While Perderabo is right about the myriad of configurations a given box can have, on just about every Unix I've dealt with, 'ifconfig -a' will list the configuration, including IP address, for all your network interfaces. Dunno about the C function.

Also, it's certainly possible to configure a machine with no loopback interface. On BSD for example, just run 'ifconfig lo0 delete'. Of course, that'll probably break a lot of stuff ;-)