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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | AVAILABILITY | NOTES | SEE ALSO | COLOPHON |
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:
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 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 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).)
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.
mmap(2)
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129 and 389-391.
This page is part of release 4.12 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 2015-08-08 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), fanotify(7), inotify(7), xfs_io(8)