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       msync - synchronize a file with a memory map


       #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 the filesystem.   Without  use
       of  this  call,  there  is  no  guarantee that changes are written back
       before munmap(2) is called.  To be more precise, the part of  the  file
       that  corresponds to the memory area starting at addr and having length
       length is updated.

       The flags argument should specify exactly one of MS_ASYNC and  MS_SYNC,
       and  may  additionally  include the MS_INVALIDATE bit.  These bits have
       the following meanings:

              Specifies that an update be  scheduled,  but  the  call  returns

              Requests an update and waits for it to complete.

              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.


       POSIX.1-2001, POSIX.1-2008.

       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


       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).)


       According to POSIX, either MS_SYNC or MS_ASYNC  must  be  specified  in
       flags,  and  indeed  failure  to  include one of these flags will cause
       msync() to fail on some systems.  However,  Linux  permits  a  call  to
       msync()  that specifies neither of these flags, with semantics that are
       (currently) equivalent to specifying MS_ASYNC.   (Since  Linux  2.6.19,
       MS_ASYNC  is  in  fact  a no-op, since the kernel properly tracks dirty
       pages and flushes them to storage as necessary.)   Notwithstanding  the
       Linux  behavior, portable, future-proof applications should ensure that
       they specify either MS_SYNC or MS_ASYNC in flags.



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


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