msync - synchronize a file with a memory map
int msync(const void *start, 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 disk.
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 start and having length length
is updated. The flags argument may have the bits
MS_ASYNC, MS_SYNC and MS_INVALIDATE set, but not both
MS_ASYNC and MS_SYNC. MS_ASYNC specifies that an update
be scheduled, but the call returns immediately. MS_SYNC
asks for 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 val
ues just written).
On success, zero is returned. On error, -1 is returned,
and errno is set appropriately.
EINVAL start is not a multiple of PAGESIZE, or any bit
other than MS_ASYNC | MS_INVALIDATE | MS_SYNC is
set in flags.
EFAULT The indicated memory (or part of it) was not
POSIX.1b (formerly POSIX.4)
mmap(2), B.O. Gallmeister, POSIX.4, O'Reilly, pp. 128-129