msync - synchronize a file with a memory map
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:
On success, zero is returned. On error, -1 is returned, and errno is set
- 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).
- MS_INVALIDATE was specified in flags, and a memory lock
exists for the specified address range.
- 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
- The indicated memory (or part of it) was not mapped.
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
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.
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128–129 and
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