There have, in fact, been multiple fork() implementation for Windows without kernel support. They weren&#x27;t very efficient, but IIRC, only in version 1.7 (at about 15 years of age), did the Cygwin fork() implementation start to use kernel support.<p>And that still does not answer my question: In what way is the &quot;NT kernel subsystem&quot; different from a user mode library? Either the underlying kernel supports the features you need, or it doesn&#x27;t. From my perspective, there&#x27;s an NT kernel (with the so-called Native API) and a few libraries that make it look like Win32 or severely limited POSIX or at one point in time OS2.