Open a COM port in C++ with number higher that 9

Question

I am using a COM port in C++. I can not open COM ports with a higher number than 9, for example 10. This is function used for COM port detection:

WCHAR port_name[7];
WCHAR num_port[4];        

for (i=1; i<256; i++)
{
    bool bSuccess = false;

    wcscpy(port_name,L"COM");
    wcscat(port_name,_itow(i,num_port,10));

    HANDLE hPort;

    //Try to open the port
    hPort = CreateFile(L"COM10", GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, 0);
    //hPort = CreateFile(port_name, GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, 0);

    if (hPort == INVALID_HANDLE_VALUE)
    {
        DWORD dwError = GetLastError();

        //Check to see if the error was because some other application had the port open
        if (dwError == ERROR_ACCESS_DENIED)
        {
            bSuccess = TRUE;
            j=j+1;  
        }
    }
    else   //The port was opened successfully
    {            
        bSuccess = TRUE;
        j=j+1;

        CloseHandle(hPort);   //closing the port
    }

    if (bSuccess)array_ports[j]=i;

}

I can not understand why for example COM10, throws FFFFFFFF back to HANDLE hPort.

hPort = CreateFile(L"COM10", GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, 0);

COM9, COM8, COM7, etc. works fine,

hPort = CreateFile(L"COM9", GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, 0);

It there a solution for this problem?

Answer 1

It is a bug and the resolution is to use the string

\\.\COM10

hPort = CreateFile("\\\\.\\COM10", GENERIC_READ | GENERIC_WRITE, 0, 0, OPEN_EXISTING, 0, 0);

check this article.

Answer 2

You need to use the following format for COM ports greater than 9:

\\\\.\\COM%d

Where %d is a printf-substitution for the port number.

Why? Well, this accesses the global NT object space, where all objects are stored. Windows only knows to alias COM0-9 in the fashion you're using it for DOS support; beyond that, they act like ordinary devices, which are accessed this way.

To explore the NT object space, I recommend WinObj which basically lets you browse around. \.\ is mapped to GLOBAL?? in this tree (as are some other areas, actually. The rest of the tree requires you have NT's, as opposed to Win32's, view of the system).

And just in case you didn't know, INVALID_HANDLE_VALUE is defined as 0xffffff... - this usually occurs when an open fails.