According to the C standard, it is undefined behaviour to have more than one references to a variable in a single sequence point (here you can think of that as being a statement, or parameters to a function) where one of more of those references includes a pre/post modification.
So:
foo(f++,f) <--undefined as to when f increments.
And likewise (I see this all the time in user code):
*p = p++ + p;
Typically a compiler will not change its behaviour for this type of thing (except for major revisions).
Avoid it by turning on warnings and paying attention to them.
