Just curious, can you give me some examples of GNU-isms/macOS-isms? I have a suspicion what they might be, but I'm not sure.
I should add that this is for performance critical software.
On an out-of-the-box installation of FreeBSD/TrueOS, most processes listed by "ps" show a wchan of things like ttyin, select, poll, pause, uwait, wait, or nanslp. On the FreeBSD machine that I am typing at right now exactly one program is waiting in kqread, one of the inner processes of Chrome.
Install my nosh toolset, and a fair amount of that changes, because my programs use kqueue+kevent and suddenly the "ps" output is full of kqreads. But what's left is still not "hardly anyone", by a long chalk.
Would that it were! There's a nasty kernel panic somewhere in the innards of the kevent system on FreeBSD, where a sleeping thread holds a non-sleepable lock, that only manifests itself (at least for me) when there are a lot of programs using kqueue+kevent on a system that is doing a lot of stuff. If more people used kqueue+kevent in more programs, there would be more people wanting it fixed. (-:
For example, the MIO library in Rust, is an abstraction over epoll, kqueue, and the MS primative. Not one over the POSIX standard. This means that if you use that in a Rust library, you get that platform independence for free.
The place you need kqueue and epoll is when most of your connections are idle -- IRC servers and web servers with large keepalive times. But the vast majority of processes are not IRC or web servers.
See http://man7.org/linux/man-pages/man3/basename.3.html for the manpage.