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.