MESSAGE_IGNORE_INSERTS

    lpBuffer = 0
    lnLength = FormatMessage(dwFlags, 0, lnErr, 0, @lpBuffer, 0,0)
    IF lnLength <> 0
        lpResult = REPLI(Chr(0), 500)
        = CopyMemory (@lpResult, lpBuffer, lnLength)
        RETURN STRTRAN(LEFT(lpResult, lnLength), Chr(13)+Chr(10), "")
    ELSE
        RETURN "[]"
    ENDIF



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2151 bytes  
Created: 2002-05-06 20:14:58  
Modified: 2004-08-17 21:56:29  
Visits in 7 days: 165  
Listed functions:
CloseHandle
CopyMemory
CreateFile
FormatMessage
GetLastError
Printer friendly API declarations
My comment:
In Windows all input/output ports are presented as files, so work with ports is performed through file functions like CreateFile, CloseHandle, ReadFile, ReadFileEx, WriteFile and WriteFileEx.

Code sample Get the owner of an NTFS file works with serial ports as well as with regular files.

Check this MSDN article "Serial Communications in Win32".

Check VB example Using CreateFile to Determine Available COM Ports on VBNet.
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