Question

Consider a simple (global in my case) variable:

int i;

Somewhere this variable is accessed

pthread_mutex_lock(i_mutex);
if(i == other value) {
  do_something();
}
pthread_mutex_unlock(i_mutex);

Another thread updates i while it holds i_mutex . Could the compiler cache the value of i so
I don’t get the recent value ? Must i be volatile ?

Answer 1

pthread locks implement memory barriers that will ensure that cache effects are made visible to other threads. You don’t need volatile to properly deal with the shared variable i if the accesses to the shared variable are protected by pthread mutexes.

from http://www.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_11:

The following functions synchronize
memory with respect to other threads:

fork()
pthread_barrier_wait()
pthread_cond_broadcast()
pthread_cond_signal()
pthread_cond_timedwait()
pthread_cond_wait()
pthread_create()
pthread_join()
pthread_mutex_lock()       // <====
pthread_mutex_timedlock()
pthread_mutex_trylock()
pthread_mutex_unlock()     // <====
pthread_spin_lock()
pthread_spin_trylock()
pthread_spin_unlock()
pthread_rwlock_rdlock()
pthread_rwlock_timedrdlock()
pthread_rwlock_timedwrlock()
pthread_rwlock_tryrdlock()
pthread_rwlock_trywrlock()
pthread_rwlock_unlock()
pthread_rwlock_wrlock()
sem_post()
sem_timedwait()
sem_trywait()
sem_wait()
semctl()
semop()
wait()
waitpid()