RTAI (Realtime Application Interface) is a Linux kernel patch, that exposes an API to userland processes, which can be used to create very low latency and jitter programs for realtime tasks.
I've been looking into compiling my own patched kernel to work with RTAI, but I've noticed that the official Ubuntu repositories contain a package called "RTAI". I'm quite sure that this package doesn't magically patch my kernel. For one thing, the current RTAI release does not even support the kernel that's running under my Ubuntu installation.
So what does that package do, and what is it useful for? It creates a folder named "kernel-patches" under /usr/src, that contains some
.patch.gz files for some 2.6.x linux kernels. It also comes with some userland libraries (binaries and headers) that you would normally use to compile your RTAI application. When I try to compile a sample with those libraries, the executable, unsurprisingly, segfaults.
The package description describes what RTAI is and not what purpose that package is meant to serve.