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The volatile keyword is a hint to informs the compiler telling it not to optimize operations on that a memory location, but instead perform memory accesses for reads and writes. This property of the volatile keyword is sometimes confused as providing atomicity of a variable that is shared between threads in a multithreaded program. A variable declared as volatile is not cached in a register leading to this confusion that it can be used safely as a synchronization primitive. When declared as volatile the compiler does not re-order the sequence of reads and writes to that memory location. However, the compiler might re-order these reads and writes with those to other memory locations. This might result in non atomic operations on the synchronization variable resulting in errors.

Noncompliant Code Example

The following This noncompliant code example uses flag as a synchronization primitive.

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

void wakeup(){
  flag = true;
}

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

In the above piece of codethis example, the value of flag is used to determine if the critical section can be executed or not. Because the flag variable 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 This noncompliant code example uses flag as a synchronization primitive but this time declared as type volatilequalifies flag as a volatile type.

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.

Compliant Solution

The compliant solution would involve using This compliant uses a mutex to protect critical sections.

#include <pthread.h>

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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