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The C Standard, 6.5, paragraph 2 [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.
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Note that not all instances of a comma in C code denote a usage of the comma operator. For example, the comma between arguments in a function call is not a sequence point. However, according to the C Standard, 6.5.2.2, paragraph 10 [ISO/IEC 9899:2011]:
Every evaluation in the calling function (including other function calls) that is not otherwise specifically sequenced before or after the execution of the body of the called function is indeterminately sequenced with respect to the execution of the called function.
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Programs cannot safely rely on the order of evaluation of operands between sequence points. In this noncompliant code example, i is evaluated twice without an intervening sequence point, and so the behavior of the expression is undefined:
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