Use pipe() and fdopen() to pass data from Python script to C++ application in Windows

In Windows, C file descriptors are inherited in the process STARTUPINFO record in the reserved fields cbReserved2 and lpReserved2. The protocol is undocumented, but the source is distributed with Visual C++. C functions that use this feature include the _[w]spawn family of functions and [_w]system. (The [_w]system function, however, is not generally useful in this regard because only the immediate cmd.exe process inherits the parent's file descriptors. CMD does not pass them on to child processes.)

In Python 2.7, os.pipe is implemented by calling CreatePipe, which returns non-inheritable file handles for the read and write ends of the pipe. These handles are then manually wrapped with inheritable file descriptors via _open_osfhandle, but the underlying OS handle is still non-inheritable. To work around this, duplicate the file descriptor via os.dup, which internally duplicates an inheritable file handle, and then close the source file descriptor. For example:

pread, pwrite = os.pipe()
pwrite_child = os.dup(pwrite)
os.close(pwrite)

Python's subprocess module is usually the preferred way to create a child process. However, we can't use subprocess in this case because it doesn't support inheriting file descriptors via STARTUPINFO (*). Here's an example that uses os.spawnv instead:

rc = os.spawnv(os.P_WAIT, 'path/to/spam.exe', ['spam', 'arg1', 'arg2'])

(*) It's an awkward situation that Windows Python internally uses the C runtime file API (e.g. _wopen, _read, _write), but in places fails to support C file descriptors. It should bite the bullet and use the Windows file API directly with OS file handles, then it would at least be consistent.