[mfukar]

You're right; it is.

According to the Standard: "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored."

What the second sentence says, is this: if an object is written to within a full expression, any and all accesses to it within the same expression must be directly involved in the computation of the value to be written. This rule effectively constrains legal expressions to those in which the accesses demonstrably precede the modification. For example, the old standby i = i + 1 is allowed, because the access of i is used to determine i's final value. The example

        *(fp++) = *fp >> 4; /* redundant parentheses */

is disallowed because one of the accesses of fp (the one in *fp) has nothing to do with the value which ends up being stored in fp (which happens over in fp++), and so there's no good way to define - either for our understanding or the compiler's - whether the access should take place before or after the modified value is stored.

Hence, the quoted statement does invoke undefined behaviour.