Improper Access to a Union

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ANSI 3.3.2.3** A member of a union object is accessed using a member of a different type

If a union of two types is declared and one value is stored, but the union is accessed with the other type, the results are unreliable.

For example, a union of float and int is declared. A float value is stored, but the program later accesses the value as an int. In such a situation, the value would depend on the internal storage of float values. The integer value would not be reliable.

Structures, Unions, Enumerations, and Bit Fields