Using Win32 functions in Visual FoxPro Image Gallery
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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: 21  
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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