NAME

       msync - synchronize a file with a memory map

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/mman.h>

       int msync(void addr[.length], size_t length, int flags);

DESCRIPTION

       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:

       MS_ASYNC
              Specifies that an update be scheduled, but the call returns immediately.

       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 to indicate the
       error.

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.

VERSIONS

       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.

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001.

       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).)

SEE ALSO

       mmap(2)

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