HOWTO: Specify Serial Ports Larger than COM9

• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.1, 3.5, 3.51, 4.0
      - Microsoft Windows 95 version 4.0
  

CreateFile() can be used to get a handle to a serial port. The "Win32 Programmer's Reference" entry for "CreateFile()" mentions that the share mode must be 0, the create parameter must be OPEN_EXISTING, and the template must be NULL.

CreateFile() is successful when you use "COM1" through "COM9" for the name of the file; however, the message "INVALID_HANDLE_VALUE" is returned if you use "COM10" or greater.

If the name of the port is \\.\COM10, the correct way to specify the serial port in a call to CreateFile() is as follows:

   CreateFile(
      "\\\\.\\COM10",     // address of name of the communications device
      fdwAccess,          // access (read-write) mode
      0,                  // share mode
      NULL,               // address of security descriptor
      OPEN_EXISTING,      // how to create
      0,                  // file attributes
      NULL                // handle of file with attributes to copy
   );