Using C/Pthreads: do shared variables need to be volatile?

Question

In the C programming language and Pthreads as the threading library; do variables/structures that are shared between threads need to be declared as volatile? Assuming that t

Answer 1

POSIX 7 guarantees that functions such as pthread_lock also synchronize memory

https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_11 "4.12 Memory Synchronization" says:

The following functions synchronize memory with respect to other threads:

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()

Therefore if your variable is guarded between pthread_mutex_lock and pthread_mutex_unlock then it does not need further synchronization as you might attempt to provide with volatile.

Related questions:

• Does guarding a variable with a pthread mutex guarantee it's also not cached?
• Does pthread_mutex_lock contains memory fence instruction?