[robespierre]

POSIX 1003.1b was ratified in 1993, after the SunOS 4 era, but the functions ( aread(), awrite(), await(), acancel() ) were already present in SunOS 4.1 in 1990. I have a full set of SunOS 4.1 hardcopy manuals and the pages for these calls do exist.
The way that Unix traditionally handled interactive applications is using signals and select(). Sockets can be opened in non-blocking mode so that reading from a socket with no new data returns to the work loop instead of waiting. SunOS also has lightweight processes (threads) so that a process can have more than one function active at once. POSIX 1003.1b threads (pthreads) were supported in Solaris 2 (SunOS 5) but not SunOS 4.