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In the above piece of code, the value of flag is used to determine if the critical section can be executed or not. flag is not declared as being of type volatile and so may be cached in registers. Before the value in the register is written to memory, another thread might be scheduled to run and so may end up reading stale data.

Noncompliant Code Example

The following code uses flag as a synchronization primitive but this time declared as type volatile

volatile bool flag = false;
void test(){
  while (!flag){
    sleep(1000);
  }
}
void wakeup(){
  flag = true;
}
void debit(int amount){
  test();
  account_balance -= amount;
}

Declaring flag as volatile solves the problem of reading stale data, but still does not provide atomicity guarantees needed for synchronization primitives to work correctly. The volatile keyword does not promise to provide the guarantees needed for synchronization primitives.

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