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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 disk.   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  may  have  the  bits   MS_ASYNC,   MS_SYNC,   and
       MS_INVALIDATE  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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