In ANSI C, you are allowed to pad as much as you feel is necessary.  In
K&R C, you are allowed to do damn near anything.

If an array of struct foo's requires word alignment, the sizeof() a given
struct foo should be a multiple of the size of a word; padding to 8 is
legit.

Code should use sizeof(struct foo) rather than 6; since sizeof() is a
compile time operator, this may be a millisecond longer of compile time,
but execution speed will not be hurt.

-s