fcntl() and packing structs with unions.

[tyler]

I'm having some trouble building a proper argument for an ioctl call.
The operation I'm working with is TUNSETIFF _IOW('T', 202, int) which
takes a struct ifreq as the arg. Could someone familiar with the
struct (usually defined in net/if.h) tell me how the heck to encode it
using the struct/array module? An example would be fantastic.

Thanks