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This requirement must be met for each allowable ordering of the subexpressions of a full expression; otherwise, the behavior is undefined. (see See undefined behavior 35.)

The following sequence points are defined in the C Standard, Annex C [ISO/IEC 9899:2011]:

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/* i is modified twice between sequence points */
i = ++i + 1;  

/* i is read other than to determine the value to be stored */
a[i++] = i;   

Note that not 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]

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