msync(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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

NAME         top

       msync - synchronize a file with a memory map

SYNOPSIS         top

       #include <sys/mman.h>

       int msync(void *addr, size_t length, int flags);

DESCRIPTION         top

       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         top

       On success, zero is returned.  On error, -1 is returned, and
       errno is set to indicate the error.

ERRORS         top

       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         top

       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.

       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).)

NOTES         top

       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 ALSO         top

       mmap(2)

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

COLOPHON         top

       This page is part of release 5.11 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                          2021-03-22                       MSYNC(2)

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