MEMBAR_OPS(3C) Standard C Library Functions MEMBAR_OPS(3C)


NAME


membar_ops, 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:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Stable |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

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.


February 14, 2005 MEMBAR_OPS(3C)