command line shown by ps

[PRIV Account]

How can a process change or obscure the command line that
ps (and other programs like w or whodo) show for it?  

This could obviously be desirable for security reasons, but also for
informational purposes.  For example, when a program forks a child
process to do something, the child proc might want to change
its "command line" (what ps shows, which by now probably has little
meaning) to something more descriptive of what its doing.

I know this is going to vary depending on system type.  I'm on 
DS3100 Ultrix, but I'd also like to know how its done on other common
systems, or where that information would be found.

Thanks for any insights.

Jock Cooper
uunet!hammer!jockc