NAME

       pmem2_get_persist_fn() - get a persist function

SYNOPSIS

              #include <libpmem2.h>

              typedef void (*pmem2_persist_fn)(const void *ptr, size_t size);

              struct pmem2_map;

              pmem2_persist_fn pmem2_get_persist_fn(struct pmem2_map *map);

DESCRIPTION

       The  pmem2_get_persist_fn()  function  returns  a  pointer  to  a function responsible for
       efficiently persisting data in the range owned by the map.

       Persisting data using pmem2_persist_fn guarantees that the data is stored durably  by  the
       time it returns.

       There  are  no  alignment  restrictions  on  the  range  described  by  ptr  and size, but
       pmem2_persist_fn  may  expand  the  range  as  necessary  to   meet   platform   alignment
       requirements.

       There  is nothing atomic or transactional about pmem2_persist_fn.  Any unwritten stores in
       the given range will be written, but some stores may have already been written  by  virtue
       of  normal cache eviction/replacement policies.  Correctly written code must not depend on
       stores waiting until pmem2_persist_fn is called to become persistent  –  they  can  become
       persistent at any time before pmem2_persist_fn is called.

       If  two  (or  more) mappings share the same pmem2_persist_fn and they are adjacent to each
       other, it is safe to call this function for a range spanning those mappings.

       Internally pmem2_persist_fn performs two operations:

       • memory flush (pmem2_get_flush_fn(3)), which can be  reordered  by  the  CPU  with  other
         flushes

       • drain  (pmem2_get_drain_fn(3)),  which makes sure that the flushes before this operation
         won’t be reordered after it

       So this code:

              pmem2_persist_fn persist_fn = pmem2_get_persist_fn(map);
              persist_fn(addr, len);

       is equivalent of:

              pmem2_flush_fn flush_fn = pmem2_get_flush_fn(map);
              pmem2_drain_fn drain_fn = pmem2_get_drain_fn(map);

              flush_fn(addr, len);
              drain_fn();

       Advanced applications may want to flush multiple discontiguous  regions  and  perform  the
       drain operation only once.

RETURN VALUE

       The pmem2_get_persist_fn() function never returns NULL.

       The  pmem2_get_persist_fn() for the same map always returns the same function.  This means
       that it’s safe to cache its return value.  However, this function is very  cheap  (because
       it returns a precomputed value), so caching may not be necessary.

SEE ALSO

       pmem2_get_drain_fn(3),    pmem2_get_flush_fn(3),    pmem2_map_new(3),    libpmem2(7)   and
       <https://pmem.io>