The C `clock()` function just returns a zero
clock() reports CPU time used. sleep() doesn't use any CPU time. So your result is probably exactly correct, just not what you want.
man clock. It's not returning what you think it is. Also man gettimeofday - it's more likely what you want.
clock_t is an integer type. You can't print it out with %f. See Fred's answer for why the difference is 0.