debugger extension question .. using IOCTL routines...

Anybody have a working example of an IOCTL call for dt command? I can see
some examples in windbgext.h for some of the routines, but am specifically
looking for a working example for the dt command. I found a writeup on
what the call should look like, typed it in, tried it, but the output
buffer was empty. I am guessing that I needed to fill in th field info
structure too. The example did not show how to setup the field info data
struct, so I am wondering what should have gone in that struct for a
successful call to work. Does anybody have a working example of what
should be in the field info structure? Does anybody have a working
example of an IOCTL call that outputs dt command like information?

Thanks in advance.