NAME

       pmem2_get_memmove_fn(), pmem2_get_memset_fn(), pmem2_get_memcpy_fn() - get a function that provides opti‐
       mized copying to persistent memory

SYNOPSIS

              #include <libpmem2.h>

              typedef void *(*pmem2_memmove_fn)(void *pmemdest, const void *src, size_t len,
                      unsigned flags);
              typedef void *(*pmem2_memcpy_fn)(void *pmemdest, const void *src, size_t len,
                      unsigned flags);
              typedef void *(*pmem2_memset_fn)(void *pmemdest, int c, size_t len,
                      unsigned flags);

              struct pmem2_map;

              pmem2_memmove_fn pmem2_get_memmove_fn(struct pmem2_map *map);
              pmem2_memset_fn pmem2_get_memset_fn(struct pmem2_map *map);
              pmem2_memcpy_fn pmem2_get_memcpy_fn(struct pmem2_map *map);

DESCRIPTION

       The  pmem2_get_memmove_fn(), pmem2_get_memset_fn(), pmem2_get_memcpy_fn() functions return a pointer to a
       function responsible for efficient storing and flushing of data for mapping map.

       pmem2_memmove_fn(), pmem2_memset_fn() and pmem2_memcpy_fn() functions provide  the  same  memory  copying
       functionalities  as  their  namesakes memmove(3), memcpy(3) and memset(3), and ensure that the result has
       been flushed to persistence before returning (unless PMEM2_F_MEM_NOFLUSH flag was used).

       For example, the following code:

                      memmove(dest, src, len);
                      pmem2_persist_fn persist_fn = pmem2_get_persist_fn(map);
                      persist_fn(dest, len);

       is functionally equivalent to:

                      pmem2_memmove_fn memmove_fn = pmem2_get_memmove_fn(map);
                      memmove_fn(dest, src, len, 0);

       Unlike libc implementation, libpmem2 functions guarantee that if destination buffer  address  and  length
       are  8  byte  aligned  then  all stores will be performed using at least 8 byte store instructions.  This
       means that a series of 8 byte stores followed by persist_fn can be safely replaced by a single memmove_fn
       call.

       The flags argument of all of the above functions has the same meaning.  It can be 0 or a  bitwise  OR  of
       one or more of the following flags:

       • PMEM2_F_MEM_NODRAIN  - modifies the behavior to skip the final pmem2_drain_fn step.  This allows appli‐
         cations to optimize cases where several ranges are being copied to persistent  memory,  followed  by  a
         single  call to pmem2_drain_fn.  The following example illustrates how this flag might be used to avoid
         multiple calls to pmem2_drain_fn when copying several ranges of memory to pmem:

         pmem2_memcpy_fn memcpy_fn = pmem2_get_memcpy_fn(map);
         pmem2_drain_fn drain_fn = pmem2_get_drain_fn(map);

         /* ... write several ranges to pmem ... */
         memcpy_fn(pmemdest1, src1, len1, PMEM2_F_MEM_NODRAIN);
         memcpy_fn(pmemdest2, src2, len2, PMEM2_F_MEM_NODRAIN);

         /* ... */

         /* wait for any pmem stores to drain from HW buffers */
         drain_fn();

       • PMEM2_F_MEM_NOFLUSH - Don’t flush anything.  This implies PMEM2_F_MEM_NODRAIN.  Using  this  flag  only
         makes sense when it’s followed by any function that flushes data.

       The remaining flags say how the operation should be done, and are merely hints.

       • PMEM2_F_MEM_NONTEMPORAL  -  Use  non-temporal  instructions.   This  flag  is  mutually  exclusive with
         PMEM2_F_MEM_TEMPORAL.  On x86_64 this flag is mutually exclusive with PMEM2_F_MEM_NOFLUSH.

       • PMEM2_F_MEM_TEMPORAL  -  Use  temporal  instructions.    This   flag   is   mutually   exclusive   with
         PMEM2_F_MEM_NONTEMPORAL.

       • PMEM2_F_MEM_WC  -  Use  write combining mode.  This flag is mutually exclusive with PMEM2_F_MEM_WB.  On
         x86_64 this flag is mutually exclusive with PMEM2_F_MEM_NOFLUSH.

       • PMEM2_F_MEM_WB - Use write back mode.  This flag is mutually exclusive with PMEM2_F_MEM_WC.  On  x86_64
         this is an alias for PMEM2_F_MEM_TEMPORAL.

       Using an invalid combination of flags has undefined behavior.

       Without  any of the above flags libpmem2 will try to guess the best strategy based on the data size.  See
       PMEM_MOVNT_THRESHOLD description in libpmem2(7) for details.

RETURN VALUE

       The pmem2_get_memmove_fn(), pmem2_get_memset_fn(), pmem2_get_memcpy_fn() functions never return NULL.

       They return the same function for the same mapping.

       This means that it’s safe to cache their return values.  However, these functions are very cheap (because
       their return values are precomputed), so caching may not be necessary.

       If two (or more) mappings share the same pmem2_memmove_fn, pmem2_memset_fn, pmem2_memcpy_fn and they  are
       adjacent to each other, it is safe to call these functions for a range spanning those mappings.

SEE ALSO

       memcpy(3),  memmove(3), memset(3), pmem2_get_drain_fn(3), pmem2_get_memcpy_fn(3), pmem2_get_memset_fn(3),
       pmem2_map_new(3), pmem2_get_persist_fn(3), libpmem2(7) and <https://pmem.io>