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MSYNC(2)                   Linux Programmer's ManualMSYNC(2)NAME       msync - synchronize a file with a memory mapSYNOPSIS       #include <sys/mman.h>       int msync(void *addr, size_t length, int flags);DESCRIPTION       msync()  flushes  changes  made  to the in-core copy of a file that was       mapped into memory usingmmap(2) back to the filesystem.   Without  use       of  this  call, there is no guarantee that changes are written back be-       foremunmap(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:       MS_ASYNC              Specifies that an update be scheduled, but the call returns  im-              mediately.       MS_SYNC              Requests 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).RETURN VALUE       On success, zero is returned.  On error, -1 is returned, and  errno  is       set appropriately.ERRORS       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.CONFORMING TO       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       ENOMEM.AVAILABILITY       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 alsosysconf(3).)NOTES       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.SEE ALSOmmap(2)       B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.COLOPHON       This page is part of release 5.05 of the Linux  man-pages  project.   A       description  of  the project, information about reporting bugs, and the       latest    version    of    this    page,    can     be     found     at       https://www.kernel.org/doc/man-pages/.Linux                             2017-09-15MSYNC(2)