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The C Standard, section 6.5 [ISO/IEC 9899:2011], states:
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. If there are multiple allowable orderings of the subexpressions of an expression, the behavior is undefined if such an unsequenced side effect occurs in any of the orderings.
(See also undefined behavior 35 in Annex J of the C Standard.)
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