Gregory's Server@resuna @jyrgenn Nope. Solaris -- as in Solaris proper, not the "Solaris 1" backported name for SunOS 4 -- was STREAMS and System V all the way.
With proper API/ABI definitions and compatibility. Look at some of the classic LSI drivers -- one binary covers 2.6 through 10 and the cut at 2.6 is only because of the jump to 64 bit splitting the driver tree.
@PCOWandre @resuna It was indeed SunOS up to 4 that was BSD-based, AFAIK from the start. (4.0.3 when I finally got access to the coveted Suns at the university, work as well as study. At my student job I got the task to install "all that GNU software" on the first SPARCstation 1 they bought — a night and a day of pure fun. No "./configure && make install", rather real configuration work at the time. The best thing: Emacs with native SunView support.)