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++);
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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