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An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Referencing a volatile object by using a non-volatile lvalue is undefined behavior. The C Standard, 6.7.3 4 paragraph 7 [ISO/IEC 9899:20112024], states

If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type, the behavior is undefined.

See undefined behavior 65.

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