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bool flag = false;

void test() {
  while (!flag) {
    sleep(1000);
  }
}

void wakeup(){
  flag = true;
}

void debit(unsigned int amount){
  test();
  account_balance -= amount;
}

volatile bool flag = false;

void test() {
  while (!flag){
    sleep(1000);
  }
}

void wakeup(){
  flag = true;
}

void debit(unsigned int amount) {
  test();
  account_balance -= amount;
}

#include <pthread.h>

int account_balance;
pthread_mutex_t flag  = PTHREAD_MUTEX_INITIALIZER;

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