APR_POLLSET_THREADSAFE has to be supported, and that is more than the
well-known capabilities of threads and polling.
APR_POLLSET_THREADSAFE means that the platform has an advanced polling
mechanism which allows pollsets to be manipulated in a certain way,
and that APR has an implementation that uses that mechanism.
Linux, Solaris, and FreeBSD/Mac OS X are platforms that satisify both
characteristics. z/OS is a platform that has an advanced polling
mechanism but without an APR implementation in the stable branches.
Neither AIX nor HP-UX have APR implementations, though I'd guess that
the OS has the feature and nobody has gotten around to exploiting it
Thank you for the clarification. I'll further clarify what I put in the docs as soon as I have a chance.