, membar_enter

, membar_exit

, membar_producer

, membar_consumer

- memory access synchronization barrier operations

Synopsis

#include <atomic.h>

void membar_enter(void);

void membar_exit(void);

void membar_producer(void);

void membar_consumer(void);

Description

The membar_enter() function is a generic memory barrier used during lock entry. It is placed after the memory operation that acquires the lock to guarantee that the lock protects its data. No stores from after the memory barrier will reach visibility and no loads from after the barrier will be resolved before the lock acquisition reaches global visibility.

The membar_exit() function is a generic memory barrier used during lock exit. It is placed before the memory operation that releases the lock to guarantee that the lock protects its data. All loads and stores issued before the barrier will be resolved before the subsequent lock update reaches visibility.

The membar_enter() and membar_exit() functions are used together to allow regions of code to be in relaxed store order and then ensure that the load or store order is maintained at a higher level. They are useful in the implementation of mutex exclusion locks.

The membar_producer() function arranges for all stores issued before this point in the code to reach global visibility before any stores that follow. This is useful in producer modules that update a data item, then set a flag that it is available. The memory barrier guarantees that the available flag is not visible earlier than the updated data, thereby imposing store ordering.

The membar_consumer() function arranges for all loads issued before this point in the code to be completed before any subsequent loads. This is useful in consumer modules that check if data is available and read the data. The memory barrier guarantees that the data is not sampled until after the available flag has been seen, thereby imposing load ordering.

Return Values

No values are returned.

Errors

No errors are defined.

Attributes

See attributes(5) for descriptions of the following attributes:

See Also

atomic_add(3C), atomic_and(3C), atomic_bits(3C), atomic_cas(3C), atomic_dec(3C), atomic_inc(3C), atomic_ops(3C), atomic_or(3C), atomic_swap(3C), attributes(5), atomic_ops(9F)

Notes

Atomic instructions (see atomic_ops(3C)) ensure global visibility of atomically-modified variables on completion. In a relaxed store order system, this does not guarantee that the visibility of other variables will be synchronized with the completion of the atomic instruction. If such synchronization is required, memory barrier instructions must be used.