Question

Why does the following expression evaluate to 0?

i > --i

Suppose i = 5. Evaluating the expression from left to right, we evaluate the left operand (i) to get 5 and we evaluate the right operand (–i) to get 4. So the expression about should evaluate to 1. But when I compile it with gcc and run it, it always evaluates to 0. Is there a flaw in my thought process?

Answer 1

It’s simply undefined behaviour, since you are modifying the value of i as well as reading it without an intervening sequence point. The relational operator < does not introduce a sequence point.

From C11, 6.5(2):

If a side effect on a scalar object is unsequenced relative to either a different side effect on the same scalar object or a value computation using the value of the same scalar
object, the behavior is undefined.