I need the IP address of the network interface which by default is
used for outgoing network traffic. The code should work under Mac OS X
(which is mostly FreeBSD under the hood). Something like
char *defaultip() { ... }
would be very nice.
If it's easier to get the name of the interface (i.e. "ppp0"), that
would be fine.

Thanks!

i'd suggest looking at the source for the route command, and the
source for the -rn argument of netstat