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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | SEE ALSO | COLOPHON |
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msync(2) System Calls Manual msync(2)
msync - synchronize a file with a memory map
Standard C library (libc, -lc)
#include <sys/mman.h>
int msync(size_t length;
void addr[length], 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:
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).
On success, zero is returned. On error, -1 is returned, and errno
is set to indicate the error.
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.
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.
POSIX.1-2008.
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).)
mmap(2)
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128–129 and 389–391.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-06-28 msync(2)
Pages that refer to this page: madvise(2), mmap2(2), mmap(2), remap_file_pages(2), sync_file_range(2), syscalls(2), nfs(5), systemd.exec(5), fanotify(7), inotify(7), xfs_io(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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