I'm working on an application that will need to be setuid root. As my first setuid-root project, I'm extremely nervous about security. I know that the most common attack on an executable is a buffer overrun, or letting them shove more into an array that fits.

My application is entirely non-interactive, the only user input coming from command line arguments. So my question is, does C and/or C++ set up argc and argv securely? Is it safe to assume that CLI args are not a threat? That is, is their creation not a threat, I realize I must still carefully validate their input.

You could compile it with Stack Smashing and Buffer overflow protection i guess, but i don't know how.
You should take a look at some of OpenBSD's projects, they are extremely good at coding securely.

Some links that might also be useful:
http://gcc.gnu.org/ml/gcc-patches/2005-05/msg01193.html
http://en.wikipedia.org/wiki/Stack-smashing_protection
http://www.trl.ibm.com/projects/security/ssp/