#include <sys/mman.h>

       int msync(void *addr, size_t length, int flags);

       msync()  flushes  changes  made  to the in-core copy of a file that was
       mapped into memory using mmap(2) back to disk.   Without  use  of  this
       call  there  is  no guarantee that changes are written back before mun-
       map(2) is called.  To be more precise, the part of the file that corre-
       sponds  to the memory area starting at addr and having length length is

       The flags argument may have the bits MS_ASYNC, MS_SYNC, and  MS_INVALI-
       DATE  set,  but not both MS_ASYNC and MS_SYNC.  MS_ASYNC specifies that
       an update be scheduled, but the call returns immediately.  MS_SYNC asks
       for  an  update  and  waits  for it to complete.  MS_INVALIDATE asks to
       invalidate other mappings of the same file (so that they can be updated
       with the fresh values just written).

       On  success,  zero is returned.  On error, -1 is returned, and errno is
       set appropriately.

       EBUSY  MS_INVALIDATE was specified in flags, and a memory  lock  exists
              for the specified address range.

       EINVAL addr  is  not  a  multiple  of  PAGESIZE;  or any bit other than
              MS_ASYNC | MS_INVALIDATE | MS_SYNC is  set  in  flags;  or  both
              MS_SYNC and MS_ASYNC are set in flags.

       ENOMEM The indicated memory (or part of it) was not mapped.


       This  call was introduced in Linux 1.3.21, and then used EFAULT instead
       of ENOMEM.  In Linux 2.4.19 this was changed to the POSIX value ENOMEM.

       On   POSIX   systems   on   which   msync()    is    available,    both
       _POSIX_MAPPED_FILES   and   _POSIX_SYNCHRONIZED_IO   are   defined   in
       <unistd.h> to a value greater than 0.  (See also sysconf(3).)


       B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.

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