return() vs. exit() vs. _exit()

[rich at mit-bugs-bunny.arpa]

I'm on a binary only 3b2/300 running SV.2.2 so...

What's the difference between leaving main by return() vs exit() vs _exit()?
I mean in reality, by proposed standards (X3J11 can you hear me?), and
functionally (like on my machine).  Who closes file descriptors?  Who reclaims
memory?  What about shared memory?  What is a gate 4, 8?

K. Richard Magill