COMPort::open should prepend port name with \\?\

[niconii]

At the moment, COMPort::open only works for COM1-COM9 unless \\?\ is prepended to the string, because the filenames COM1-COM9 are handled specially by the Win32 subsystem, whereas others aren't, causing COMPort::open to attempt to open a file instead.

In contrast, a path like \\?\COM10 specifies a verbatim path, which will work with any COM port.

[dcuddeback]

@niconii Interesting. I didn't know this about Windows. I'll have to find a way to get 10 COM ports installed on Windows so that I can reproduce this.

[niconii]

I personally ran into this when using com0com, which is a driver that can create pairs of virtual COM ports. The newest version (3.0.0.0) isn't signed, thus it doesn't work easily on newer versions of Windows, but there is an older version (2.2.2.0) which is signed.

By default, it creates pairs of ports with names like CNCA0 and CNCB0, CNCA1 and CNCB1, and so on. Since these names don't start with COM at all, they fail to work without a \\?\ prefix. When looking into it, I found out that the same is true of ports that do start with COM, yet don't fall within the COM1-COM9 range.

[dcuddeback]

@niconii Thank you. I was able to reproduce it using com0com. I'm pushing up a fix that will go into the next release. I found this documentation on MSDN useful (https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx):

For example, if you want to open the system's serial communications port 1, you can use "COM1" in the call to the CreateFile function. This works because COM1–COM9 are part of the reserved names in the NT namespace, although using the "\." prefix will also work with these device names. By comparison, if you have a 100 port serial expansion board installed and want to open COM56, you cannot open it using "COM56" because there is no predefined NT namespace for COM56. You will need to open it using "\.\COM56" because "\." goes directly to the device namespace without attempting to locate a predefined alias.

[niconii]

Alright. AFAIK it shouldn't matter whether \\?\ or \\.\ is used (the difference has something to do with path canonicalization which shouldn't be relevant for this), but yeah, you might as well use the one Microsoft recommends.