What is the volatile keyword in C?

volatile tells the compiler that the value of a variable can change at any time, from outside the normal program flow. So the compiler must read it from memory each time, and cannot keep it in a register.

volatile int flag = 0;      // may be changed by an interrupt or another thread

while (flag == 0) {
    // wait
}

Without volatile the compiler may see that the loop never changes flag and turn it into an infinite loop. It is used for hardware registers, interrupt handlers and signal flags. It does not make the code thread safe.

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