There are a couple of ways to figure out.

open a terminal (OSX/Linux) and type:

    man dup

open a browser window and search for:

    man dup

Both will bring up the man page for the function call.

To get recursive, you can try:

    man man unix

(the unix is important, otherwise it gives you manly men)

I love that this situation occured.

you may also consider gnu info

  info dup