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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.

Compliant

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Solution

The compliant code example solution would involve using a mutex to protect critical sections.

#include <pthread.h>

volatile int account_balance;
pthread_mutex_t flag  = PTHREAD_MUTEX_INITIALIZER;

void debit(int amount) {
  pthread_mutex_lock(&flag);
  account_balance -= amount;//Inside critical section
  pthread_mutex_unlock(&flag);
}

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