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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. Asynchronous signal handling falls under these conditions. Without this type qualifier, unintended optimizations may occur.

Wiki Markupvolatile eliminates this confusion by imposing restrictions on access and cacheing \[C99 Rationale\cacheing. According to the C99 Rationale]:

No cacheing through this lvalue: each operation in the abstract semantics must be performed (that is, no cacheing assumptions may be made, since the location is not guaranteed to contain any previous value). In the absence of this qualifier, the contents of the designated location may be assumed to be unchanged except for possible aliasing.

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