> Windows lacks fork(), or at least it's not documented.
The NT kernel supports forking, but the Win32 subsystem does not. Because Interix lives outside the Win32 subsystem, it can provide a proper fork() implementation, whereas Cygwin has to live with its somewhat brittle emulation.
As I understand it, the problem lies with making the different subsystems interact: Forking itself is reasonably easy using NtCreateProcess(), but the Win32 subsystem won't know how to deal with the forked process and stuff will break, including console output.
I don't see Microsoft adding forking support to the Win32 subsystem any time soon, so you'd end up rewriting Cygwin from scratch by reverse engineering Interix...
The NT kernel supports forking, but the Win32 subsystem does not. Because Interix lives outside the Win32 subsystem, it can provide a proper fork() implementation, whereas Cygwin has to live with its somewhat brittle emulation.