Using Win32 functions in Visual FoxPro Image Gallery
Code examples:
Custom GDI+ class
Storing content of the Clipboard to a bitmap file
How to print FoxPro form
Storing screen shot of a form to bitmap file
How to print a bitmap file
Capturing keyboard activity of another application with the Raw Input API (VFP9)
Displaying icons in the system tray (VFP9)
GDI+: Implementing image scrolling with inertia
Vertical Label control
Displaying dimmed window behind VFP top-level form
Splash Screen for the VFP application
Monitoring clipboard content changes (VFP9)
Pocket PC: custom RAPI class for operating with files and folders on mobile device
Copying picture of the active form to the Clipboard using Enhanced Metafile API functions
Obtaining names and positions for shortcuts located on the Windows Desktop
Using the LoadImage function to have a bitmap file loaded and displayed on VFP main window
Reading the structure of VFP main menu
Custom HttpRequest class (WinINet)
GDI+: sending image of FoxPro form to printer
Loading a string resource from an executable file
How to make a VFP form fading out when released (GDI version)
How to ping a remote site using IP Helper API calls
Printing Image File, programmatically set print page orientation to landscape
How to view icons stored in executable files (Icon Viewer) - II
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: 168  
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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