180. Static Memory

Question.9

A static variable is shared between an ISR and the main loop:

static volatile bool flag = false;

void UART_IRQHandler() {
   flag = true;  // Set in ISR
}

int main() {
   while (!flag) {}  // Wait in main loop
   process_data();
}

Why is volatile necessary?

Need Help? Refer to the Quick Guide below

volatile makes the variable thread-safe
Without volatile, the compiler may optimize the while loop into an infinite loop -- it assumes flag never changes because no visible code in main modifies it
volatile prevents the ISR from being called
volatile is only needed for hardware registers, not variables

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