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MSYNC(2) Linux Programmer's Manual MSYNC(2)


     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.


     This page is part of release 4.16 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

Linux 2017-09-15 MSYNC(2)

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