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MEMBAR_OPS(3)              Library Functions Manual              MEMBAR_OPS(3)

NAME
     membar_ops, membar_enter, membar_exit, membar_producer, membar_consumer,
     membar_sync -- memory access barrier operations

SYNOPSIS
     #include <sys/atomic.h>

     void
     membar_enter(void);

     void
     membar_exit(void);

     void
     membar_producer(void);

     void
     membar_consumer(void);

     void
     membar_datadep_consumer(void);

     void
     membar_sync(void);

DESCRIPTION
     The membar_ops family of functions provide memory access barrier
     operations necessary for synchronization in multiprocessor execution
     environments that have relaxed load and store order.

     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.

SEE ALSO
     atomic_ops(3)

HISTORY
     The membar_ops functions first appeared in NetBSD 5.0.  The data-
     dependent load barrier, membar_datadep_consumer(), first appeared in
     NetBSD 7.0.

NetBSD 7.1.2                   November 20, 2014                  NetBSD 7.1.2