Add #ifndef to sort.h to have capability redefine this behaviour in your...

... code

Now you can redefine swap function, but it's optional.
It's useful fot tasks like this:
sort two dimensional arrays by first and second column:
x1 y1 x2 y2:

30 20 10 40 
10 50 40 30 
50 60 20 30 
sort them like this:

10 50 40 30 
30 20 10 40 
50 60 20 30