I don't mind the POSIX module getting bigger. In C, these functions are all in a flat namespace, also. I like the view "if it's specified by POSIX, you find it in the POSIX module" (although this specific API was rejected for inclusion into POSIX). The functions are all very small, as the real functionality is in the C library, or even the OS kernel.

As for the ifdefery: most of it is straight-forward: functionality is either present or it isn't. It gets messy when it tries to use alternative underlying APIs, e.g. for Win32 or OS/2. If the code is to be refactored, this should be the way to go (i.e. move all Win32 and OS/2 implementations out of the module)

As for PyMem_Malloc: I see no need to use that API; it doesn't improve the code to do so, compared to malloc/free. All that matters it is symmetric.