What is evaluation order of function parameters in C?

It is compiler dependent in C. It is never safe to depend on the order of evaluation of side effects. For example, a function call like below may very well behave differently from one compiler to another:

void func (int, int);

int i = 2;
func (i++, i++);

There is no guarantee (in either the C or the C++ standard language definitions) that the increments will be evaluated in any particular order. Either increment might happen first. func might get the arguments `2, 3′, or it might get `3, 2′, or even `2, 2′.

Source: http://gcc.gnu.org/onlinedocs/gcc/Non_002dbugs.html