I thought you were speaking about some Linux specific APIs.

There are some Linux specific APIs that build on this and I've got the most experience with Linux so I was refering to that.

But Async IO is part of POSIX.

I was mostly responding to the comment which cited synchronous IO as a problem with POSIX. Linux is most widely used so on top of being my primary domain on the issue, it's going to ensure best understanding.

That's not to say it's implemented as efficiently as it could be.

See here: https://linux.die.net/man/7/aio

AFAIK, system-specific APIs such as epoll or kqueue are the way to go if you're serious about performing asynchronous IO?