Constvolatile

Constvolatile is a keyword combination in some programming languages, notably C and C++. It serves to modify the interpretation of a variable by the compiler. The 'const' qualifier indicates that the variable's value should not be modified by the program after initialization. The 'volatile' qualifier signals to the compiler that the variable's value may change at any time, even if no code explicitly modifies it within the current scope. This typically occurs when the variable is accessed by hardware or by an interrupt service routine.

When a variable is declared as both const and volatile, it means that the program must treat the variable as read-only, yet be aware that its contents might be externally altered. The compiler must therefore avoid optimizing away reads from such a variable, as each read might yield a different value. This is crucial for interacting with hardware registers, shared memory in multi-threaded applications where another thread or process might modify the data, or in scenarios involving memory-mapped I/O. Without 'volatile', a compiler might cache the value of a const volatile variable and reuse the cached value, leading to incorrect behavior if the external source has changed the variable's actual content.