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DESCRIPTION

membar_enter()

Any store preceding membar_enter() will reach global visibility before all loads and stores following it.

membar_enter() is typically used in code that implements locking primitives to ensure that a lock protects its data.

membar_exit()

All loads and stores preceding membar_exit() will reach global visibility before any store that follows it.

membar_exit() is typically used in code that implements locking primitives to ensure that a lock protects its data.

membar_producer()

All stores preceding the memory barrier will reach global visibility before any stores after the memory barrier reach global visibility.

membar_consumer()

All loads preceding the memory barrier will complete before any loads after the memory barrier complete.

membar_datadep_consumer()

Same as membar_consumer(), but limited to loads of addresses dependent on prior loads, or ‘data-dependent’ loads:

int **pp, *p, v;

p = *pp;
membar_datadep_consumer();
v = *p;
consume(v);

Does not guarantee ordering of loads in branches, or ‘control-dependent’ loads -- you must use membar_consumer() instead:

int *ok, *p, v;

if (*ok) {
	membar_consumer();
	v = *p;
	consume(v);
}

Most CPUs do not reorder data-dependent loads (i.e., most CPUs guarantee that cached values are not stale in that case), so membar_datadep_consumer() is a no-op on those CPUs.

membar_sync()

All loads and stores preceding the memory barrier will complete and reach global visibility before any loads and stores after the memory barrier complete and reach global visibility.