msync(2) — Linux manual page


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:

              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).

RETURN VALUE         top

       On success, zero is returned.  On error, -1 is returned, and errno is
       set appropriately.

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
       <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


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

COLOPHON         top

       This page is part of release 5.09 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                            2020-06-09                         MSYNC(2)

Pages that refer to this page: arm_sync_file_range(2)madvise(2)mmap2(2)mmap(2)munmap(2)remap_file_pages(2)sync_file_range2(2)sync_file_range(2)syscalls(2)mmap64(3)nfs(5)systemd.exec(5)fanotify(7)inotify(7)xfs_io(8)