msync - synchronize a file with a memory map
int msync(void *
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
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 immediately.
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).
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.
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_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
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
B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128–129 and 389–391.
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