We looked into the /usr/include/fcntl.h file on an Apple Campus UNIX
machine (apple.com, a BSD 4.3 UNIX system), and the FASYNC flag is defined
in the /usr/include/fcntl.h header file. In A/UX, instead of defining in
the /usr/include/fcntl.h file, it is defined in the
"/usr/include/sys/file.h" file with this statement:
#define FASYNC 0x00000040 /* signal pgrp when data ready */
Therefore, we think it is supported in A/UX because the BSD socket library
is supported under A/UX.