According to the C Standard, Annex J, J.1 [ISO/IEC 9899:2011], the behavior of a program is unspecified in the circumstances outlined the following table. The descriptions of unspecified behaviors in the "Description" column are direct quotes from the standard. The parenthesized numbers refer to the subclause of the C Standard (C11) that identifies the unspecified behavior. The "Guideline" column in the table identifies the coding practices that address the specific case of unspecified behavior (USB).
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